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Bycatch
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Shark entangled in a net on board a fishing vessel
A finetooth shark (Carcharhinus isodon) caught as bycatch by a fishing vessel

Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juveniles of the target species. The term "bycatch" is also sometimes used for untargeted catch in other forms of animal harvesting or collecting. Non-marine species (freshwater fish not saltwater fish) that are caught (either intentionally or unintentionally) but regarded as generally "undesirable" are referred to as rough fish (mainly US) or coarse fish (mainly UK).

In 1997, the Organisation for Economic Co-operation and Development (OECD) defined bycatch as "total fishing mortality, excluding that accounted directly by the retained catch of target species".[1] Bycatch contributes to fishery decline and is a mechanism of overfishing for unintentional catch.[2]

The average annual bycatch rate of pinnipeds and cetaceans in the US from 1990 to 1999 was estimated at 6215 animals with a standard error of 448.[3]

Bycatch issues originated with the "mortality of dolphins in tuna nets in the 1960s".[4]

There are at least four different ways the word "bycatch" is used in fisheries:[5]

  • Catch which is retained and sold but which is not the target species for the fishery [citation needed]
  • Species/sizes/sexes of fish which fishers discard[a]
  • Non-target fish, whether retained and sold or discarded[6]
  • Unwanted invertebrate species, such as echinoderms and non-commercial crustaceans, and various vulnerable species groups, including seabirds, sea turtles, marine mammals and elasmobranchs (sharks and their relatives).[7]

Additionally, the term "deliberate bycatch" is used to refer to bycatch as a source of illegal wildlife trade (IWT) in several areas throughout the world.[8]

There are several tools to estimate bycatch limits—the maximum number of animals that could be sustainably removed from a population impacted by bycatch. These include the 'potential biological removal' (PBR) and the 'sustainable anthropogenic mortality in stochastic environments' (SAMSE), which incorporates stochastic factors to determine sustainable limits to bycatch and other human-caused mortality of wildlife.[9]

Examples

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Recreational fishing

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Given the popularity of recreational fishing throughout the world, a small local study in the US in 2013 suggested that discards may be an important unmonitored source of fish mortality.[10]

Shrimp trawling

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Photo of boat moving forward at sea. On each side, the boat has one pole pointing away from boat with nets attached
Double-rigged shrimp trawler hauling in nets
Photo of hundreds of dead fish lying on ship deck
Shrimp bycatch

The highest rates of incidental catch of non-target species are associated with tropical shrimp trawling. In 1997, the Food and Agriculture Organization of the United Nations (FAO) documented the estimated bycatch and discard levels from shrimp fisheries around the world. They found discard rates (bycatch to catch ratios) as high as 20:1 with a world average of 5.7:1.[11]

Shrimp trawl fisheries catch two percent of the world total catch of all fish by weight, but produce more than one-third of the world total bycatch. US shrimp trawlers produce bycatch ratios between 3:1 (3 bycatch:1 shrimp) and 15:1 (15 bycatch:1 shrimp).[4]

Trawl nets in general, and shrimp trawls in particular, have been identified as sources of mortality for cetacean and finfish species.[12] When bycatch is discarded (returned to the sea), it is often dead or dying.[13]

Tropical shrimp trawlers often make trips of several months without coming to port. A typical haul may last four hours after which the net is pulled in. Just before it is pulled on board the net is washed by zigzagging at full speed. The contents are then dumped on deck and are sorted. An average of 5.7:1 means that for every kilogram of shrimp there are 5.7 kg of bycatch. In tropical inshore waters the bycatch usually consists of small fish. The shrimps are frozen and stored on board; the bycatch is discarded.[14]

Recent sampling in the South Atlantic rock shrimp fishery found 166 species of finfish, 37 crustacean species, and 29 other species of invertebrate among the bycatch in the trawls.[12] Another sampling of the same fishery over a two-year period found that rock shrimp amounted to only 10% of total catch weight. Iridescent swimming crab, dusky flounder, inshore lizardfish, spot, brown shrimp, longspine swimming crabs, and other bycatch made up the rest.[12]

Despite the use of bycatch reduction devices, the shrimp fishery in the Gulf of Mexico removes about 25–45 million red snapper annually as bycatch, nearly one-half the amount taken in recreational and commercial snapper fisheries.[15][16]

Cetacean

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Main article: Cetacean bycatch
Group of Fraser's dolphins

Cetaceans, such as dolphins, porpoises, and whales, can be seriously affected by entanglement in fishing nets and lines, or direct capture by hooks or in trawl nets. Cetacean bycatch is increasing in intensity and frequency.[17] In some fisheries, cetaceans are captured as bycatch but then retained because of their value as food or bait.[18] In this fashion, cetaceans can become a target of fisheries.

A Dall's porpoise caught in a fishing net

One example of bycatch is dolphins caught in tuna nets. As dolphins are mammals and do not have gills, they may drown while stuck in nets underwater. This bycatch issue has been one of the reasons of the growing ecolabelling industry, where fish producers mark their packagings with disclaimers such as "dolphin friendly" to reassure buyers. However, "dolphin friendly" does not mean that dolphins were not killed in the production of a particular tin of tuna, but that the fleet which caught the tuna did not specifically target a feeding pod of dolphins, but relied on other methods to spot tuna schools.[citation needed] The bycatch of the Caspian seal may be recognized as one of the biggest entanglements of pinnipeds as bycatch in the world [19][20]

Albatross

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See also: Longline fishing
Photo of bird struggling to fly away
Black-browed albatross hooked on a long-line

Of the 22 albatross species recognised by IUCN on their Red List, 15 are threatened with extinction, six species are considered as Near Threatened, and only one of Least Concern.[21] Two species, the Tristan albatross and the waved albatross, are considered as Critically Endangered.[21] One of the main threats is commercial longline fishing,[22] because albatrosses and other seabirds which readily feed on offal are attracted to the set bait, after which they become hooked on the lines and drown. An estimated 100,000 albatross per year are killed in this fashion. Unregulated pirate fisheries exacerbate the problem.

A research study examined the impact of illegal longline fishing vessels on albatrosses, by using environmental criminology as a guiding theoretical framework.[23] The results indicated that potentially illegal longline fishing activities are highly concentrated in areas of illegally-caught fish species, and the risk to bycatch albatrosses is significantly higher in areas where these illegal longline fishing vessels operate.[23] These findings provide strong grounding that illegal longline fishing poses a particularly serious threat to the survival of seabirds.

Sea turtles

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Sea turtle swimming near a diverse group of fish.
Loggerhead sea turtle

Sea turtles, already critically endangered, have been killed in large numbers in shrimp trawl nets. Estimates indicate that thousands of Kemp's ridley, loggerhead, green, and leatherback sea turtles are caught in shrimp trawl fisheries in the Gulf of Mexico and the US Atlantic annually[24] The speed and length of the trawl method is significant because, "for a tow duration of less than 10 minutes, the mortality rate for sea turtles is less than one percent, whereas for tows greater than sixty minutes the mortality rate rapidly increases to fifty to one hundred percent".[25]

Sea turtles can sometimes escape from the trawls. In the Gulf of Mexico, the Kemp's ridley turtles recorded most interactions, followed in order by loggerhead, green, and leatherback sea turtles. In the US Atlantic, the interactions were greatest for loggerheads, followed in order by Kemp's ridley, leatherback, and green sea turtles.[24]

Fishing gear

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Bycatch is inevitable wherever there is fishing. The incidental catch is not limited to only fish species: dolphins, sea turtles, and seabirds are also victims of bycatch. Longlines, trawls and purse seine nets are driving factors in the endangerment of no fewer than fifteen shark species. Bycatch may also affect reproduction of populations as juveniles are also victims of bycatch. Bycatch happens most commonly with the use of gillnetting, longlines, or bottom trawling. Longlines with bait hook attachments can potentially reach lengths of dozens of kilometres, and, along with gill nets in the water and bottom trawls sweeping the sea floor, can catch essentially everything in their path.[26] There are thousands of kilometres of nets and lines cast into the world's oceans daily. This modern fish gear is robust and invisible to the eye, making it efficient at catching fish and bycatching everything that happens to be in the way.[27] Hook-and-line fishing could limit bycatch to a certain extent as the non-target animals can be released back to the ocean fairly quickly.[26]

Mitigation

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A turtle excluder device

Concern about bycatch has led fishers and scientists to seek ways of reducing unwanted catch.[28] There are two main approaches.

One approach is to ban fishing in areas where bycatch is unacceptably high. Such area closures can be permanent, seasonal, or for a specific period when a bycatch problem is registered. Temporary area closures are common in some bottom trawl fisheries where undersized fish or non-target species are caught unpredictably. In some cases fishers are required to relocate when a bycatch problem occurs.

