<Abstract>
The Indian Ocean is the second-largest tuna fishing ground in the world, accounting for approximately 1.2 million tonnes
(23%) of the estimated 5.2 million tonnes of global commercial tuna catch in 2023. This study examined the relationship
between tuna catches, specifically skipjack, bigeye, and yellowfin tunas, and prey biomass (Nautical Area Scattering
Coefficient, NASC) estimated from acoustic surveys conducted in the southwestern Indian Ocean from 20 April to 15 May
2019. Environmental variables were derived from the Copernicus Ocean Model, and tuna length data from the IOTC. The
estimated total tuna catch in the study area was approximately 166,400 tonnes, with the northwestern region showing the
highest catches and NASC values. Tuna catches increased with NASC; however, the relationship was non-linear. While
skipjack showed no significant correlation with NASC, bigeye and yellowfin tunas exhibited weak but significant positive
correlations. Environmental analysis revealed that the northern waters had high surface temperatures, low salinity, and low oxygen levels, with mid and deep layers characterized by low temperature, salinity, oxygen, and chlorophyll. These findings
offer a foundation for understanding tuna distribution in relation to prey and environmental conditions, highlighting the
need for future species- and fishery-specific studies to support sustainable tuna resource management.