Rastrelliger

Rastrelliger is a genus of small to medium-sized mackerels in the family Scombridae and order Scombriformes, comprising four valid species: R. brachysoma, R. chrysozonus, R. faughni, and R. kanagurta.^[1] These semi-pelagic fish, which rarely exceed 30 cm in fork length, are characterized by streamlined bodies, high numbers of gill rakers (typically 32–59 total) adapted for filter-feeding on zooplankton and phytoplankton, and a coastal distribution primarily in the Indo-West Pacific region.^[1] They form schools in turbid, plankton-rich waters such as bays, harbors, and lagoons, where surface temperatures are generally above 17°C.^[2] The species exhibit morphological distinctions, including variations in body depth, upper jaw length relative to head length, and intestine proportions; for instance, R. brachysoma has a deeper body (24.2–28.3% of fork length) and longer intestine (3.2–3.6 times fork length) compared to the more slender R. faughni.^[1] Distribution patterns differ across species: R. chrysozonus is endemic to the Red Sea, R. kanagurta is widespread in the Indo-West Pacific (excluding the Red Sea) to the central Pacific, R. brachysoma ranges from India to Samoa, and R. faughni occurs from India to Fiji.^[1] Recent taxonomic revisions have resurrected R. chrysozonus from synonymy with R. kanagurta based on morphometric and genetic analyses, confirming genetic divergence of about 3% between these species.^[1] Rastrelliger species hold significant commercial value in South and Southeast Asian fisheries, where they are caught using gill nets, purse seines, and bag nets, contributing substantially to local economies; for example, R. brachysoma is a primary target in Philippine and Thai fisheries, valued at over 190 million USD annually in Thailand alone as of 2013.^[3] Their ecological role as plankton feeders supports marine food webs, though overfishing poses management challenges in key habitats like the Java Sea and Coral Triangle.^[1][4]

Taxonomy

Etymology

The genus name Rastrelliger was established by David Starr Jordan and Edwin Chapin Starks in 1908 to accommodate mackerels characterized by exceptionally numerous and elongated gill rakers, distinguishing them from other scombrids like those in the genus Scomber.^[1] The name derives from the Latin rastrum (or rastrellum, meaning "rake") combined with gerere ("to bear" or "to carry"), alluding to the rake-like arrangement of the densely packed gill rakers that facilitate filter-feeding on plankton.^[5] This etymological reference highlights the adaptive morphology enabling the genus's specialized planktivorous diet, a key diagnostic trait noted in the original description.^[6] Jordan and Starks' description was based on specimens collected during expeditions in the Indo-Pacific, particularly from Fiji waters, where the type species Rastrelliger brachysoma (originally described as Scomber brachysoma by Bleeker in 1851) was designated.^[7] The genus falls within the family Scombridae, the tunas and mackerels. Subsequent species assignments within Rastrelliger retained or adapted vernacular roots; for instance, R. kanagurta, the Indian mackerel, derives its specific epithet from "kanagurta," a local name used along the Coromandel Coast of India as recorded by Patrick Russell in 1803.^[8]

Classification

Rastrelliger is classified within the phylum Chordata, class Actinopterygii, order Scombriformes, family Scombridae, subfamily Scombrinae, and tribe Scombrini.^[9][1] Within the tribe Scombrini, Rastrelliger forms a sister genus to Scomber, the true mackerels, supported by phylogenetic analyses that highlight their close evolutionary ties. Molecular studies from the 2020s, utilizing mitochondrial DNA sequences such as the control region and complete mitogenomes, have confirmed the monophyly of Rastrelliger, distinguishing it as a cohesive clade separate from other scombrids like Scomberomorus.^[10][11] The taxonomic history of Rastrelliger includes key revisions that established its current composition of four valid species. Prior to 1967, the genus was primarily recognized with two species, but Matsui's description of R. faughni in that year expanded it to three based on morphological distinctions. Recent systematic reviews in 2025 have further refined this by resurrecting R. chrysozonus from synonymy with R. kanagurta and synonymizing R. neglectus (originally described as Scomber neglectus van Kampen, 1907) under R. brachysoma, resolving prior uncertainties through integrated morphometric and genetic evidence.^[12][1]

