Largehead hairtail stocks off southern Java show signs of overfishing, three-year study finds

A three-year biological survey of the largehead hairtail, Trichiurus lepturus, in the upwelling-rich waters off southern Java, Indonesia, has concluded that the species there is experiencing both growth overfishing and recruitment overfishing, with an exploitation rate of 0.72 well above the commonly accepted optimum of 0.50. The findings, based on nearly 20,000 fish measured at a single landing site between 2019 and 2021, provide the most detailed picture yet of a stock that supports a substantial Indonesian export industry but has until now been assessed with very limited local data. The largehead hairtail is a benthopelagic predator of the family Trichiuridae, order Scombriformes, distributed across tropical and subtropical waters of the Indian, Atlantic and Northwestern Pacific Oceans.

It commands high economic value for its taste and nutritional profile, supplying essential amino acids and polyunsaturated fatty acids. Indonesia ships the fish to China, the United States, Malaysia, Japan, Vietnam, Thailand, South Korea, Italy, Taiwan and Singapore; between 2017 and 2021 export volume reached 43,894 tons, an increase of 25.26 percentage points. That demand, the study's authors note, is expected to intensify fishing pressure in the southern Java region of the Indian Ocean, an area where strong seasonal upwelling brings nutrient-rich deep water to the surface and supports vigorous fish growth and reproduction.

Assessing the stock has been difficult. The research area is vast, and holistic approaches such as demersal trawl surveys are costly, while classical analytical models struggle in tropical waters characterized by seasonal variation, multispecies fisheries and a diversity of fishing methods. The team therefore adopted a length-based, data-driven strategy, building the assessment from monthly length-frequency measurements of fishermen's catches landed at Palabuhanratu, West Java Province, within Fisheries Management Area 573.

The dominant gears were handline and bottom longline rigged with hook sizes 7, 8 and 9, operated at depths of 10 to 100 meters. Alongside length and weight, researchers recorded sex and gonad condition, with laboratory work on length, weight and gonads carried out at the Research Institute for Marine Fisheries Laboratory of the Ministry of Marine Affairs and Fisheries in 2019. In total 19,587 individuals were measured over the three years.

Total lengths ranged from 7.0 to 135.0 centimeters, with a mean of 78.33 plus or minus 8.35 centimeters and a distribution mode at 79.0 centimeters. Females spanned 69.0 to 107.0 centimeters with a mean of 82.05 plus or minus 7.52 centimeters, while males ranged from 68.9 to 101.5 centimeters with a mean of 80.00 plus or minus 5.95 centimeters. Length-weight analysis showed positive allometric growth in females, males and the combined sample, meaning weight increases faster than length as the fish grows, with exponents of 3.2994, 3.5557 and 3.2747 respectively and correlation coefficients between 0.81 and 0.93.

The authors interpret the positive allometry as an indicator of good body condition, consistent with reports from Mexico and Oman. Reproductive biology emerged as a central thread of the assessment. Of 377 sexed fish, 214, or 57 percent, were male and 163, or 43 percent, female, a pooled ratio of 1.31 to 1 that chi-square testing showed to be significantly unbalanced toward males.

The imbalance was not constant through the year: in February, May, October and November the ratio did not differ significantly from 1:1, whereas in June, July, August and December males significantly outnumbered females. The authors attribute such spatial and temporal variation in sex ratios elsewhere and in this fishery to environmental conditions, fishing pressure and reproductive dynamics, including sex-specific migration to and from fishing grounds and the selectivity of the gears deployed. Gonadal maturity stages and the gonadosomatic index together pinpointed the spawning season.

Females showed all maturity stages from immature to running ripe, with the highest gonadosomatic index values for both sexes occurring in July, reaching 21.59 in females and 5.13 in males. The study concludes that spawning in the southern Java waters peaks in July and August, a timing that differs markedly from other regions: March to June in Ghanaian waters, May to June in the Arabian Sea, January to March in Mexico, December to March in the northern Arabian Sea and Bay of Bengal, spring to summer in southern Brazil, and March to September in southeastern Australia. The authors suggest that the region's strong upwelling conditions may underpin this locally adapted reproductive schedule, and they flag a seasonal fishing closure during the July-August peak as a candidate management measure.

