Bigeye tuna, Thunnus obesus, is a large marine pelagic fish species and highly valuable. It mainly feeds on fishes and cephalopods. The bigeye tuna is increasingly important to fisheries in recent years in the Atlantic, Indian, and Pacific Ocean. We only have limited information about their trophic dynamics. Studies on its feeding behavior can provide basic information on trophic structure and dynamics in the pelagic marine ecosystem. A comparative analysis of feeding behavior of Thunnus obesus in different waters can highlight spatial variation of trophic dynamics of Thunnus obesus and provides insights on the function and position of Thunnus obesus in the pelagic marine ecosystem.
In situ stomach content method is a commonly used approach for collecting basic data of fish trophic dynamics. Because trophic data are collected real time at sampling sites, parameters estimated using the method for feeding behavior tend to more realistically reflect what occurs in the field. The parameters often used to evaluate the importance of different food items include the percentage of food item in weight, the percentage of food item in number, the percentage of occurrence frequency in the food item and relative importance index. Stomach fullness index is used to describe feeding of fishes. Both stomach fullness index and percent of empty stomach should be used to describe feeding intensity of fishes. The dietary niche breadth of food item is another important parameter for studying feeding behavior. Shannon-Wiener diversity index H′ and Pielou evenness index J′ are often used to describe the breadth of fish dietary niche.
Based on the data collected in the tuna longline fishery in the western Atlantic Ocean from August 2004 to March 2005 and the west-central Indian Ocean from December 2003 to May 2004, we studied the feeding ecology of Thunnus obesus. The results indicated that the prey species of Thunnus obesus in the west-central Indian Ocean included sardine, squid, cuttlefish and other species with the main preys being squid and sardine. In the western Atlantic Ocean, the prey species of Thunnus obesus included sardine, squid, shrimp and other small species with the main preys being sardine and squid. The percentage of empty stomachs in the west-central Indian Ocean was high, reaching up to 60%. The percentage of empty stomachs is, however, below 30% in the western Atlantic Ocean. The mean fullness stomach index fluctuated between 040-0.55 in the western Atlantic Ocean among all the months except for September 2004. Small monthly fluctuation in mean stomach fullness index also occurred in the west-central Indian Ocean. Monthly mean stomach fullness index of Thunnus obesus in the west-central Indian Ocean was higher than that of Thunnus obesus in the western Atlantic Ocean. The percentage of empty stomachs of Thunnus obesus in the west-central Indian Ocean were, however, higher. Shannon-Weiner diversity index H′ to the diet of Thunnus obesus varied between 1.50-2.00 in the west-central Indian Ocean and the western Atlantic Ocean. In a given month, Pielou evenness index J′ to the diet of Thunnus obesus in the west-central Indian Ocean were higher than those in the western Atlantic Ocean.