Description

Morphology

Rastrelliger species exhibit a streamlined fusiform body shape, which facilitates rapid movement through the water column, an adaptation suited to their pelagic lifestyle. The body is covered with small ctenoid scales, with larger cycloid scales behind the head and below the pectoral fins, providing flexibility and hydrodynamic efficiency. The lateral line runs straight along the body, aiding in the detection of water movements and prey in open water environments.^[1] The fin configuration supports agile swimming and stability in mid-water. There are two dorsal fins, widely separated, with the first consisting of VIII–XI spines and the second with I spine and 10–12 soft rays. The anal fin is preceded by 1 or 2 detached spines and includes I spine followed by 11–13 soft rays, with 4–6 dorsal and 4–6 anal finlets behind. Pectoral fins are notably long and falcate, extending beyond the origin of the second dorsal fin, enhancing maneuverability during schooling and foraging. The caudal fin is deeply forked, contributing to propulsion in pelagic habitats.^[1] A key diagnostic feature of the genus is the presence of extremely long gill rakers, which form a sieve-like structure for filtering planktonic food particles from the water. These gill rakers, visible through the open mouth, bear numerous fine bristles—up to approximately 150 on the longest raker in adults—enabling efficient particle retention during feeding. Gill raker counts vary slightly among species, but the elongated structure is consistent across Rastrelliger.^[13][1] Internally, Rastrelliger possess a swim bladder, which provides buoyancy control essential for maintaining position in the water column without constant swimming effort, further adapting them to pelagic existence.^[13]

Size and coloration

Species of the genus Rastrelliger are medium-sized pelagic fishes, typically attaining maximum fork lengths of up to 35 cm in R. kanagurta and R. brachysoma, 24 cm in R. faughni, and approximately 20 cm in R. chrysozonus, with recorded weights reaching 750 g in R. faughni.^{[14][13][15][1]} Their coloration features a silvery-blue dorsum that grades to white on the ventrum, accompanied by a distinctive blackish spot behind the operculum and another below the pectoral fin base.^[1] In live specimens, oblique dark bars or spots are evident along the back, contributing to camouflage and schooling cohesion.^[1] Sexual dimorphism is subtle, with females generally achieving slightly larger sizes than males.^[16] Juveniles exhibit more prominent spotting patterns, which fade in adults to a more uniform silvery appearance.^[1]

Distribution and habitat

Geographic range

The genus Rastrelliger is distributed across tropical and subtropical coastal waters of the Indo-West Pacific, ranging from the Red Sea and the east coast of Africa through the Indian Ocean to the western Pacific Ocean, encompassing regions such as Indonesia, the Philippines, and extending eastward to Fiji and Samoa.^[17] This broad expanse reflects the pelagic nature of these mackerels, which inhabit epipelagic zones influenced by warm surface currents.^[18] Among the four species, R. chrysozonus is endemic to the Red Sea, while R. kanagurta exhibits the widest distribution, spanning from the east coast of Africa and the Persian Gulf across the Indian Ocean to the western Pacific, including Southeast Asia and as far east as Tonga.^[1][17] In contrast, R. brachysoma is primarily found from the eastern Indian Ocean (Andaman Sea) to the western Central Pacific, including Thailand, Indonesia, Papua New Guinea, the Philippines, Solomon Islands, and Fiji.^[13] R. faughni has a more restricted range, occurring from the eastern coast of India to Fiji and north to Taiwan, often associated with oceanic islands in the Indo-West Pacific. All four species overlap significantly in Southeast Asian waters, such as the Java Sea, where they co-occur in mixed assemblages due to shared neritic habitats.^[19] Genetic studies indicate that post-Pleistocene range expansions facilitated connectivity among Rastrelliger populations via Indo-Pacific ocean currents, contributing to the current panmictic patterns observed in species like R. kanagurta.^[18] Late Pleistocene climate fluctuations, including sea-level changes, likely drove historical isolation followed by recolonization, with evidence of population expansions around 100,000–20,000 years ago influencing genetic homogeneity across the Indian and Pacific Oceans.^[20] These dynamics underscore the role of major current systems, such as the Indian Monsoon Current and the Indonesian Throughflow, in shaping the genus's contemporary distribution.^[18]

