Feeding ecology of Thunnus obesus in 

the tropical Western and Central Pacific Ocean

Abstract: Bigeye tuna Thunnus obesus, widely distributed in the tropical and subtropical waters of the Atlantic, Indian and Pacific oceans, is one of major targets of tuna longline fisheries of almost all countries. The Western and Central Pacific Ocean (WCPO) is the largest tuna fishing area in the world. Global climate change aggravates series of changes in the physical and chemical environment and marine ecosystems, as well as alters the abundance and distribution of marine key populations, which profoundly affects marine fisheries globally, especially for fisheries in the Western and Central Pacific. In addition, the increasing management measures for tuna fisheries by the relevant regional fisheries management organizations (RFMOs) have also caused changes in the distribution of fisheries. To understand the feeding ecological characteristics of Thunnus obesus can provide an invaluable reference for further studies on the biology, population structure and resource assessment, as well as scientific basis for conservation and management of Thunnus obesus resources in tropical WCPO. We collected Thunnus obesus from a tuna longline fishery operating in the tropical WCPO (2°03′S11°17′S，163°14′E173°35′E) between May 2018 and February 2019. All samples were dissected and measured for fisheries biology. A visual method was used to observe the gastric fullness of sampled Thunnus obesus to identify and count individual gastric contents, the feeding ecological indicators such as gastric diversity (ShannonWiener diversity index H′ and Pielou evenness index J) of sampled individuals were also analyzed. On this basis, analysis of variance (ANOVA) was used to investigate the effects of biological factors (fork length, sex, gonadosomatic index) and spatiotemporal factors (lunar phase and fishing depth) on the ShannonWiener diversity index H′ and Pielou evenness index J of the stomach contents of both sexes of Thunnus obesus. In addition, the polytomous logistic regression for the ordinal response model was used to analyze the relationship between different biological and spatial factors on the feeding intensity of Thunnus obesus. A total of 1 123 Thunnus obesus samples were obtained during the survey, both sexes of 881 species were identified, and 682 were identified for individual feeding intensity and stomach composition. Results showed that: 1) In sampled Thunnus obesus populations, 309 (35.07%) were females and 572 (64.93%) were males with a ratio of 1∶1.85. Fork length ranged from 81 cm to 195 cm with a mean of 129.2 cm (SD=±23.0 cm) , and the dominant fork length group ranged from 120 cm to 150 cm (120 cm150 cm for males; 120140 cm for females). ANOVA analysis showed that significant differences existed in fork length between the sexes (P<0.001). 2) Analysis of the stomach contents of 509 nonempty individuals showed that Loligo sp., shrimps, Sardina sp. and Hemisalanx prognathous were the frequent species, with a ratio of 40.32%, 39.74%, 30.06% and 26.98%, respectively (Sardina sp. was used as bait). 3) The observation of feeding intensity showed that the empty stomach rate of Thunnus obesus individuals was 25.37%, and the feeding intensity was mainly composed of level 02, accounting for 81.9% of all samples. Male samples were dominated by level 1, accounting for 31.0% of males , followed by level 0, 2 and 3, accounting for 24.0%, 22.4% and 15.4%, respectively. While female samples were dominated by empty stomach, accounting for 27.91% of females, followed by level 1, 2 and 3, accounting for 25.0%, 23.7% and 14.2% of females, respectively. The empty stomach rate of the sample was high due to gastric inversion when Thunnus obesus was dragged to the deck by pressure changes or active evasion behavior of the fish. 4) ANOVA results showed that both sex and fork length had no significant effect on the feeding ecological niche width index (P>0.05).Fishing depths had also no significant effect on ShannonWiener diversity index H′ (P=0.200), but had a significant effect on Pielou evenness index J (P=0.024), with fishing depths increasing, there was few change in bait species richness while a gradual increase exsited in evenness. Temporal factor (including lunar phase and month) had no significant effect on the feeding niche breadth of both sexes of Thunnus obesuss (P>0.05). 5) The polytomous logistic regression for the ordinal response model and the circular statistic fit showed that females with the same fork length had higher feeding intensity than that of males, but had no significant difference (P=0.293). While with the increasing of fork length, individual feeding intensity decreased significantly (P=0.000 4).There was a significant effect of fishing depths on individual feeding intensity (P=0.002). The distribution of individual feeding intensity at different water depths showed that the empty stomach rate decreased with the increasing of fishing depths, which was inconsistent with the high feeding intensity. The vertical movement behavior of Thunnus obesus was to maximize the energy harvesting rate, and larger individuals had a more pronounced vertical movement habit than smaller ones. Lunar phase had a marginally significant effect on individual feeding intensity (P=0.058<0.1, marginally significant). It suggested that individual feeding intensity was relatively high during the full moon phase , while showed relatively low around the new moon. There was also a significant effect of individual gonad development on feeding intensity (P=0.030), indicating that individual feeding intensity decreased with the increased gonadosomatic index. 

Keywords: tropical Western and Central Pacific Ocean; Thunnus obesus; feeding intensity; niche breadth; lunar phase; fishing depth