The other approach is alternative fishing gear. A technically simple solution is to use nets with a larger mesh size, allowing smaller species and smaller individuals to escape. However, this usually requires replacing the existing gear. In some cases, it is possible to modify gear. Bycatch reduction devices (BRDs) and the Nordmore grate are net modifications that help fish escape from shrimp nets.

Bycatch reduction devices

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BRDs allow many commercial finfish species to escape. The US government has approved BRDs that reduce finfish bycatch by 30%. Spanish mackerel and weakfish bycatch in the South Atlantic was reduced by 40%.[12] However, recent surveys suggest BRDs may be less effective than previously thought.[15] A rock shrimp fishery off Florida found the devices failed to exclude 166 species of fish, 37 crustacean species, and 29 species of other invertebrates.[12]

A pulsed electric field-based shark and ray bycatch mitigation device, SharkGuard, was reported by 2022 study to have reduced bycatch of blue shark by 91% and of stingrays by 71% with commercial fishing gear in a French longline tuna fishery in the Mediterranean.[29][30]

Turtle excluder devices

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In 1978, the National Marine Fisheries Service (NMFS) started to develop turtle excluder devices (TED). A TED uses a grid which deflects turtles and other big animals, so they exit from the trawl net through an opening above the grid. US shrimp trawlers and foreign fleets which market shrimp in the US are required to use TEDs. Not all nations enforce the use of TEDs.

For the most part, when they are used, TEDs have been successful reducing sea turtle bycatch.[12][31][32] However, they are not completely effective, and some turtles are still captured.[12][24] NMFS certifies TED designs if they are 97% effective. In heavily trawled areas, the same sea turtle may pass repeatedly through TEDs.[24] Recent studies indicate recapture rates of 20% or more, but it is not clear how many turtles survive the escape process.[24]

Conservation engineering of trawl nets

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The size selectivity of trawl nets is controlled by the size of the net openings, especially in the "cod end". The larger the openings, the more easily small fish can escape. The development and testing of modifications to fishing gear to improve selectivity and decrease impact is called "conservation engineering".

Photo of hundreds of seabirds on water surface around boat
Seabirds chase longline fishing vessel

Longline fishing is controversial in some areas because of bycatch. Mitigation methods have been successfully implemented in some fisheries. These include:

  • weights to sink the lines quickly
  • streamer lines to scare birds away from baited hooks while deploying the lines
  • setting lines only at night with minimal ship lighting (to avoid attracting birds)
  • limiting fishing seasons to the southern winter (when most seabirds are not feeding young)
  • not discharging offal while setting lines.

However, gear modifications do not eliminate bycatch of many species. In March 2006, the Hawaiʻi longline swordfish fishing season was closed due to excessive loggerhead sea turtle bycatch after being open only a few months, despite using modified circle hooks.

One of the mitigation methods is using streamer lines (in orange).

No discards policy

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One solution that Norway came up with to reduce bycatch is to adopt a 'no discards' policy. This means that the fishermen must keep everything they catch.[33] This policy has helped to "encourage bycatch research", which, in turn has helped "encourage behavioral changes in fishers" and "reduce the waste of life" as well.[4][33]

Seabirds

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Seabirds get entangled in longlines by flocking around vessels, this eventually leads to drowning because they try to catch baits on the hooks. Fisheries had been using "streamer lines" as a cost effective solution to mitigate this type of bycatch, and it has dramatically reduced seabird mortality. These streamer lines have bright colors and are made of polyester rope, they are positioned alongside the longlines on both sides. Their bright colors and constantly flapping of water frightens the seabirds and they fly away before reaching the baited hooks. A successful example would be the use of streamer lines in Alaskan groundfish longline fisheries, as the deaths of seabirds declined by about 70% after the deployment of these lines.[26]

Alternative to discarding

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Some fisheries retain bycatch, rather than throwing the fish back into the ocean. Sometimes bycatch is sorted and sold as food,[34] especially in Asia, Africa and Latin America, where cost of labour is cheaper. Bycatch can also be sold in frozen bags as "assorted seafood" or "seafood medley" at cheaper prices. Bycatch can be converted into fish hydrolysate (ground up fish carcasses) for use as a soil amendment in organic agriculture or it can be used as an ingredient in fish meal. In Southeast Asia bycatch is sometimes used as a raw material for fish sauce production. Bycatch is also commonly de-boned, de-shelled, ground and blended into fish paste or moulded into fish cakes (surimi) and sold either fresh (for domestic use) or frozen (for export). This is commonly the case in Asia or by Asian fisheries. Sometimes bycatch is sold to fish farms to feed farmed fish, especially in Asia.

Non-fisheries bycatch

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The term "bycatch" is used also in contexts other than fisheries. Examples are insect collecting with pitfall traps or flight interception traps for either financial, controlling or scientific purposes (where the bycatch may either be small vertebrates[35] or untargeted insects) and control of introduced vertebrates which have become pest species like the muskrat in Europe (where the bycatch in traps may be European minks[36] or waterfowl).

See also

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Notes

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Bycatch

View on Grokipedia
from Grokipedia
Bycatch refers to the incidental capture of non-target marine during operations aimed at specific commercial targets, including undersized or non-commercial , , rays, seabirds, sea turtles, marine mammals, and other organisms that become entangled or hooked in gear such as trawls, gillnets, and longlines. This phenomenon arises from the inherent selectivity limitations of methods, where gear captures organisms based on spatial overlap, , and rather than precise species targeting, often leading to immediate mortality, , or post-release stress that contributes to population declines in vulnerable taxa. In commercial fisheries, bycatch represents a significant portion of total catch—frequently discarded at sea due to regulatory restrictions, low market value, or unsuitability—resulting in wasted biomass and lost ecological roles, as affected species like apex predators help regulate prey populations and maintain biodiversity. Empirical assessments indicate that bycatch exacerbates overexploitation risks for non-target stocks and disrupts marine food webs, with cumulative effects on ecosystem function observed in regions of high fishing intensity, such as pelagic longline fisheries impacting seabird and turtle assemblages. While some bycatch species are retained for secondary markets or processing, the majority in trawl-dominated fisheries like shrimp harvesting yields disproportionate non-target captures, underscoring causal links between gear design and unintended harvests that challenge sustainable yield principles. Efforts to mitigate bycatch emphasize gear modifications, such as turtle excluder devices in trawls or bird-scaring lines in longlines, alongside temporal and spatial closures informed by observer data, though effectiveness depends on compliance and adaptive management, with experimental trials showing reductions in specific interactions but variable real-world outcomes due to behavioral variability among species. These measures highlight ongoing tensions between economic imperatives of fishing efficiency and empirical imperatives for ecosystem preservation, where underreporting in self-monitored fleets can obscure true scales, prompting calls for enhanced monitoring via electronic systems to ground policies in verifiable data rather than assumptions.

Definition and Scope

Core Definition and Components

Bycatch constitutes the incidental harvest of non-target marine organisms during activities directed at specific , encompassing both retained and discarded portions of the catch. According to the (FAO), bycatch is defined as " or other marine caught unintentionally while trying to catch another type of ," representing the portion of the catch captured in addition to the intended target. This definition aligns with U.S. regulatory frameworks under the Magnuson-Stevens Act, where bycatch includes harvested but not sold or retained for personal use, extending to economic discards (undersized or low-value catch) and regulatory discards ( exceeding quotas or protected by law). Core components of bycatch involve a spectrum of biological taxa and interaction outcomes, including finfish, , elasmobranchs, seabirds, marine mammals, and reptiles like sea turtles, which become hooked, entangled, or entrapped in gear such as trawls, longlines, gillnets, or pots. Retained bycatch consists of viable non-target sold commercially despite not being the primary objective, often comprising up to 20-30% of total catch in mixed-species fisheries like shrimp . Discarded bycatch, conversely, includes dead or dying organisms released at sea due to lack of market value, regulatory prohibitions, or gear limitations, contributing to unobserved mortality from stress, , or predation post-release. Unobserved bycatch mortality, such as from lost "ghost gear" or interactions evading direct observation, further amplifies ecological impacts, though quantification remains challenging without onboard monitoring. These components arise primarily from gear selectivity limitations and behavioral overlaps between target and non-target species in shared habitats, underscoring bycatch as an inherent byproduct of capture fisheries rather than isolated errors. Empirical data from global assessments indicate that bycatch can exceed target catch volumes in certain operations, such as purse-seine tuna fisheries where incidental captures of sharks or billfish occur alongside skipjack tuna.