Environmental preferences

Species of the genus Rastrelliger are primarily epipelagic-neritic fishes, inhabiting the upper layers of coastal and shelf waters from the surface down to depths of approximately 20–200 m, depending on the species. For instance, R. kanagurta is commonly found between 20 and 90 m, while R. brachysoma extends to 200 m, reflecting their adaptation to continental shelf environments where they form schools in open water columns.^[14] These depth preferences allow access to plankton-rich zones, though they occasionally tolerate shallower estuarine areas with reduced salinity.^[21] Optimal temperatures for Rastrelliger species range from 20°C to 30°C, with surface waters below 17–18°C generally avoided to prevent physiological stress. R. kanagurta, the most widespread species, shows peak abundance in waters of 23.7–28.3°C (mean 27.3°C), aligning with tropical Indo-Pacific conditions, while R. brachysoma thrives specifically between 20°C and 30°C.^[14] Such thermal tolerances influence seasonal migrations toward warmer coastal currents, ensuring metabolic efficiency in their pelagic lifestyle.^[14] Salinity preferences center on marine levels of 30–35 ppt, though Rastrelliger species exhibit tolerance to slightly reduced salinities in brackish estuarine habitats, particularly R. brachysoma. Off the Mumbai coast, R. kanagurta abundances correlate with sea surface salinities of 33–34.5 ppt, highlighting their adaptability to coastal gradients without venturing into fully freshwater systems.^[21][22] These fishes favor turbid coastal waters enriched with plankton, as indicated by chlorophyll-a concentrations around 0.35 mg/m³ for R. kanagurta, which supports their filter-feeding habits in productive neritic zones.^[22] They show sensitivity to low dissolved oxygen, avoiding areas below approximately 4 mg/L as observed in upwelling-influenced fisheries where oxygen-poor waters displace pelagic schools.^[23] This preference for well-oxygenated environments underscores their reliance on stable, high-productivity coastal systems.

Ecology

Feeding habits

Species of the genus Rastrelliger are ram-filter feeders that utilize their elongated gill rakers to strain plankton from the water column while swimming forward. This feeding mechanism allows them to process large volumes of water efficiently, capturing small particles such as zooplankton and phytoplankton. The gill rakers form a sieve-like structure that retains prey items while permitting water to pass through, enabling continuous filtration during active swimming.^[24][25] The diet of Rastrelliger primarily consists of planktonic organisms, with zooplankton comprising 34-44% and phytoplankton 40-66% of the gut contents, depending on availability and location. Dominant zooplankton include calanoid copepods, cladocerans, and larval forms, while phytoplankton such as diatoms and dinoflagellates are opportunistically consumed, particularly during algal blooms. These fish exhibit no significant piscivory, focusing exclusively on particulate plankton rather than larger prey.^[26][27] Foraging occurs in schools that target dense patches of plankton, enhancing encounter rates and filtration efficiency through collective swimming. This schooling behavior facilitates exploitation of localized prey concentrations in surface waters. While specific diurnal patterns vary, feeding intensity often aligns with plankton availability, supporting their role as key consumers in pelagic food webs.^[25][28]

Reproduction

Rastrelliger species are gonochoristic, lacking hermaphroditism, and reproduce via external fertilization in pelagic environments.^[14] Individuals typically reach sexual maturity at lengths of 15-21 cm total length, corresponding to ages of 1-2 years, depending on environmental conditions and population.^[29][30] Spawning occurs in multiple batches throughout the year in tropical regions, with peaks aligned to monsoon seasons such as June-July or August in the Indian Ocean and February, May-June, or August-September in the Gulf of Thailand.^[31][32][33] Eggs are pelagic and buoyant, with diameters ranging from 0.91-0.98 mm and containing a single oil globule of 0.23-0.25 mm.^[34] Fecundity varies by species and size but generally falls between 25,000 and 120,000 eggs per female, positively correlated with body length and ovary weight.^[32][35] Eggs hatch after approximately 2-3 days under tropical water temperatures, yielding yolk-sac larvae measuring 1.5-1.8 mm in standard length.^[34] Larvae remain planktonic for 20-30 days, progressing through pre-flexion (1.8-3.3 mm), flexion (2.85-4.85 mm), and post-flexion (4.6-11.3 mm) stages, during which they develop fins, teeth, and feeding capabilities on microzooplankton before transitioning to larger prey like copepods.^[34][36] This early life stage experiences high mortality primarily due to predation, with survival rates influenced by growth rates of 0.2-0.34 mm per day.^[34]

Species

Rastrelliger brachysoma

Rastrelliger brachysoma, commonly known as the short mackerel, is characterized by a short, deep body with a fusiform shape and oval cross-section, featuring 8-11 dorsal spines and 47–57 total gill rakers (14–20 upper + 32–38 lower) that facilitate filter-feeding on small particles.^[1][13] It reaches a maximum fork length of 34.5 cm, though commonly 20 cm, with sexual maturity attained at around 17.3 cm.^[13] Like other species in the genus, it possesses a swim bladder and a notably long intestine, measuring 3.2-3.6 times the fork length, adapted for processing a diet rich in plankton.^[13] This species inhabits the eastern Indian Ocean and western Pacific, ranging from the Andaman Sea eastward to Fiji, between approximately 18°N and 18°S latitude and 93°E to 180°E longitude.^[13] It prefers pelagic-neritic environments in marine and brackish waters, typically at depths of 15-200 m but favoring shallower zones under 50 m, with surface temperatures between 20°C and 30°C, including estuarine areas with reduced salinity.^[13] Ecologically, R. brachysoma forms large schools of similarly sized individuals, often larger than those observed in its congeners, enabling efficient foraging and predator avoidance in open coastal waters.^[37] Its diet primarily consists of smaller zooplankton such as copepods, alongside a significant component of phytoplankton and diatoms, reflecting its role as a planktivorous filter-feeder.^[13] Reproduction occurs through batch spawning, typically from March to September in coastal bays, with peaks varying by region but aligning with warmer monsoon periods.^[13] Genetic analyses based on mtDNA control region sequences have identified distinct populations within Indonesian waters, including a separate cluster in the Java Sea corresponding to eastern Jawa origins, indicating limited gene flow and potential stock separation.

