# Image-Based Analysis of Morphometric Differences Between Sea-Caught and Farmed Large Yellow Croaker (Larimichthys crocea)

## TL;DR

This study uses AI and photos to objectively compare body shapes of wild and farmed large yellow croaker, finding that farmed fish have larger bodies but smaller fins and tails.

## Contribution

A fast, non-invasive image-based method for quantifying morphological differences between wild and farmed fish using AI segmentation.

## Key findings

- Farmed large yellow croaker have larger body areas but smaller fins and tails compared to sea-caught individuals.
- The image-based method achieved high accuracy (mAP@50 > 98%) in segmenting and measuring fish body parts.
- Sea-caught fish show greater proportional investment in locomotor structures, likely due to natural environmental demands.

## Abstract

Fish raised on farms often look different from fish captured from the sea, but these differences are usually judged subjectively rather than measured objectively. In this study, we used ordinary photographs and artificial intelligence to compare the body shapes of sea-caught and farmed large yellow croaker, an important marine food fish in China. A computer program was trained to automatically outline different external body parts, such as the head, body, fins, tail, and eyes, and then measure their surface areas. The results showed that farmed fish had much larger body areas, while their fins and tails were relatively smaller. In contrast, sea-caught fish retained relatively larger fins and tails, which are important for swimming ability and survival in the open ocean. These findings indicate that farming environments with abundant food and limited space promote body mass growth, whereas natural marine conditions favor body shapes that support movement and escape. The method used in this study is fast, non-invasive, and low-cost, and does not require handling live animals. It can support fish breeding, product grading, and quality control, and also provides a practical tool for understanding how aquaculture alters fish body shape.

Morphological differences between sea-caught and farmed fish reflect environmental conditions and long-term domestication. However, standardized and objective quantification of these differences remains limited for many commercially important species. The large yellow croaker (Larimichthys crocea) represents a typical marine fish with clear contrasts between natural and aquaculture production systems. In this study, an image-based phenotyping workflow was developed to quantify external morphological traits of sea-caught and farmed L. crocea. Visible-light images were acquired under standardized conditions. A YOLOv11-based instance segmentation model was applied to automatically delineate major anatomical regions, including the body, head, eyes, pectoral fins, and tail. Surface areas and proportional indices were calculated following geometric calibration to ensure measurement consistency. The segmentation model achieved high accuracy on the test dataset (mAP@50 > 98%). Morphometric analyses revealed clear differences between the two groups. Farmed individuals exhibited larger body-related surface areas, whereas the relative proportions of pectoral fins and tail regions were reduced. Sea-caught fish showed higher proportional investment in locomotor structures, consistent with the physical demands of natural marine environments. These results indicate a shift in morphological allocation associated with aquaculture, characterized by enhanced trunk growth and reduced relative development of propulsion-related structures. The proposed workflow provides a rapid, non-invasive, and reproducible approach for fish morphometric analysis. It offers practical potential for phenotypic monitoring and stock assessment, while contributing quantitative evidence for domestication-driven morphological divergence in marine fishes.