🧬 biology

Length–weight relationships and growth patterns of eleven marine finfish species from Thondi, Palk Bay, southeast coast of India

This study presents the first length–weight relationship and growth pattern data for eleven marine finfish species landed at the Thondi fish market in Palk Bay, India, revealing that most species exhibit negative allometric growth with varying degrees of correlation between total length and weight.

Original authors: Sathiya C

Published 2026-07-15

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sathiya C

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine the Palk Bay, a shimmering stretch of water off the southeast coast of India, as a giant, bustling underwater city. In this city, eleven different species of fish are the residents, and a team of scientists decided to take a census at the local market in Thondi to figure out how these fish are growing up.

Think of a fish's growth like a balloon being inflated. Usually, as a balloon gets longer, it gets wider and heavier in a predictable way. Scientists call this the "Length-Weight Relationship." They want to know: if you know how long a fish is, can you guess how heavy it is? And more importantly, is the fish getting "fatter" or "skinnier" as it gets bigger?

The Great Fish Count
Between January 2021 and December 2022, the researchers gathered a massive crowd of 1,206 fish from 8 different families. These weren't just any fish; they were collected from the fish market in Thondi, where local fishermen landed their catch using nets with holes of different sizes (20, 40, 60, and 80 mm), ensuring they sampled a mix of small kids and big adults.

The fish they measured were quite a mixed bag in size:

The tiniest fish was a tiny 2 cm long (about the length of a paperclip).
The longest fish stretched to 29.5 cm (almost a foot long).
The lightest fish weighed a mere 0.5 grams (less than a paperclip).
The heaviest fish tipped the scales at 180 grams (about the weight of a small apple).

The "B" Factor: The Secret Growth Code
To understand how these fish grow, the scientists used a special math formula: W = aTLb. Don't let the letters scare you! Think of it as a recipe. The most important ingredient here is the number b.

If b equals 3, the fish is growing perfectly balanced. It's like a perfect sphere getting bigger; every inch of length adds a proportional amount of weight.
If b is less than 3, the fish is growing "negatively allometric." This is the big finding of the paper: every single one of the eleven species studied was growing this way.

What does that mean in plain English? It means these fish are getting longer faster than they are getting heavier. Imagine a teenager who shoots up in height but hasn't quite filled out their muscles yet. They are getting tall and lanky, not stocky.

The Star Performers and the Stragglers
Not all fish followed the same growth script, and the scientists measured exactly how well their math worked for each one.

The Superstar: The fish called Sillago sihama had the most predictable growth pattern. The math fit so perfectly that the correlation score was 0.9451. It was like a clockwork machine; if you knew its length, you could almost perfectly guess its weight.
The Wild Card: On the other end, the fish Tricanthus biaculatus was a bit of a mystery. Its correlation score was only 0.4928. This means its length and weight didn't line up as neatly. Maybe some were chubby and some were skinny, even if they were the same length.
The Almost-Perfect Grower: The milkfish, Chanos chanos, had a b value of 2.9995. That is incredibly close to the perfect "3" balance. It's growing almost exactly as nature intended, with length and weight matching up perfectly.
The Skinniest Grower: The fish Atherinomorus insularum had the lowest b value of 1.2893. This one is stretching out its body length way faster than it's adding weight. It's the ultimate "lanky" fish of the bunch.

Why Does This Matter?
The paper suggests that these growth patterns aren't random. They are likely influenced by the water temperature, what the fish are eating, and whether they are ready to have babies. Just like how a human might grow differently depending on their diet and environment, these fish are reacting to the specific conditions of the Palk Bay.

The scientists found that most of their math models were very strong (with scores above 0.70), meaning they can trust these numbers to help manage the fish population. This study is the first time anyone has reported these specific growth numbers for these eleven species in the Thondi area.

The Bottom Line
This research doesn't solve every mystery of the ocean, but it provides a crucial baseline. By knowing exactly how these eleven species grow—from the tiny 2 cm swimmers to the 180 g giants—fisheries managers can better estimate how much fish is in the water and how to keep the population healthy. The data shows that in this part of the bay, fish are generally getting tall and lanky rather than short and stout, a pattern that will help scientists monitor the health of the ecosystem for years to come.