Rastrelliger chrysozonus

Rastrelliger chrysozonus, commonly known as the golden-striped mackerel, is a moderately deep-bodied species with a laterally compressed, fusiform shape, featuring 8–11 dorsal spines, 12 dorsal soft rays, 5 anal finlets, and 53–59 total gill rakers (17–21 upper + 35–39 lower, mode 56) adapted for filter-feeding on zooplankton.^[1] It reaches a maximum fork length of 24.0 cm, with specimens examined ranging from 10.2–24.0 cm. The species has a large oblique mouth with the upper jaw extending posterior to the eye's rear margin, a swim bladder, and an intestine length of 1.5–1.7 times fork length; coloration includes golden stripes along the sides.^[1] Endemic to the Red Sea, R. chrysozonus is distributed from the Gulf of Aqaba to the Bab-el-Mandeb Strait, primarily in coastal waters of Saudi Arabia, Egypt, and Sudan.^[1] It inhabits epipelagic to mesopelagic marine environments at depths of 0–70 m, preferring warm, plankton-rich coastal bays and lagoons with surface temperatures above 20°C and salinities of 38–41 ppt.^[1][38] Ecologically, R. chrysozonus forms schools in protected inshore areas, feeding primarily on zooplankton such as copepods and appendicularians by ram-filtering with extended gill rakers. Reproductive biology is poorly documented, but spawning likely occurs year-round in tropical Red Sea conditions, with maturity reached at smaller sizes similar to congeners. Genetic analyses show approximately 3% divergence from R. kanagurta, supporting its status as a distinct species.^[1]

Rastrelliger faughni

Rastrelliger faughni, commonly known as the island mackerel, is a species of small pelagic fish in the family Scombridae, distinguished by its slender, fusiform body that is moderately compressed laterally. The body depth at the posterior margin of the operculum measures 19.2–22.1% of fork length, rendering it noticeably slimmer than congeners such as R. brachysoma. It possesses 8–11 dorsal fin spines (mode 10), 12 dorsal soft rays, no anal spines, 12 anal soft rays, and 32–38 total gill rakers (10–16 upper + 21–24 lower, mode 34), a count notably fewer than in other Rastrelliger species (≥47 total). Additional morphological traits include a head longer than body depth at the opercular margin, a small single interpelvic process, a rudimentary anal spine, and a swim bladder; coloration features a yellowish-silver belly, black blotch behind the pectoral fin base, and 2–6 dark spots at the base of the first dorsal fin. Maximum size reaches 24.0 cm fork length and 750 g in weight.^[39][1] This species exhibits a restricted distribution within the tropical Indo-West Pacific, ranging from the eastern coast of India eastward to Fiji and northward to Taiwan, with records from coastal areas including the Philippines, Indonesia, Malaysia, and the Andaman Islands. It inhabits epipelagic-neritic marine environments at depths of 2–150 m (typically shallower, 10–70 m in observed occurrences), preferring oceanodromous migrations in waters where surface temperatures remain above 17°C. Unlike more widespread congeners, R. faughni shows affinity for island-associated coastal habitats, contributing to its designation as the "island mackerel." Recent 2025 morphometric analyses, based on specimens from the Philippines, have reinforced its taxonomic separation from R. brachysoma through ratios such as body depth relative to fork length and head length, alongside genetic markers, underscoring its distinct slender form adapted to insular ecosystems.^[39][1][40] Ecologically, R. faughni forms schools comprising individuals of similar size, though these aggregations tend to be smaller and more dispersed in oceanic settings compared to continental shelf species. It is a filter-feeder, targeting the largest available zooplankton, which aligns with its reduced gill raker count facilitating coarser filtration. Reproductive details remain incomplete, with maturity length and spawning patterns undocumented in detail, though the species likely spawns in tropical waters year-round given its habitat stability.^[39][1]

