Abstract: A total of 33 individuals of Kawakawa (Euthynnus affinis), 34 individuals of Skipjack tuna (Katsuwonus pelamis),and 40 individuals of Yellowfin tuna (Thunnus albacares)were collected by light traps and falling nets operated in the central and western South China Sea in spring 2018. Carbon and nitrogen stable isotope techniques and multivariate statistical models were used to investigate isotopic characteristics, shifts of ontogenic diet, and trophic niche breadths and overlaps of the three tuna species. The results showed that the mean values of δ¹³C were (-17.83±0.15)‰ forE. affinis, (-17.68±0.18)‰ for K. pelamis, and (-18.05±0.24)‰ for T. albacares, and their mean δ¹⁵N values were (10.78±0.47)‰,(10.26±0.82)‰, and (8.72±0.88)‰, respectively. Results of Mann-WhitneyU test showed significant difference in δ¹³C (P<0.01) and δ¹⁵N (P<0.05) between E. affinis and K. pelamis, and for δ¹³C (P<0.01) and δ¹⁵N (P<0.01) between T. albacares and eitherE. affinisorK. pelamis. A significant positive correlation was found between fork length and δ¹⁵N values of each species, indicating that their diets shifted with growth to higher trophic level. T. albacares exhibited the largest trophic niche breadth, followed by K. pelamis and E. affinis, indicating that T. albacares had a higher trophic diversity than E. affinis and K. pelamis. No overlaps in trophic niche were detected betweenT. albacares and either E. affinisorK. pelamis. There was, however, a low trophic overlap rate (0.19)between E. affinis and K. pelamis, indicating a certain extent of dietary competitionbetween them.

Key words: Euthynnus affinis, Katsuwonus pelamis, Thunnus albacares, ontogenetic diet shift, standard ellipse area (SEA).