Differentiation from Target Catch and Discards

Target catch refers to the species and sizes of marine organisms that commercial or artisanal fishers intentionally pursue and retain for landing, sale, or utilization, typically comprising the primary economic objective of a fishing operation. This portion is selected based on market demand, regulatory quotas, and gear selectivity, with retention rates often exceeding 90% for primary target species in well-managed fisheries. Bycatch, in contrast, encompasses the incidental capture of non-target , immature or oversized individuals of target , or protected alongside the intended catch, arising from the non-selective nature of gear such as trawls, gillnets, or longlines. According to the (FAO), bycatch constitutes "incidental catch taken in addition to the target ," which may include both retained incidental (if economically viable) and those discarded, distinguishing it from target catch by its unintended composition rather than retention status. NOAA Fisheries further clarifies that bycatch involves animals "not wanted, cannot sell, or are not allowed to keep," emphasizing its origin in operational inefficiencies or gear limitations rather than deliberate targeting. Discards represent the subset of total catch—encompassing both target and bycatch components—that is returned to the sea without being landed, often due to regulatory minimum sizes, poor quality, low , or excess quotas. Unlike bycatch, which focuses on identity (non-target), discards pertain to the disposal action, with global estimates indicating that discards account for 8-40% of total marine catch depending on gear type and region, including "high-grading" where higher-value target fish replace lower ones. This overlap occurs as much bycatch is discarded (e.g., 100% of captured seabirds or in some longline fisheries), but discards also include non-bycatch elements like undersized target , highlighting that while all discards contribute to mortality and waste, not all bycatch is discarded if marketable.

Historical Development

Traditional Fishing Eras Pre-1950

Prior to 1950, operations were predominantly artisanal and small-scale, utilizing selective gear such as handlines, pole-and-line, traps, spears, and localized nets that targeted specific sizes and species, thereby inherently limiting bycatch compared to later industrial methods. These techniques, employed since and persisting through the 19th and early 20th centuries, relied on labor and sail-powered vessels with restricted range and capacity, resulting in low overall effort and minimal unintended captures relative to post-war expansions. For instance, in North American Pacific fisheries, early 20th-century practices like setnetting for and often incidentally captured non-target fish, but volumes were managed through rudimentary rules allowing limited retention for crew food rather than systematic discard. Bycatch was recognized as an issue in specific contexts, such as the 19th-century Columbia River salmon wheel fisheries, where sturgeon (Acipenser spp.) were frequently discarded as a nuisance by fishermen targeting salmon, leading to substantial population reductions that persist today. Similarly, in Alaskan halibut fisheries, incidental catches prompted international agreements like the 1923 U.S.-Canada Convention, which addressed bycatch through prohibitions on wasteful practices, followed by the 1932 closure of nursery grounds and gear bans, including dories in 1935 and setnets in 1938 for halibut south of Cape Spencer. The 1937 "One-in-Seven Rule" further permitted retaining up to 1 pound of halibut per 7 pounds of other species, reflecting early efforts to utilize rather than waste incidental catches amid disputes over resource allocation. Ecological impacts from pre-1950 bycatch remained localized due to the absence of mechanized trawling fleets and high-seas operations, with discards often repurposed for local consumption, bait, or fertilizer rather than contributing to widespread ocean waste. Quantitative data on bycatch rates are scarce, as systematic observer programs and stock assessments emerged only later, but historical records indicate that non-target mortality did not drive broad biodiversity collapses until intensified effort post-World War II. Management emphasized target species conservation over bycatch mitigation per se, with regulations like the 1807 Upper Canada law restricting salmon capture methods to protect spawning runs, indirectly curbing incidental harms.

Post-War Expansion and Issue Emergence (1950s-1990s)

Following , the global industry underwent rapid expansion driven by technological innovations and economic recovery. Wartime developments in , (echo sounders), and diesel propulsion were adapted for civilian use, enabling vessels to locate fish schools more efficiently and operate farther offshore for longer durations. Synthetic nets, stronger and less prone to damage than natural fibers, increased gear durability and catch capacity, while the proliferation of factory ships—particularly in Soviet and Eastern European fleets, which comprised nearly 60% of large-vessel tonnage by the 1970s—allowed processing of massive hauls at sea. This period saw the global fishing fleet grow substantially, with vessel numbers doubling from 1.7 million in 1950 to higher levels by the late , fueled by subsidies and demand for protein in post-war populations. Consequently, marine capture production surged from approximately 16.8 million metric tons in 1950 to around 81 million metric tons by the late 1980s, reflecting intensified effort across distant waters previously underfished. These advances, while boosting target yields, amplified bycatch through less selective methods like otter trawling and purse seining, which swept broad swaths of ocean and captured non-target species in high volumes relative to kept catch—often exceeding 50% in trawls for shrimp or demersal fish. The shift to industrial-scale operations extended fishing pressure into deeper, more diverse ecosystems, where gear incidentally ensnared marine mammals, seabirds, and juveniles of commercial stocks, contributing to serial depletions observed from the 1950s onward. Early documentation of such impacts appeared in regional studies, but systemic underreporting in national statistics—due to discards at sea and focus on landed value—masked the scale until observer programs emerged. Bycatch gained prominence as a distinct environmental concern in the and amid rising ecological awareness, catalyzed by high-profile cases of mortality. In the eastern tropical Pacific tuna purse-seine , U.S. vessels targeting associated with spotted and spinner resulted in tens of thousands of dolphin deaths annually by the late 1960s, prompting public campaigns and scientific surveys that quantified encirclement bycatch rates exceeding 100,000 individuals per year. This led to the U.S. of 1972, which imposed strict bycatch limits and mandated gear modifications, marking the first major policy response to incidental capture. Similarly, shrimp trawling in the U.S. drew scrutiny in the 1970s for discarding vast quantities of finfish—estimated at 90-95% of catch by weight—fueling litigation that spurred development of turtle excluder devices by the . By the and , international bodies like the FAO began compiling bycatch data, revealing global patterns of ecosystem strain, though management lagged due to jurisdictional disputes and economic reliance on high-discard . These events shifted bycatch from an incidental to a recognized threat to and sustainability, with calls for selectivity improvements gaining traction despite resistance from industry stakeholders prioritizing efficiency.

Causal Mechanisms

Primary Fishing Gear and Techniques

Trawling represents one of the most significant contributors to bycatch, employing large nets dragged through the or along the seafloor to capture demersal or pelagic . Demersal or bottom trawls, which scrape the bottom, indiscriminately capture benthic organisms, , and non-target in their path, often resulting in bycatch ratios exceeding target catch by factors of 3 to 15 in shrimp fisheries. Midwater trawls, targeting schooling higher in the , still ensnare marine mammals, seabirds, and due to the net's wide mouth and fast towing speeds. Globally, trawl fisheries account for substantial discards, with estimates indicating high volumes of unutilized catch from non-selective gear contact with diverse . Longline fishing deploys extensive lines with thousands of baited hooks, either pelagic (near-surface for and ) or demersal (bottom-set for and ), leading to bycatch of seabirds, , turtles, and rays attracted to or mistaking hooks for prey. Pelagic longlines alone are estimated to kill approximately 160,000 seabirds annually worldwide through hooking or entanglement during setting or hauling. Demersal longlines exacerbate bycatch of bottom-dwelling species and scavenging marine mammals due to prolonged soak times, with challenges persisting despite measures like weighted lines or bird-scaring devices. Gillnets, consisting of vertical panels of netting that entangle , , and marine mammals by their gills or fins, pose severe bycatch risks due to their near-invisibility underwater and passive deployment. These fixed or drift nets capture an estimated 400,000 seabirds yearly and over 500,000 marine mammals globally, including dolphins, porpoises, and seals unable to detect the fine . Bycatch rates remain high in small-scale and artisanal fisheries, where gear is often unmonitored, though modifications like larger sizes have reduced interactions with species such as sturgeon by over 60% in some U.S. regions. Purse seine operations encircle dense schools of pelagic fish like with a deep curtain-like net, frequently capturing associated non-target such as , , and small pelagics due to behavioral aggregations. Historical bycatch in tropical tuna purse seines included massive dolphin mortalities, though rates have declined with gear modifications; current estimates show bycatch comprising about 5% of total catch in these fisheries. Despite improvements, unintended captures of and persist, driven by the method's reliance on visual or school detection without selectivity for co-occurring .

