Assessing trophic interactions among three tuna species in the Solomon Islands based on stomach contents and stable isotopes

Trophic research is essential to the conservation and management of fishery resources. This study analyzed the feeding habits and nutritional interactions among three tuna species in the waters of the Solomon Islands (5 degrees 12 ' S-15 degrees 20 ' S, 157 degrees 31 ' E-172 degrees 19 ' E). A total of 103 bigeye tuna (Thunnus obesus), 296 yellowfin tuna (Thunnus albacores), and 264 albacore tuna (Thunnus alalunga) samples were collected from September to December 2019. Samples for stomach content and stable isotope analysis were randomly selected. The stomach content analysis results showed that the stomach contents of the three tuna species were rich in prey, and 48 prey species were identified, including fish, cephalopods, and crustaceans, with high between-phenotype component (BPC). Yellowfin tuna consumed the largest variety of food and bigeye tuna consumed the smallest. The feeding intensity of bigeye tuna were higher than that of yellowfin and albacore tuna. Yellowfin tuna had the highest empty stomach rate (35.69%) and lowest average stomach fullness index (0.064). The degree of stomach fullness in the three tuna species was mainly 1, and the difference in the stomach fullness index among them was significant (P<0.001). Food overlap (0.3-0.6) and Levins index (<0.6) among the three species were low, indicating a specialized feeding tendency. Bigeye tuna mainly feed on Ommastrephes bartrami and Myctophidae, yellowfin tuna mainly feed on Hyperiidea and Aluterus monoceros, and albacore tuna mainly feed on Gempylus serpens and Loligo chinensis. The stable isotope results showed that the delta C-13 and delta N-15 values of the three tuna species were significant (P<0.001). The standard ellipse area corrected for small sample sizes (SEAc) and total niche area (TA) of bigeye tuna, range of delta N-15 (NR) of yellowfin tuna, and range of delta C-13 (CR) of albacore tuna were the highest. The results of this study will help improve our understanding of the feeding ecology of these three tuna species and their important roles in the ecosystem and food web structure.