Growth and mortality parameters were estimated with the Electronic Length Frequency Analysis method implemented in the TropFishR package in R. Among the fitting options, the ELEFAN optimize method gave the best goodness-of-fit, with an Rn of 0.27 against 0.26 for ELEFAN SA and 0.25 for ELEFAN GA. The resulting von Bertalanffy parameters were an asymptotic length of 131.67 centimeters, a growth coefficient of 0.22 per year, and a theoretical age at zero length of minus 0.5094 years, yielding the growth equation Lt = 131.67(1 minus e to the power of minus 0.22(t + 0.5094)). Maximum age was estimated at 14 years.

The growth coefficient places the southern Java population in the slow-growth category, consistent with populations in southeastern Australia, the northern Arabian Sea, the Persian Gulf and the East China Sea, and implying that the fish needs a long time to reach both its maximum size and full sexual maturity. The mortality figures carry the assessment's sharpest warning. Natural mortality was estimated at 0.44 per year, fishing mortality at 1.16 per year, and total mortality at 1.60 per year, giving an exploitation rate of 0.72.

Fishing mortality thus accounts for roughly 72 percent of all deaths in the exploited population and exceeds natural mortality by a wide margin, a combination the study identifies as characteristic of overfishing. The M/K ratio of 2.0 still sits within the 1.5 to 2.5 range considered typical, but the imbalance between fishing and natural mortality, combined with the slow growth rate, means depleted fish are replaced slowly and heavy exploitation disrupts recruitment. Two reference-point analyses quantify how far the fishery sits from sustainable targets.

Yield-per-recruit analysis found maximum yield per recruit of 8,068 grams per recruit at a fishing mortality of 1.13 per year, only marginally below the current fishing mortality of 1.16 per year, so fishing effort would need to fall by about 2 percent to reach that optimum. More tellingly, the current length at first capture of 81.0 centimeters is below the 88.0 centimeters that would maximize yield per recruit, meaning fish are being taken before they can contribute fully to biomass; an 8 percent increase in selectivity would close that gap. The spawning potential ratio, the ratio of spawning stock biomass per recruit under fishing to that with no fishing, was estimated at 26 percent, between the target reference point of 40 percent and the limit of 20 percent.

Current fishing mortality is 48 percent above the level that would deliver a 40 percent spawning potential ratio, which the authors estimate would require a 48 percent reduction in effort, or alternatively a rise in selectivity length to 89 centimeters. There is a genuine bright spot in the data. The length at 50 percent selectivity of the handline and bottom longline fishery, 81.0 centimeters, exceeds the female length at 50 percent maturity of 77.0 centimeters, although it remains below the male maturity length of 84.5 centimeters.

Because most captured fish have therefore matured and had at least one opportunity to spawn, the authors describe the gear selectivity as relatively favorable, and they contrast it with shrimp trawls, which in Ghanaian waters caught hairtail at a mean first-capture length of only 47.11 centimeters and which elsewhere dominantly take fish under 25 centimeters. The 77.0 centimeter female maturity length, higher than values reported from Brazil, India, the Persian Gulf, Ghana and the Sunda Strait but lower than those from Oman and southeastern Australia, could serve as a basis for a minimum legal size. The study's overall prescription combines effort reduction of roughly 48 percent, size-based catch limits, and temporary closures during the July-August spawning peak, alongside continued involvement of fishers and traders, to keep this economically vital species productive in one of the Indian Ocean's most fertile fishing grounds.

Subject of Research: Stock assessment and reproductive biology of largehead hairtail in southern Java waters, Indonesia

Article Title: Unveiling the status of Trichiurus lepturus ( Actinopterygii , Scombriformes , Trichiuridae ) stocks in the southern Java waters, Indonesia: A biological and length-based assessment approach

Article References: Prihatiningsih, P., Tirtadanu, T., Taufik, M., Rosmiati, R., Nurdin, E., Panggabean, A. S., Kembaren, D. D., Nurulludin, N., Chodrijah, U., Mahulette, R. T., & Nugraha, B. (2025). Unveiling the status of Trichiurus lepturus (Actinopterygii, Scombriformes, Trichiuridae) stocks in the southern Java waters, Indonesia: A biological and length-based assessment approach. Acta Ichthyologica et Piscatoria, 55, 31-43. https://doi.org/10.3897/aiep.55.138745

Image Credits: AI Generated

DOI: 10.3897/aiep.55.138745

Keywords: largehead hairtail, Trichiurus lepturus, stock assessment, Indonesia, Indian Ocean, overfishing, spawning potential ratio, yield per recruit, von Bertalanffy growth, fisheries management, upwelling, length-frequency analysis