Biological and Environmental Contributors

Biological contributors to bycatch primarily stem from the behavioral, physiological, and ecological traits of non-target species that increase their interaction with fishing gear. For example, many marine mammals, seabirds, and sea turtles exhibit foraging behaviors that lead them to aggregate near target fish schools or baited hooks, such as dolphins herding prey into nets or albatrosses scavenging longline bait during surface feeding. These traits, including curiosity-driven approaches to novel stimuli like nets or lights, heighten entanglement risks, particularly in gillnets where species with poor maneuverability or slow reaction times—such as harbor porpoises—are disproportionately captured due to their echolocation limitations in turbid waters. Life history characteristics, like juvenile stages with underdeveloped escape responses or migratory schooling behaviors in species such as and rays, further exacerbate vulnerability by aligning their spatial distributions with high-effort fishing zones targeting tunas or billfishes. Environmental factors amplify these biological susceptibilities by dynamically altering species distributions and gear efficacy. Seasonal variations in and ocean currents, for instance, drive prey aggregations that co-locate non-target species with commercial fisheries; in the drift gillnet fishery, warmer waters correlate with elevated bycatch due to thermal preferences overlapping with sets. Chlorophyll-a concentrations, indicative of , influence bycatch rates by signaling hotspots—common dolphin entanglements in Pacific fisheries rise with abundance tied to spring blooms, peaking in March-April. Bathymetric features like depth gradients and zones concentrate pelagic species vertically and horizontally, increasing encounters in midwater trawls or longlines, while wind speed and moonlight phases modulate diving into gillnets by affecting visibility and efficiency. Climate-driven shifts, including poleward migrations of temperate species, are projected to intensify bycatch in poleward fisheries by 2050 through altered predator-prey dynamics, underscoring the interplay between abiotic forcings and biological responses.

Quantitative Assessment

Global and Regional Bycatch Rates

Global estimates of bycatch in marine capture fisheries vary due to differences in definitions, availability, and whether retained non-target are included alongside discards. A benchmarking study calculated annual global discards at 9.1 million tonnes (95% uncertainty interval: 7–16 million tonnes), equating to 10.8% (95% UI: 10–12%) of total reported catch for the period 2010–2014, with trawl fisheries contributing the largest share. Earlier FAO assessments from the suggested discards alone could exceed 20 million tonnes annually, or up to 25% of catch, though methodological improvements have refined these figures downward while highlighting underreporting in small-scale fisheries. Broader bycatch estimates, encompassing both discarded and retained incidental catch, range higher, with some analyses proposing up to 40% of global catch (approximately 63 billion pounds or 28.6 million tonnes annually as of early 2000s ), though such figures from advocacy sources warrant caution for potential overestimation without observer verification. Regional disparities reflect gear types, target species, and regulatory enforcement. Tropical shrimp trawl fisheries exhibit the highest bycatch ratios, with discards comprising over 27% of global totals despite shrimp landings representing less than 2% of world catch; ratios often exceed 5:1 (bycatch to shrimp by weight) in areas like the Gulf of Mexico and Southeast Asia. In contrast, industrial tuna purse-seine fisheries in the Pacific and Atlantic show lower overall discard rates (typically under 5%), but elevated incidental captures of sharks, billfish, and turtles, with bycatch varying from 1–10% of total catch depending on sets on free-swimming schools versus fish-aggregating devices. Longline tuna fisheries in the Atlantic report average discard rates of 21%, influenced by species composition and market factors, while gillnet operations in coastal regions like the Black Sea or Baltic yield bycatch rates up to 20–30% for non-target fish and marine mammals.
Fishery Type/RegionEstimated Bycatch/Discard Rate (% of total catch)Key NotesSource
Global Marine Capture10.8% (discards only)9.1 Mt annually; trawls dominantNature, 2020
Tropical Shrimp Trawl>27% of global discardsRatios 5:1+ by weight; high mortalityFAO, 2005
Pacific/Atlantic Purse-Seine1–10%Higher with FADs; species-specific (e.g., )Wiley, 2023
Atlantic Longline ~21%Varies by year and tuna ScienceDirect, 2023
These rates underscore the need for gear-specific , as underreporting—estimated at 20–50% in data-poor regions—likely understates true impacts, particularly in developing-world artisanal fleets. Post-2010 trends show modest declines in discard ratios in regulated areas like the Northeast Atlantic due to landing obligations, but global stability persists amid rising capture volumes.

Data Collection Methods and Limitations

Bycatch data are primarily collected through at-sea observer programs, in which trained personnel deploy on vessels to directly observe and record catches, including non-target , by weighing, counting, or sampling bycatch during hauls. These observers document gear types, locations, environmental conditions, and interactions, providing the raw data for estimating total bycatch across unobserved trips via expansion methods that scale observed rates to fleet-wide effort reported in logbooks. Electronic monitoring systems, including video cameras and sensors affixed to vessels, serve as an alternative or complement, capturing footage of hauls for post-processing analysis to identify and quantify bycatch without continuous human presence. Vessel logbooks and self-reported data from fishers contribute supplementary information on effort and catches, though these are often less reliable due to incentives for underreporting. Estimation procedures typically involve statistical models that raise observer-sampled bycatch rates—adjusted for factors like gear selectivity and spatiotemporal variability—to the total effort derived from mandatory entries or vessel monitoring systems. For instance, in U.S. fisheries managed by NOAA, bycatch totals are calculated by multiplying observed proportions by fleet-wide metrics such as days fished or hooks set, with delta-lognormal or stratified models accounting for zero-inflated data common in low-bycatch scenarios. International bodies like the FAO recommend integrating these with independent scientific surveys, such as trawl or acoustic assessments, to validate commercial data, though such surveys are sporadic and region-specific. Observer coverage remains a core limitation, with many fisheries achieving only 1-5% of trips monitored due to high costs—estimated at $500-1,000 per observer sea day—resulting in wide confidence intervals for rare or patchy bycatch events like entanglements. Biases arise from the "observer effect," where fishers may alter gear, locations, or practices to minimize detections, leading to underestimated rates; studies show protocol variations, such as fish-focused versus bycatch-prioritized sampling, can skew observed drop-out and interaction ratios by up to 20-30%. Self-reported logbooks exacerbate underestimation through deliberate omission or recall errors, while electronic monitoring faces challenges in species identification from footage and data storage overload in high-volume fisheries. Global inconsistencies in protocols hinder comparability, with high-seas fisheries often lacking any monitoring, and estimates for infrequently encountered carrying uncertainties exceeding 50% due to small sample sizes. Additional issues include observer safety risks and —reported in up to 20% of deployments as of 2024—potentially deterring participation and introducing toward compliant vessels. Despite improvements like AI-assisted EM analysis reducing processing times by 40-60%, coverage below 20% sustains high variability in precision, underscoring the need for risk-based stratification to target high-bycatch fleets.

Impacts on Ecosystems

Population-Level Effects on Non-Target Species

Bycatch mortality imposes substantial pressure on populations of non-target , particularly long-lived marine with low reproductive rates, such as marine mammals, seabirds, sea turtles, and elasmobranchs, often exceeding sustainable levels and driving declines. For with K-selected histories—characterized by delayed maturity, few , and high —bycatch removes breeding adults or juveniles, reducing and amplifying demographic imbalances through mechanisms like reduced and Allee effects, where low densities hinder success. Empirical assessments indicate that bycatch rates can surpass maximum sustainable yields for affected populations, leading to exponential declines absent mitigation. In marine mammals, bycatch in gillnets has nearly extirpated the (Phocoena sinus), with acoustic surveys documenting an average annual of 34% from 2015 to 2018, reducing numbers from approximately 100 individuals in 2015 to fewer than 10 by 2024, primarily due to entanglement in illegal fisheries. This collapse exemplifies how localized bycatch hotspots can eradicate endemic species, as vaquita habitat confines them to the northern , where gillnet bycatch accounts for over 90% of documented mortality. Seabird populations have similarly suffered, with longline fisheries implicated in annual bycatch mortality exceeding 200,000 individuals in European waters alone and global estimates reaching hundreds of thousands to over one million, correlating with observed declines in species like albatrosses. In Alaskan longline fisheries, bycatch has driven poor recovery in vulnerable taxa, with demographic models linking incidental mortality to sustained population trajectories downward despite regulatory efforts since the . For elasmobranchs, bycatch in pelagic longlines and trawls has contributed to global population reductions, including an 85% decline in dusky sharks (Carcharhinus obscurus) off the U.S. Atlantic coast since the , driven by post-release mortality rates often exceeding 50% in non-targeted captures. populations overall have plummeted over the past two decades, with bycatch exacerbating by removing individuals across size classes, disrupting age structures and hindering rebound due to their slow growth and low fecundity. Sea turtle populations face analogous threats from trawl and longline bycatch, which constitutes a primary driver of global declines; for instance, loggerhead (Caretta caretta) and leatherback (Dermochelys coriacea) nesting assemblages have decreased by 60-90% in regions with high incidental capture, as bycatch disproportionately affects subadults and adults during migration. Mitigation like turtle excluder devices has reduced U.S. trawl mortality by up to 94% in some fisheries since the , yet persistent bycatch in unregulated areas continues to impede recovery. These cases underscore that while sublethal injuries compound effects, direct mortality from bycatch remains the dominant population-level driver, necessitating gear-specific interventions to avert further erosions.