Rastrelliger kanagurta

Rastrelliger kanagurta, commonly known as the Indian mackerel, is characterized by a robust, fusiform body with an oval cross-section, adapted for fast swimming in coastal waters. It possesses 8–11 dorsal spines and 12 dorsal soft rays, along with 5–6 dorsal and anal finlets. The species features long gill rakers, with 140–160 bristles on the longest raker in adults, numbering approximately 51–65 total on the first gill arch (17–24 upper, 34–41 lower), which aid in filter-feeding on plankton. Maximum length reaches 35–36 cm total length (TL), though common sizes are around 25 cm TL, with maturity attained at about 21 cm TL.^[41][42][43] This species exhibits the broadest distribution within the genus, spanning the Indo-West Pacific from the Red Sea and East Africa eastward to Samoa, including key regions like the Arabian Sea, Bay of Bengal, and Andaman Sea; it has also entered the eastern Mediterranean via the Suez Canal. Tolerant of varying salinities, R. kanagurta inhabits coastal bays, harbors, estuaries, and deep lagoons at depths of 20–90 m, preferring turbid, plankton-rich waters in tropical environments (17–30°C). It is oceanodromous, undertaking migrations along continental shelves.^[41][42] Ecologically, R. kanagurta forms large schools numbering in the thousands, often segregated by size, which facilitates efficient foraging on phytoplankton (especially diatoms) and small zooplankton. A 2024 geometric morphometric study revealed high genetic diversity and distinct subpopulations in the eastern Indian Ocean, with shape variations indicating limited gene flow: deeper-bodied forms in the Bay of Bengal, streamlined shapes in the Andaman Sea, and intermediate traits in the Arabian Sea. Spawning occurs in batches within coastal lagoons, with peaks from November to March in some regions. The maximum reported age is 4 years.^[41][44][29]

Fisheries and conservation

Commercial importance

Rastrelliger species, particularly R. kanagurta, represent a significant component of global marine capture fisheries, with annual landings of approximately 600,000 tonnes as of 2022 according to FAO data.^[45][46] About half of this catch comprises R. kanagurta, the Indian mackerel, harvested primarily through purse seine and gillnet methods that target dense schools in coastal waters. These fishing techniques exploit the species' schooling behavior, enabling efficient capture in the Indo-Pacific region.^[17] The fish are utilized in various forms, including fresh markets, canning for export, and occasionally as baitfish in pole-and-line fisheries. In major producing countries like Indonesia, India, and the Philippines, canning is prominent, with products processed for domestic consumption and international trade, while fresh sales dominate local coastal economies. India and Indonesia are the leading producers for R. brachysoma, while the Philippines leads for R. kanagurta. Harvest levels exhibit seasonal peaks aligned with monsoon patterns, which influence water temperatures and plankton blooms that aggregate fish schools, leading to higher yields during post-monsoon periods in these regions.^[47][48]

Conservation status

The species of the genus Rastrelliger exhibit varying conservation statuses according to the IUCN Red List. Rastrelliger kanagurta is classified as Least Concern, reflecting its wide distribution and relatively stable global populations, while R. brachysoma and R. faughni are assessed as Vulnerable due to inferred population reductions from overexploitation.^[14][13][15] Despite these global assessments, local declines have been observed, particularly from overfishing; for instance, small pelagic stocks including Rastrelliger species in the Java Sea have decreased significantly since the 1990s, reaching historically low levels amid high exploitation rates exceeding sustainable thresholds.^[49][50] Key threats to Rastrelliger populations include bycatch in non-selective fisheries such as trawling, which captures juveniles and contributes to recruitment overfishing, and habitat degradation from coastal development and pollution that alters nearshore spawning and nursery areas.^[51][52] Climate change exacerbates these pressures by warming sea surface temperatures, which disrupt plankton communities—the primary prey for Rastrelliger—potentially leading to shifts in distribution and reduced productivity; models indicate poleward range expansions for tropical pelagic species like these mackerels in response to such changes.^[53][54][55] Management efforts focus on mitigating these threats through regulatory measures. In Indonesia, quota-based fisheries management has been implemented for small pelagics, including Rastrelliger species, to limit catches and promote sustainability within fisheries management areas.^[56] Similar quota systems operate in India to control harvests of R. kanagurta, alongside efforts to improve enforcement and monitoring.^[57] Marine protected areas (MPAs) in the Pacific, such as those in the Bay of Bengal region, provide refugia for Rastrelliger stocks by restricting fishing and preserving habitats.^[58] Additionally, genetic monitoring has advanced stock assessments since 2022, utilizing population structure analyses to inform mixed-stock fisheries management and detect overexploitation risks.^[59][60]