Technical Summary: Length–Weight Relationships and Growth Patterns of Eleven Marine Finfish Species from Thondi, Palk Bay

Problem Statement
The Palk Bay on the southeast coast of India is a critical marine ecosystem supporting diverse finfish resources and coastal livelihoods. However, biological data regarding commercially exploited species in this region, particularly their length–weight relationships (LWRs), remain inadequate. While LWRs are fundamental tools in fisheries biology for estimating biomass, assessing stock status, and understanding growth patterns, region-specific estimates are necessary because growth characteristics vary due to environmental conditions, population structure, and fishing pressures. Existing data from other Indian coastal regions cannot be directly applied to Palk Bay due to these local variations. Consequently, there is a lack of baseline biological information required for accurate stock assessment and sustainable fisheries management in the Thondi coastal area.

Methodology
The study was conducted between January 2021 and December 2022 at the Thondi fish market in Palk Bay.

Sample Collection: A total of 1,206 individuals representing 11 finfish species and 8 families were collected from landings by local artisanal and commercial vessels. The primary fishing gear used was multimesh composite gill nets (100 m × 2 m) with mesh sizes ranging from 20 to 80 mm.
Measurements: Total length (TL) was measured to the nearest 0.1 cm from the snout tip to the end of the longest caudal fin ray. Total body weight (W) was recorded to the nearest 0.01 g using a digital electronic balance.
Identification: Specimens were preserved in 10% buffered formalin and identified to the species level using FAO Species Identification Sheets.
Statistical Analysis: The length–weight relationship was modeled using the equation $W = aTL^b$ . Data were log $_{10}$ -transformed to linearize the relationship. Parameters $a$ (intercept) and $b$ (allometric coefficient) were estimated using ordinary least-squares regression. The coefficient of determination ( $R^2$ ) and Pearson's correlation coefficient ( $r$ ) were calculated to assess model fit. The significance of the regression slope was tested at $P < 0.05$ . Growth types were determined by comparing estimated $b$ values against the theoretical isometric value of 3 using a Student's $t$ -test. Outliers were identified via visual inspection of scatter plots and standardized residuals and excluded prior to analysis.

Key Results
The study successfully established LWRs for 11 species, including Siganus canaliculatus, Sardinella gibbosa, Terapon jarbua, Tricanthus biaculeatus, Gerres abbreviatus, Gerres cinereus, Atherinomorus insularum, Mugil cephalus, Chanos chanos, Sillago sihama, and Upeneus tragula.

Size and Weight Ranges: Total lengths ranged from 2.0 cm (Atherinomorus insularum) to 29.5 cm (Sillago sihama), while weights varied from 0.5 g (Tricanthus biaculeatus) to 180 g (Mugil cephalus).
Growth Patterns: All 11 species exhibited negative allometric growth ( $b < 3$ $b < 3$ ), indicating that body length increased at a faster rate than body weight.
- Chanos chanos showed the growth pattern closest to isometry ( $b = 2.9995$ ).
- Atherinomorus insularum exhibited the strongest negative allometric growth ( $b = 1.2893$ ).
Model Fit: The coefficient of determination ( $R^2$ $R^{2}$ ) varied between 0.4928 and 0.9451.
- Sillago sihama demonstrated the highest correlation ( $R^2 = 0.9451$ ).
- Tricanthus biaculeatus showed the lowest correlation ( $R^2 = 0.4928$ ), which the authors attribute to greater variability in individual size, physiological condition, or limited representation of certain size classes.
Dominant Families: The family Gerridae was the most dominant in the sample, followed by Clupeidae.

Significance and Claims
The paper claims to provide the first report on the length–weight relationships for these eleven marine finfish species specifically from the Thondi coastal area of Palk Bay. The authors state that the generated parameters serve as essential baseline biological information for the region. The study asserts that these LWR estimates will:

Facilitate accurate biomass estimation and stock assessment for local fisheries.
Support comparative population studies and ecosystem modeling.
Aid in the sustainable management and conservation of marine fishery resources in Palk Bay.
Provide a reference for monitoring temporal changes in fish populations.

The authors emphasize that while the results are specific to the sampled populations and timeframes, they offer a necessary foundation for fisheries management in an area heavily dependent on small-scale and artisanal fishing sectors. The study does not propose new experimental applications or future management strategies beyond the provision of these baseline data for use by resource managers and researchers.

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