Broader Biodiversity and Food Web Consequences

Bycatch exacerbates loss by disproportionately affecting vulnerable taxa such as apex predators, , and rare endemics, which reduces overall and in marine ecosystems. For instance, incidental capture of top predators like and seabirds disrupts ecological roles that maintain balance across trophic levels, leading to simplified community structures and diminished resilience to environmental stressors. Empirical studies indicate that bycatch accounts for approximately 10% of global annual marine catches, or 9.1 million tons, directly threatening biodiversity hotspots where non-target species overlap with grounds. In food webs, bycatch-induced mortality triggers trophic cascades by removing regulators of prey populations, resulting in alternate stable states such as jellyfish blooms or algal overgrowth. Evidence from overfished systems shows that declines in predatory fish and elasmobranchs—often via bycatch—allow mesopredator and herbivore surges, which in turn depress primary producers and lower-trophic fisheries yields. For example, in regions with high shark bycatch, ray populations have increased, exerting predation pressure on bivalves and causing localized shellfish depletions that cascade upward to affect commercial stocks. These shifts alter energy flow and nutrient cycling, with modeling of 400 food webs demonstrating that heterogeneous bycatch reductions can preserve biomass in non-target species by up to median levels observed in unmanaged scenarios. Long-term ecosystem restructuring from bycatch includes flattened size spectra and lowered mean trophic levels in fished communities, as selective removal favors smaller, lower-trophic organisms over larger predators. This homogenization reduces functional diversity, impairing services like and engineering by species such as sea turtles, whose bycatch mortality compounds habitat loss effects. FAO assessments confirm that high discard rates from bycatch negatively influence both target and non-target populations, amplifying regime shifts observed in empirical data from gillnet and longline fisheries. Such dynamics underscore bycatch's role in eroding stability, with recovery hindered by ongoing incidental harvests exceeding natural mortality in many depleted guilds.

Economic and Human Dimensions

Direct Costs to Fishing Industries

Bycatch imposes direct economic costs on industries primarily through the discard of non-target that could otherwise be sold, operational inefficiencies from handling and sorting, gear damage caused by entangled animals, and enforced closures when bycatch quotas are exceeded. In the United States, commercial fisheries discard approximately 2 billion pounds of annually due to bycatch, representing a dockside value loss of at least $1 billion based on data analyzed in 2014. This discarded catch equates to forgone revenue, as much of it consists of marketable undersized or outside quota limits, exacerbating opportunity costs for fishermen targeting primary . Handling bycatch adds labor and fuel expenses, as crews spend time sorting and releasing non-target , which reduces overall and effective catch rates. For instance, in groundfish fisheries, on-deck sorting of like increases fuel consumption and incurs opportunity costs from delayed or shortened trips. Gear damage from bycatch, particularly entanglements with marine mammals, , or , necessitates repairs or replacements, though quantitative estimates remain limited; such incidents commonly tear nets and lines, directly elevating maintenance expenditures in trawl and gillnet operations. Excess bycatch frequently triggers early closures of fisheries to avoid quota overruns, resulting in substantial revenue shortfalls. A analysis estimated that bycatch-related closures in U.S. fisheries could cost up to $453 million annually in lost landings, with national bycatch discards representing a broader potential sales loss of approximately $4.2 billion if fully utilized. In specific cases, such as Pacific whiting fisheries, hitting bycatch limits has led to multimillion-dollar seasonal revenue losses for vessels unable to continue operations. These costs underscore bycatch as a direct drag on industry profitability, independent of broader ecological impacts.

Opportunities from Bycatch Utilization and Markets

Utilization of bycatch transforms incidental catches, often discarded due to low or regulatory constraints, into viable products, thereby mitigating economic losses estimated at approximately $4.2 billion annually in potential U.S. sales from discards. Primary outlets include processing into fishmeal and for feeds, livestock , and industrial applications, where bycatch supplements reduction fisheries' inputs in mixed-trawl operations. In 2023, global fishmeal production, partially derived from such low-value catches, supported expanding demands, with prices for 65% protein fishmeal averaging between $385 and $554 per ton. Efforts to develop direct human consumption markets target underutilized , particularly in regions like the U.S. Northeast, where discards exceed 100 million pounds yearly. For instance, a 2014 assessment of fisheries projected $57 million in fisher revenue and up to $1.96 billion in downstream value from marketing 126 million pounds of bycatch such as skate (90.4 million pounds discarded) and (21.65 million pounds), leveraging low ex-vessel prices ($0.21–$0.34 per pound) against higher retail potential through and consumer education. Initiatives promote like , Atlantic , silver , and as sustainable alternatives, with frameworks assessing market readiness based on abundance, nutritional profiles, and culinary versatility to build demand in restaurants and retail. In tropical shrimp trawling, bycatch—often comprising finfish and —is commercialized locally or processed into meal, as exemplified by Guyana's 1970s mandate requiring trawlers to land at least one of bycatch per trip for value addition, fostering processing and supplementary sales. The U.S. National Bycatch Reduction , updated in 2024, explicitly supports such utilization to enhance economic viability while aligning with goals, provided monitoring ensures it does not exacerbate fishing effort. These approaches generate ancillary income for fleets facing quota restrictions, though success hinges on , quota flexibility, and targeted to overcome entrenched preferences for high-value targets.

Prominent Examples by Species

Marine Mammals

Marine mammals experience substantial bycatch mortality, with cetaceans comprising the majority of documented cases globally. At least 300,000 cetaceans die annually from entanglement in fishing gear, primarily gillnets, purse seines, and trawls. Estimates for all marine mammals exceed 500,000 individuals per year, excluding and walruses, underscoring bycatch as the leading direct anthropogenic threat to these populations. Cetaceans, including dolphins, porpoises, and small whales, are particularly vulnerable in gillnet and purse fisheries. Gillnets alone account for approximately 50,000 bycatch deaths annually from 1990 to 2020, often in coastal and small-scale operations where monitoring is limited. In the Eastern Tropical Pacific purse seine fishery, dolphin-associated sets historically caused tens of thousands of deaths per year in the 1960s-1970s, but observer programs and techniques like the backdown maneuver reduced observed mortalities to under 1,500 annually by the 2010s, though unreported interactions persist. Pinnipeds such as seals and sea lions face risks primarily in trawl fisheries, where they enter nets during hauling. Bycatch rates vary by region and season; for instance, in New Zealand's hoki trawl fishery, fur seal interactions increase in winter and spring, with historical incidences exceeding 0.1 seals per tow in some cases. In U.S. fisheries from 1990 to 2017, bycatch totaled over 62,000 individuals, roughly equal to cetacean losses. Specific cases highlight extinction risks from bycatch. The porpoise (Phocoena sinus) in Mexico's has declined over 98% since the 1990s due to gillnet entanglement, especially illegal nets for ; early estimates pegged annual bycatch at 39-84 individuals, now threatening with fewer than 10 remaining as of 2023. Harbor porpoises in European and North American gillnet fisheries similarly suffer high rates, contributing to population declines in areas like the . These incidents demonstrate how even low absolute numbers of bycatch can devastate small, slow-reproducing populations when exceeding replacement rates.

Seabirds

Seabirds experience high bycatch mortality primarily in longline fisheries, where species such as albatrosses (Diomedea spp.), (Procellaria spp.), and (Puffinus spp.) are attracted to baited hooks floated on the surface during setting, leading to or tangling that results in or . Global estimates from fleet-specific data project at least 160,000 seabirds killed annually in longline operations, with potential totals surpassing 320,000, representing a major threat to long-lived, slow-reproducing procellariiforms that comprise the majority of victims. These figures derive from observer programs and extrapolations, though underreporting in unregulated fleets likely understates true impacts. In trawl fisheries, seabirds face risks from collisions with high-speed warp cables during net deployment or hauling, or entanglement in , particularly in midwater or bottom trawls targeting or groundfish. A 2024 global review synthesized observer data from 25 fleets, estimating minimum annual trawl bycatch at 44,000 seabirds, with higher figures probable due to sparse coverage in developing regions and non-observed vessels. Gillnet fisheries contribute further, with incidental captures exceeding 400,000 seabirds yearly across configurations, though precise apportionment to seabirds remains data-limited. Prominent examples include the wandering albatross (Diomedea exulans), whose populations in the have declined partly from bycatch in Patagonian toothfish (Dissostichus eleginoides) longlines, where hooking rates reached 0.3-1.0 birds per 1,000 hooks in early 2000s operations before mitigations. Similarly, black-browed albatrosses (Thalassarche melanophris) suffer elevated mortality in sub-Antarctic demersal longlines, contributing to 20-30% annual adult losses in some colonies. In the North Pacific, Laysan albatrosses (Phoebastria immutabilis) and black-footed albatrosses (P. nigripes) are incidentally hooked in Hawaii-based longlines, with genetic analyses confirming fishery impacts on specific breeding subpopulations. These cases underscore bycatch as a key driver of declines for 17 of 22 albatross species classified as threatened by the IUCN, exacerbating vulnerabilities from low and K-strategist life histories. Bycatch disproportionately affects scavenging and surface-foraging , with indices larger procellariiforms twice as susceptible to pelagic longlines due to behavioral traits like plunge-diving for bait. In regions like the Mediterranean, scopoli's shearwaters (Calonectris diomedea) and yelkouan shearwaters (Puffinus yelkouan) face ongoing risks from driftnet and interactions, despite regulatory efforts. Empirical studies link these mortalities to reduced breeding success and recruitment, with overlap models predicting sustained pressure without enhanced monitoring.

