# Impacts of Winter and Spring Water Masses on Demersal Fish Community Structure Around Hainan Island

## TL;DR

This study shows how seasonal changes in ocean water masses affect fish communities around Hainan Island, with implications for marine conservation and sustainable fishing.

## Contribution

The study reveals how seasonal water mass movements influence fish diversity and distribution patterns around Hainan Island.

## Key findings

- Fish communities in winter showed distinct habitat specificity across different water masses.
- In spring, warmer and saltier water intrusion led to migratory species dominating mixed water zones.
- Fish diversity centers shifted northward and shoreward from winter to spring.

## Abstract

Fish populations are highly sensitive to changes in the ocean environment. This study investigated how different “water masses”—large bodies of water with distinct temperature and salinity levels—affect fish diversity around Hainan Island. By conducting surveys in winter and spring, we analyzed which fish species were present and how their distribution shifted with seasonal water changes. We found that the movement of water masses significantly dictates where fish live, and which species are most abundant. For instance, fish communities in winter were very distinct across different areas, whereas in spring, the intrusion of warmer, saltier water brought more migratory species into mixed water zones. Additionally, the areas with the highest fish diversity moved northward from winter to spring. Understanding these seasonal patterns is vital for protecting marine biodiversity. This knowledge helps fishery managers and conservationists make better decisions to ensure sustainable fishing and healthy ocean ecosystems in the face of environmental changes.

To elucidate water mass impacts on fish diversity in Hainan Island, bottom trawl surveys were conducted at 50 stations around the Island in December 2023 (winter) and April 2024 (spring). K-means clustering identified three water masses: Coastal Water (CW), Mixed Water (MW), and Offshore Water (OW). A total of 396 species were collected. Winter communities demonstrated clear habitat specificity, with distinct dominant species in each water mass (OW, MW, and CW). Conversely, the spring intrusion of warm, saline water facilitated the dominance of migratory species in the MW. Diversity centers shifted significantly northward and shoreward, transitioning from the MW region in winter to the CW region in spring. These findings highlight the critical influence of seasonal hydrodynamics on fish community structure, providing essential baselines for regional fisheries management and conservation.