Simple Fish Measurements Reveal Which Malaysian Pelagic Species Are Thriving and Which Are Struggling

Off the east coast of Peninsular Malaysia, the waters of Terengganu support one of the region’s most economically important small pelagic fisheries, landing thousands of tonnes of mackerel, scad, sardinella and toothpony every year. Yet until recently, fisheries scientists had almost no published baseline data on how these fish grow and how healthy they are in this specific stretch of the South China Sea. A new open-access study published in Discover Oceans by Nurul Iman Athirah Hamdan, Mastura Mustapha and Mohd Samsul Rohizad Maidin closes that gap with an unusually direct approach: measuring thousands of individual fish and letting the relationship between length and weight tell the story of each species’ physiological condition.

The research team sampled four commercially vital species: the toothpony Gazza minuta, the Indian mackerel Rastrelliger kanagurta, the Indian scad Decapterus russelli and the goldstripe sardinella Sardinella gibbosa. Between September 2024 and April 2025, the researchers conducted eight sampling trips across nine stations in Terengganu waters, working from two purse-seine vessels. The nets used were substantial, spanning 550 metres in length and 130 metres in depth, with a one-inch mesh size. A designated observer monitored every sampling location and catch method to ensure the protocol was followed, and the total catch was thoroughly homogenised before subsampling with a standardised fixed-volume scoop to minimise bias.

In total, 3,523 individual fish were measured. Each specimen had its total length recorded on a measuring board accurate to one millimetre and its body weight taken on an electronic balance precise to 0.1 grams. The haul included 537 toothpony, 683 Indian mackerel, 1,497 Indian scad and 806 goldstripe sardinella. The scale of the dataset matters, because length-weight analysis is only as reliable as the sample behind it, and these numbers give the resulting growth parameters genuine statistical weight for a region where such information was previously absent.

The analytical core of the study is the length-weight relationship, expressed through the classic power equation W = aL^b, where W is weight, L is length, a is a constant and b is the exponent describing how weight scales with length. After logarithmic transformation, the data were fitted by least-squares linear regression, with 95 percent confidence intervals and coefficients of determination estimated for each species. The exponent b is the star of the show: a value of exactly 3 indicates isometric growth, meaning the fish maintains its proportions as it grows; a value below 3 signals negative allometric growth, where the fish becomes more elongated and lighter for its length; and a value above 3 indicates positive allometric growth, where the fish becomes plumper and heavier as it lengthens.

The results split the four species into two contrasting camps. Gazza minuta produced a b value of 2.98, just shy of isometry but still classified as negative allometric growth, supported by a strong length-weight correlation of r = 0.98. The other three species all exceeded the magic number: Rastrelliger kanagurta at 3.29, Decapterus russelli at 3.10 and Sardinella gibbosa at 3.11, all indicating positive allometric growth, meaning these fish gain weight faster than length as they mature. The maximum sizes recorded were 134 millimetres for toothpony, 240 millimetres for Indian mackerel, 219 millimetres for Indian scad and 185 millimetres for goldstripe sardinella, with the mackerel and sardinella figures exceeding some maxima reported from neighbouring waters such as Pakistan’s coast and Sri Lanka.

The second key metric, the relative condition factor Kn, compares each fish’s observed weight to the weight predicted by the fitted length-weight equation. Values above 1.0 indicate fish in excellent condition, values near 1.0 suggest good condition, and values below 1.0 point to fish that are underweight for their length. Here the divergence between species was stark. The toothpony averaged just 0.84, well below unity and a clear sign of suboptimal physiological condition. The Indian mackerel, by contrast, averaged 1.14, while the Indian scad and goldstripe sardinella both sat at 1.04, all suggesting fish in favourable health. The researchers chose Kn over Fulton’s condition factor deliberately, because Fulton’s index assumes isometric growth, an assumption violated by three of the four species studied.

Why would one species fare so much worse than its neighbours? The authors point to a web of interacting biological and environmental influences. The growth exponent b is not a fixed property of a species; it fluctuates with age, sex, gonadal maturity, disease and parasite burden, seasonal shifts in stomach fullness, food availability, habitat quality and environmental stressors such as variations in pH, temperature and dissolved oxygen. An earlier study of toothpony in Pabean Bay, Indonesia, reported a far lower b value of 2.39, illustrating how much populations of the same species can differ across their range. The lowest condition factors in the Terengganu dataset appeared in specific size classes, including 81 to 95 millimetre toothpony and 80 to 94 millimetre Indian scad, hinting that juvenile stages of some species may face particular nutritional or environmental pressure.

The economic stakes of these measurements are considerable. According to Malaysian fisheries statistics for 2024, Terengganu landings reached 4,695 tonnes of Indian scad, 2,964 tonnes of Indian mackerel, 248 tonnes of goldstripe sardinella and 117 tonnes of toothpony. Artisanal and commercial fishers in the state depend on the year-round availability of these species for steady monthly income, and small pelagic fishes occupy a critical trophic position, linking planktonic production at the base of the food web to the larger predatory fish, seabirds and marine mammals above them. Their rapid turnover rates and sensitivity to environmental variability also make them valuable indicators of overall ecosystem productivity, which means a fish in poor condition can be an early warning sign for the whole coastal system.

The practical implications extend directly into fisheries management. Species showing negative allometry and low condition, such as the toothpony in this study, may be more vulnerable to environmental stress or overfishing, and the authors argue such findings justify stricter size-at-entry regulations or seasonal closures to allow biomass to recover. The condition factor also serves as a proxy for the energy reserves available for reproduction, so monitoring Kn over time could help managers anticipate recruitment success and adjust total allowable catches based on the actual physiological health of the stock rather than catch numbers alone. The species-specific reference points generated here could inform minimum legal landing sizes and help evaluate the effectiveness of marine protected areas in Terengganu.

What makes this study a quiet triumph is its demonstration that humble, low-cost measurements can yield management-grade insight. No genetic sequencing or acoustic telemetry was required, just a measuring board, a balance, careful sampling design and rigorous regression analysis. The authors call for future work to broaden the geographical scope across Malaysia and to investigate the environmental drivers behind the growth patterns they documented, particularly the apparent stress in the toothpony population. As climate variability increasingly perturbs tropical shelf seas, building these baseline biometric datasets across the Indo-West Pacific will be essential for detecting change early, and Terengganu’s fish have now officially joined the record.

Subject of Research: Growth patterns and condition factors of commercially important small pelagic fishes in Terengganu waters, Malaysia

Article Title: Growth patterns and condition factors of selected marine pelagic fishes from Terengganu waters, Malaysia

Article References: Growth patterns and condition factors of selected marine pelagic fishes from Terengganu waters, Malaysia. (n.d.). https://doi.org/10.1007/s44289-026-00147-z

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00147-z

Keywords: length-weight relationship, allometric growth, relative condition factor, small pelagic fishes, Terengganu, Malaysia, Indian mackerel, Indian scad, goldstripe sardinella, toothpony, fisheries management, South China Sea