Sea Turtles and Sharks

Sea turtles face significant mortality from bycatch in commercial fisheries, particularly in pelagic longlines targeting and , as well as shrimp trawls and gillnets. Global estimates indicate annual sea turtle bycatch ranging from 85,000 to 250,000 individuals, with underreporting likely inflating true figures due to limited observer coverage in many regions. Loggerhead turtles (Caretta caretta) dominate bycatch in certain fisheries, such as U.S. shrimp trawls, where they comprise the majority of observed captures. Over the past two decades, cumulative bycatch has reached millions, exacerbating pressures on already vulnerable populations. Bycatch interactions often result in from prolonged gear entanglement or severe injuries like limb amputations and internal trauma, even among released individuals, leading to reduced post-release survival rates estimated below 50% in many cases. Fisheries bycatch ranks as the primary anthropogenic to populations worldwide, surpassing habitat loss in some assessments, with demographic models showing hidden impacts on nesting females and juveniles that delay recovery. In gillnet fisheries, survival upon release varies by species, exceeding 60% for Kemp's ridley and green turtles but reaching over 90% for loggerheads; however, trawls and longlines impose higher lethality due to exhaustion and hook ingestion. Sharks encounter high bycatch rates across global fisheries, including tuna longlines, purse seines, and gillnets, contributing to estimated annual fishing mortality of 80 million individuals as of 2019, up from 76 million in 2012 despite regulatory efforts. In western and central Pacific purse seine fisheries, silky sharks (Carcharhinus falciformis) alone numbered over 92,000 bycatch captures in 2019, representing a substantial portion of non-target elasmobranch interactions. Pelagic longline fisheries report sharks comprising about 7% of total catch, with roughly 81% released alive in observed sets, though at-sea mortality from stress and injuries elevates effective death rates. Shark bycatch often intersects with finning practices, where fins are harvested from captured individuals—many incidental—while bodies are discarded, amplifying waste and population declines given sharks' slow maturation and low . Global shark catches, blending targeted and bycatch sources, approached 1.44 million metric tons annually in recent assessments, with unreported discards and obscuring full impacts. Since the , overall shark and ray abundances have plummeted 71%, attributable largely to intensified pressures rather than natural factors, underscoring bycatch's role in hindering recovery. Data limitations persist, as many fisheries lack comprehensive monitoring, potentially underestimating mortality in data-poor regions.

Reduction Strategies

Technological Modifications

Technological modifications to fishing gear aim to minimize bycatch by altering net designs, shapes, and adding deterrents that allow non-target to escape or avoid capture while sustaining yields of commercially valuable . These include excluder grids, adjustments, specialized hooks, and visual or acoustic barriers, often tested through collaborative programs between scientists and fishers. In trawl fisheries, particularly shrimp trawling, turtle excluder devices (TEDs) consist of rigid or flexible grids installed ahead of the codend, directing larger animals like s toward escape openings. TEDs have demonstrated a 97% reduction in captures in U.S. shrimp trawls since their development in the late , with testing in 1987 confirming this efficacy alongside minimal shrimp loss of less than 5%. Complementary bycatch reduction devices (BRDs), such as fisheye or Jones-Davis designs, feature openings or panels that enable finfish to exit the net, achieving at least 30% reduction in total finfish bycatch weight as required for in U.S. fisheries. In Australia's northern prawn fishery, combined TEDs and BRDs reduced turtle bycatch by 99%, by 5%, and by 17.7%, though impacts on rays and smaller elasmobranchs varied. For longline fisheries, circle hooks, which feature a curved shape that promotes jaw hooking rather than gut hooking, reduce and interactions compared to traditional J-hooks. Trials in the U.S. Atlantic pelagic longline indicated circle hooks significantly lowered bycatch rates, though effectiveness can interact with factors like type and setting location. Weighted branch lines or sinkers accelerate hook submersion, further deterring surface-feeding s. Tori lines, or streamer lines, towed astern from poles with flapping ribbons, create a visual barrier over baited hooks, preventing access; paired lines reduced bycatch by 88% to 100% in Alaskan fisheries, while single lines achieved 71% to 91% reductions. Other innovations include codend mesh size increases or orientation changes in trawls, which selectively retain larger target fish while releasing juveniles, and sensory deterrents like acoustic pingers for marine mammals in gillnets. These modifications often entail initial costs and minor target catch reductions—typically under 10% for TEDs and BRDs—but empirical evaluations confirm net benefits for sustainability when properly implemented. Ongoing research through programs like NOAA's Bycatch Reduction Engineering emphasizes adaptive testing to address fishery-specific challenges.

Operational and Behavioral Adjustments

Operational adjustments to reduce bycatch encompass strategic changes in fishing location, timing, depth, and speed to minimize overlaps with vulnerable non-target distributions. For instance, temporal closures during peak aggregation periods of marine mammals, such as harbour porpoises in gillnet , have demonstrated effectiveness in lowering incidental captures without substantially affecting target yields. Similarly, dynamic area closures in trawl can achieve up to 57% bycatch reductions on average while preserving target catches, outperforming static measures that yield only 16% declines. These approaches rely on empirical data from logs and observer programs to identify high-risk zones, though their success hinges on accurate spatiotemporal modeling and fisher compliance, which varies across fleets. In longline fisheries, night-time setting of baited hooks—combined with increased sinking rates through depth adjustments—significantly curtails seabird bycatch by exploiting diurnal foraging patterns, particularly for albatrosses that forage primarily during daylight. Studies across multiple regions confirm night setting reduces interactions by limiting bait visibility, with one analysis showing substantial declines under diverse environmental conditions, though efficacy drops if dawn setting predominates due to low global compliance rates of 3-5.5%. Deep-setting hooks below surface layers further mitigates risks for species like billfishes and sharks, as evidenced by trials indicating lower incidental captures while maintaining economic viability for target tuna species. Such operational shifts often incur minimal target catch losses but require vessel-specific adaptations, with real-world outcomes sometimes underperforming controlled experiments owing to variable sea states and crew practices. Behavioral adjustments focus on fisher-level actions during capture and handling to boost post-release survival, including rapid sorting, de-hooking, and revival techniques tailored to species physiology. In tuna purse seine operations, fisher-designed sorting grids facilitate quick, safe release of threatened manta and devil rays, directly supporting population conservation by minimizing handling trauma. Best handling protocols, such as immediate return to water and avoiding air exposure, have been shown to elevate survival rates for discarded fish and elasmobranchs, with vitality assessments enabling prioritized releases. These practices, often disseminated via observer training and guidelines from bodies like ICCAT, prove highly individualized and effective in reducing discard mortality when integrated into daily routines, though barriers like time pressures in high-volume fisheries can limit adoption. Empirical evaluations underscore that such interventions complement gear technologies, yielding cumulative bycatch mortality drops without necessitating broad regulatory overhauls.

