# The Spatial and Temporal Distribution of Bigeye Tuna and Yellowfin Tuna in the Northwest Indian Ocean and Their Relationship with Environmental Factors

## TL;DR

This study compares the distribution patterns of bigeye and yellowfin tuna in the Indian Ocean and how they relate to environmental factors.

## Contribution

The study provides new insights into the spatial and vertical distribution patterns of bigeye and yellowfin tuna in relation to environmental variables in the NWIO.

## Key findings

- Bigeye tuna occupy deeper and broader vertical habitats compared to yellowfin tuna.
- Yellowfin tuna show a stronger preference for warmer waters and a wider horizontal distribution.
- Both species exhibit distinct spatial clustering patterns influenced by environmental factors like temperature and chlorophyll a.

## Abstract

Bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) are the primary target species of longline fisheries in the northwestern Indian Ocean (NWIO). Their population distribution patterns and environmental preferences are critical to fisheries production, resource management, and the elucidation of their migratory behaviors. This study used scientific survey data from two research cruises to comparatively analyze the spatial distribution, vertical distribution, and environmental preferences of the two species. It was confirmed that T. obesus occupies deeper and broader vertical habitats, whereas T. albacares exhibits a wider horizontal distribution range. The distribution hotspots of the two species also differ significantly, with T. albacares showing a stronger preference for warmer waters than T. obesus. Environmental factors across different water layers exert a substantial influence on the distributions of both species. Our findings provide crucial insights for further investigations into the environmental preferences and underlying migratory mechanisms of these tuna species, particularly with respect to their transoceanic connectivity.

The Northwestern Indian Ocean (NWIO) serves as a primary fishing ground for tuna longline fisheries, with bigeye tuna (Thunnus obesus) and yellowfin tuna (Thunnus albacares) constituting the main target species. Investigating their spatiotemporal distribution and relationship with environmental factors is of significant importance for fishery management and fishing. This study analyzed and compared the distribution patterns and environmental preferences of these two species across different depth layers, based on fisheries scientific survey data collected during the 2023/2024 and 2024/2025 fishing seasons. Key findings include: The hook rate in 2023/2024 was higher than in 2024/2025, and the hook rate for T. obesus exceeded that of T. albacares. T. obesus were predominantly concentrated within 63° E–69° E and 7° N–9° N, while T. albacares exhibited a broader yet more dispersed distribution range. T. obesus primarily occupied depth layers of 130–140 m (12.20%), 180–190 m (9.76%), and 270–280 m (9.76%). T. albacares were mainly found at 110–120 m (15%), 140–150 m (15%), and 200–210 m (15%). Both species exhibit distinct spatial clustering patterns, and their hotspot distribution areas are, respectively, 63° E–69° E, 5° N–10° N and 64° E–68° E, 0° N–4° N. Correlation analysis revealed significant relationships between T. obesus distribution and latitude, zooplankton abundance, water temperature at various depths, and chlorophyll a concentration. Our research provides reference for understanding the distribution of T. obesus and T. albacares across different water layers and their habitat preferences, laying a scientific foundation for achieving sustainable utilization of both species.