Regulatory Approaches

International Treaties and Frameworks

The Convention on the Law of the Sea (UNCLOS), ratified by 168 parties as of 2023, establishes foundational obligations for states to conserve and manage living marine resources, including indirect provisions addressing bycatch through requirements to consider impacts on associated or dependent when setting conservation measures. Specifically, Article 61(4) mandates coastal states to evaluate effects on non-target to maintain populations above levels threatening , though relies on national without dedicated bycatch quotas or global monitoring. Complementing UNCLOS, the (FAO) Code of Conduct for Responsible Fisheries, adopted in 1995, promotes ecosystem approaches that incorporate bycatch reduction via selective gear and , serving as voluntary influencing over 190 member states. FAO's International Guidelines for the Management of Bycatch and Reduction of Discards, endorsed in following expert consultations, provide technical advice on regulatory frameworks, including mandatory reporting, gear modifications, and discard limits to minimize waste and non-target mortality, aligned with the and applicable to both capture fisheries and regional bodies. These guidelines emphasize empirical on bycatch rates and composition to inform evidence-based policies, though their non-binding nature limits direct enforceability, with adoption varying by jurisdiction. Similarly, the 1995 Agreement for the Implementation of the Provisions of the Convention on the relating to the Conservation and Management of Straddling and Highly Migratory (UNFSA) requires cooperation on bycatch in transboundary fisheries, obligating states to assess and mitigate incidental catches through stock assessments that include impacts. Species-specific multilateral environmental agreements under the Convention on Migratory Species (CMS) target bycatch threats to vulnerable taxa; the 2001 Agreement on the Conservation of Albatrosses and (ACAP), with 13 parties as of 2023, coordinates for seabirds by promoting best practices such as bird-scaring lines and night setting in longline fisheries, drawing on data showing bycatch as a primary driver of population declines. ACAP's binding resolutions require parties to implement national plans reducing fishery interactions, with monitoring via observer programs, though coverage remains partial outside member states. Regional Fisheries Management Organizations (RFMOs), operating under UNFSA frameworks, adopt legally binding conservation and management measures (CMMs) tailored to bycatch in high-seas fisheries; for instance, five tuna RFMOs mandate combined mitigation strategies like weighted branch lines and tori lines to curb incidental mortality in longline operations overlapping with breeding areas. Shark-focused CMMs in organizations like the International Commission for the Conservation of Atlantic Tunas (ICCAT) prohibit and require live release of non-target elasmobranchs, informed by species-specific vulnerability assessments, while cetacean bycatch protocols in bodies like the Western and Central Pacific Fisheries Commission (WCPFC) include gear restrictions and acoustic deterrents based on observer data indicating annual mortalities exceeding sustainable levels for some populations. Effectiveness of RFMO measures depends on compliance monitoring, with audits revealing gaps in data reporting and variable adoption rates among distant-water fleets.

Domestic Policies and Enforcement Challenges

In the United States, the Magnuson-Stevens Fishery Conservation and Management Act (MSA), originally enacted in 1976 and reauthorized multiple times, including in 2006 with enhanced bycatch provisions, mandates regional fishery management councils to minimize bycatch and its mortality through measures such as gear modifications, time-area closures, and quotas for protected . The Act requires annual reports on bycatch levels and the development of standardized bycatch reporting methodologies, while the National Bycatch Reduction Strategy, updated in 2024, coordinates efforts across NOAA Fisheries to reduce bycatch via technological incentives like the Bycatch Reduction Engineering Program, which funds gear innovations demonstrated to cut unintended catch by up to 50% in trials for like sea turtles. These policies apply to federal waters up to 200 nautical miles, emphasizing science-based limits to prevent while addressing bycatch of non-target . In the , the (CFP), reformed in 2013 and further updated through technical measures, prohibits discards of catches above specified quotas and implements a landing obligation phased in since 2015, fully effective by 2019 for most stocks, to curb bycatch waste estimated at 1.3 million tonnes annually pre-reform. Regulations include mandatory use of selective gear like turtle excluder devices in certain fisheries and real-time closures when bycatch thresholds for protected species, such as cetaceans, are exceeded, with the 2023 revised Fisheries Control Regulation enhancing traceability via electronic reporting and inspections to enforce compliance. Member states must align national laws, though implementation varies, with exemptions allowed for high-survivability bycatch under strict monitoring. Enforcement of domestic bycatch policies faces significant hurdles, including insufficient observer coverage—often below 10% in U.S. fisheries—leading to underreporting and unreliable bycatch , as highlighted in a 2024 Government Accountability Office review criticizing NOAA for inadequate monitoring in high-bycatch sectors like shrimp . Vast exclusive economic zones complicate patrols, with limited vessels and personnel; for instance, the U.S. boards only a fraction of domestic fleets annually, exacerbating issues from illegal, unreported, and unregulated (IUU) activities that evade gear restrictions and contribute to unmonitored bycatch. Economic pressures incentivize non-compliance, such as misreporting or discarding , while technological gaps in real-time tracking persist despite electronic monitoring pilots showing violation rates up to 20% in observer programs from 2000 to 2021. In the EU, inconsistent national capacities and cross-border fleet movements undermine uniform application, with studies indicating cetacean bycatch measures remain insufficient due to delayed responses and deficiencies.

Evaluation of Effectiveness

Evidence from Empirical Studies

Empirical evaluations of bycatch reduction measures, primarily through controlled experiments and fishery-dependent data, indicate substantial reductions in incidental captures for specific taxa and gear types, though outcomes vary by implementation fidelity and environmental context. A of 42 technical mitigation measures across seabirds, elasmobranchs, marine mammals, and sea turtles found average bycatch reductions of 50-80% for many devices, with minimal impacts on target catch in optimized designs, based on data from over 100 studies spanning global fisheries. However, real-world efficacy often trails experimental results due to inconsistent gear modifications and operational adherence. For sea turtles in shrimp trawls, turtle excluder devices (TEDs) consistently demonstrate high exclusion rates. Testing by NOAA Fisheries across U.S. Gulf and South Atlantic waters showed TEDs excluding 97% of turtles while retaining 90-95% of , with data from thousands of hauls confirming these rates under commercial conditions since mandatory adoption in 1987. In northern Australian prawn fisheries, a 2006 study of 1,200+ hauls reported 99% turtle exclusion with TEDs, alongside 5-6% loss mitigated by design refinements. Seabird bycatch in pelagic longline fisheries has been mitigated effectively by tori lines (streamer devices). A 2024 network of experimental and observational data from multiple oceans ranked paired tori lines with weighted branch lines as reducing and captures by 89% relative to controls, drawing from 20+ trials involving over 10,000 sets. Field experiments in the eastern South Pacific confirmed 60-80% reductions in strikes during line setting, with efficacy increasing in high-abundance areas. Marine mammal bycatch responds to acoustic deterrents like pingers. A 1997 controlled experiment in U.S. sink gillnet fisheries achieved 92% reduction in harbor entanglements using pingers emitting 10 kHz pulses, validated across 200+ nets. In Peruvian driftnet fisheries targeting , pingers reduced small cetacean bycatch by 37-50% in trials with 500+ sets, though risks were noted in longer-term deployments. For dolphins, deterrent devices in tuna purse seines yielded over 90% bycatch frequency reductions per haul in Mediterranean trials from 2018-2022. Shark and ray bycatch in longlines shows moderate success with gear modifications. Meta-analytic evidence indicates circle hooks and bait optimization reduce elasmobranch captures by 20-40% compared to J-hooks, with data from 59 experiments across tropical fisheries preserving target tuna yields. Global assessments of implemented measures, however, reveal persistent challenges, with bycatch rates for declining only 10-30% post-regulation in non-compliant fleets.

Trade-Offs with Target Species Yields

In trawling fisheries, turtle excluder devices (TEDs) exemplify gear modifications that reduce bycatch by up to 97% while imposing modest losses on target yields. A reanalysis of field trials in Georgia waters indicated loss rates of 5.5% with TEDs equipped with accelerator funnels and 7.5% without, primarily due to the escape of smaller or juvenile through the exclusion grid. These losses stem from the physical barrier design, which allows larger non-target species to exit but can inadvertently permit some target to escape, though subsequent improvements in TED geometry and integration with bycatch reduction devices (BRDs) have mitigated quality degradation, reducing damaged proportions by 41% in combined setups despite a 6% overall catch drop. In pelagic longline fisheries targeting tunas and , hooks reduce bycatch of seabirds, , and certain by altering hook-up patterns and decreasing deep hooking, but they often lower catch rates of target species owing to differences in hook shape and gape width. Empirical data from Hawaii-based longline sets showed 18/0 hooks reducing catch rates of and other pelagics compared to J-hooks, attributed to the hook's wider minimum width (up to 57% broader), which affects bait presentation and mouth during strikes. Similar trials in Australian equatorial waters confirmed no significant overall target catch decline with non-offset 18/0 hooks, though species-specific effects varied, with potential retention decreases in shallow-set operations using larger circles. These trade-offs arise from causal mismatches between hook design optimized for bycatch avoidance and the predatory behaviors of high-value targets, though post-release survival improvements for incidentally caught can offset some yield impacts in contexts. Beyond gear, spatial management like dynamic area closures demonstrates variable trade-offs, with empirical analyses across 15 global fisheries revealing 57% average bycatch reductions without target yield losses, contrasting static closures' 16% bycatch cuts alongside minor target declines (0-4%). Such approaches leverage spatiotemporal species overlaps, minimizing effort displacement costs, but require to avoid unintended yield shortfalls from over-restrictive . Overall, while initial bycatch mitigation prototypes incurred substantial target losses (e.g., 38-53% reductions in early TEDs), refined designs increasingly balance conservation gains against economic yields, with net benefits emerging when reduced sorting time and market access enhancements are factored in. Empirical variability underscores the need for fishery-specific testing, as source biases toward regulatory optimism in agency reports may understate persistent trade-offs in data-poor contexts.

Debates and Criticisms

Questions on Impact Overstatement

Some analyses suggest that bycatch impacts are overstated by assuming uniform 100% mortality for all discarded catch, whereas empirical studies demonstrate variable rates influenced by , gear type, handling duration, and environmental factors. For example, in Northeast Atlantic fisheries, post-discard for has been estimated at 48-72% under moderate crowding conditions, while can reach 50-90% depending on net burst simulations and air exposure. Similarly, reviews of discards indicate that many exhibit short-term exceeding 50%, with long-term rates varying from 10-80% based on tagging and recapture data, challenging models that default to total mortality. These findings imply that aggregate bycatch mortality estimates, often used to justify regulations, may inflate ecological harm by 20-50% or more in discard-heavy fisheries. Bycatch quantification for rare or protected species is further susceptible to overestimation due to sampling biases in observer programs, where low coverage (typically under 10-20%) amplifies rare-event encounters. Statistical models show a high tendency for total bycatch overestimation when observer underrepresent fleet-wide variability, as rare species captures cluster in specific hauls, skewing extrapolations upward by factors of 2-10 times for low-prevalence events. Recent assessments of elasmobranch bycatch in mixed fisheries highlight how underestimated spatial —failing to account for heterogeneous distributions—leads to inflated estimates, with corrected models reducing projected impacts by up to 30-40%. Observer effects, including behavioral changes by fishers or protocol-induced drop-outs, can exacerbate this, though empirical corrections for such biases reveal that uncorrected systematically overestimate bycatch rates by 3-25% in some protocols. Population-level evidence sometimes fails to corroborate alarmist bycatch narratives, as stable or recovering stocks coexist with documented incidental catch. For the , genetic and demographic analyses found no empirical link between cryptic bycatch and observed declines, attributing trajectories more to environmental factors than fisheries interactions despite modeled risks. Broader critiques note that over-reliance on bycatch mortality assumptions in management models can lead to erroneous conclusions about risks, with sensitivity analyses showing that even modest reductions in assumed post-release survival (e.g., from 100% to 60% mortality) alter population viability projections substantially. Such discrepancies underscore the need for fishery-specific discard mortality validation, as generalized high-mortality priors—prevalent in conservation literature—may prioritize bycatch over evidence-based assessments of actual demographic threats.

Regulatory Burdens and Socioeconomic Trade-Offs

Regulations intended to mitigate bycatch, such as mandatory gear modifications and species-specific quotas, impose direct compliance costs on operations, including retrofitting and monitoring requirements. For instance, turtle excluder devices (TEDs) mandated in U.S. trawls since the late cost between $325 and $550 per net, with installation and maintenance adding to operational expenses for vessels. These devices have been associated with a 2-6% reduction in target catch rates in southeastern U.S. offshore waters, translating to forgone for fishers already facing volatile markets. Bycatch quotas often trigger early fishery closures when limits are reached, curtailing target species harvests and exacerbating economic losses through regulatory discards—catches legally required to be released due to size, quota, or protected status restrictions. In U.S. commercial fisheries, such regulatory discards annually diminish potential ex-vessel revenue by approximately $427 million, primarily from unlanded fish that could otherwise enter markets. Observer programs, enforced to verify bycatch reporting, further elevate burdens by increasing vessel operational costs—up to several thousand dollars per trip—and administrative reporting demands, particularly in multispecies fisheries like where post-reduction regulations have amplified paperwork without proportional bycatch gains. Socioeconomic trade-offs manifest in coastal communities reliant on , where measures like area closures or gear restrictions reduce and income, potentially leading to out-migration or shifts to less sustainable practices. closures for bycatch protection, such as those targeting cetaceans in gillnet operations, can impose substantial disruptions in regions where accounts for a majority of household earnings, as seen in small-scale European and developing-world fleets. Internationally, purse-seine regulations in Pacific island nations have modeled macroeconomic contractions, with GDP losses up to 1-2% from curtailed access, highlighting tensions between conservation goals and in export-dependent economies. These burdens raise questions of proportionality, as empirical cost-benefit analyses reveal uneven returns: while some measures like TEDs demonstrably lower turtle mortality, broader quota systems may yield diminishing ecological gains relative to forgone yields, especially when bycatch rates are low or data-limited, prompting industry critiques of overregulation driven by precautionary biases in bodies. In developing countries, externally imposed standards often overlook local adaptive capacities, amplifying inequities by favoring industrial fleets with resources for compliance while marginalizing artisanal fishers, thus trading short-term aims for long-term socioeconomic resilience.

Recent Advances and Prospects

Developments Since 2020

The National Bycatch Reduction Strategy Implementation Plan, released by NOAA Fisheries in 2020, outlined priorities for monitoring, research, gear modifications, and incentives to minimize bycatch across U.S. fisheries through 2024, emphasizing data-driven approaches to identify high-risk areas and test devices. This built on empirical assessments showing persistent bycatch impacts on like sea turtles and marine mammals, with annual funding allocated to the Bycatch Reduction Engineering Program, which in 2020 supported 13 projects totaling $2.3 million for technologies such as modified nets and sorting grids. By 2025, the program continued with similar funding levels, focusing on scalable solutions like the Flexigrid—a dual sorting system that reduced undersized bycatch by up to 50% in West Coast groundfish trawls without significantly affecting target catches. Innovations in gear technology advanced notably post-2020, including LED-based NetLights integrated into gillnets, which trials in the Mediterranean demonstrated reduced sea turtle bycatch by 42% and batoid bycatch by 50% while maintaining catches of target species like pandora fish. In shark-prone fisheries, the SharkGuard device—an electrified bait system—showed promise in field tests, potentially cutting shark bycatch by over 70% in longline operations, with scalability assessments indicating broader adoption could avert millions of shark deaths annually if integrated into global fleets. Similarly, refined bycatch release devices (BRDs), such as dehookers and stretchers co-developed with tropical tuna fishers, improved post-capture survival rates for sharks and rays by minimizing handling time and injury, with adoption tracked in purse seine fisheries since 2021. Regional strategies highlighted targeted reductions; Australia's Northern Prawn Fishery introduced three new BRDs under its 2020-2026 plan, achieving 37-44% decreases in small bycatch species compared to conventional square mesh panels, verified through onboard monitoring. For marine mammals, acoustic pingers on nets yielded a 92% drop in entanglements in U.S. Northeast trials, prompting regulatory pushes for wider use by 2025. The EU-funded ECO-CATCH project, launched in 2023, deployed AI-assisted mapping to help Baltic and vessels avoid bycatch hotspots, integrating real-time environmental data for up to 30% fewer unintended captures in demersal trawls. Despite these gains, empirical reviews noted uneven adoption due to cost barriers and variable efficacy across gear types, with bycatch trends showing sustained declines in Atlantic regions but stagnation in parts of the Pacific through 2017 data extended to recent monitoring.

Emerging Research and Unresolved Questions

Research into bycatch mitigation has increasingly incorporated and for onboard species identification, enabling automated sorting and release to reduce mortality rates in trawl fisheries, with prototypes demonstrating up to 90% accuracy in distinguishing target from non-target catch in controlled tests as of 2024. Similarly, sensory deterrents such as acoustic and light-based systems have been trialed to repel non-target species like and rays from baited hooks, showing preliminary reductions in incidental capture by 30-50% in small-scale operations. In tropical purse fisheries, co-developed bycatch release devices—including stretchers, velcro restraints for , and deck-based hoppers—have improved survival rates for released and , with field evaluations in 2025 confirming lower stress-induced fatalities compared to traditional handling. NOAA's Bycatch Reduction Engineering Program has supported gear modifications like the Flexigrid, a dual-sorting system that decreased undersized bycatch by over 50% in Pacific trawls during 2024-2025 trials, while maintaining target yields. Long-term datasets from fisheries, analyzed in 2025, highlight spatiotemporal patterns in bycatch, revealing hotspots linked to seasonal migrations and suggesting predictive modeling for dynamic closures. Projects such as ECO-CATCH in the Baltic and North Seas are testing integrated approaches combining gear tech with real-time monitoring to address multispecies interactions, with initial data from 2025 indicating potential for ecosystem-based reductions. Despite these advances, unresolved questions center on accurate global bycatch estimation, as definitional inconsistencies—such as distinguishing discarded target species from true non-target captures—and underreporting in unregulated fleets hinder reliable quantification, with estimates varying by factors of 2-10 across methods. The causal links between bycatch and population declines in -limited species remain debated, particularly where observer coverage is below 5% and indirect effects like disruption confound direct mortality . Empirical validation of efficacy in variable conditions, including climate-driven shifts in species distributions, lacks longitudinal studies beyond short-term trials, raising doubts about . Socioeconomic trade-offs, such as yield losses from strict gear restrictions in small-scale fisheries, require further cost-benefit analyses integrating fisher behavior and market dynamics.

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