Feeding Habits and Ontogenetic Diet Shifts of Yellowfin Tuna (Thunnus albacores) in the South China Sea

Abstract:

To understand the diet composition of wild yellowfin tuna (Thunnus albacores) and its ontogenetic variations, some surveys were conducted from February 2012 to January 2013 in the South China Sea. Both stomach content and stable isotope analysis were used to study the feeding ecology of yellowfin tuna. A total of 94 yellowfin tuna samples, ranging between 41 cm and 180 cm in fork length (FL), were measured according to the national standards on specifications for oceanographic survey (GB/T 12763.6–2007). Results showed that diet compositions of Thunnus albacores mainly consisted of Sthenoteuthis oualaniensis, Uroteuthis sp., Sepia sp., Decapterus sp., Gempylus, Alepisaurus, Brama japonica, Cypselurus sp., unidentified crustacean and fishes, etc. S. oualaniensis was the primary prey components of the diet, averaging 51.07% by percentage of Index of Relative Importance (IRI%). Decapterus was the second most important diet, with IRI of 39.81%. The percentage of empty stomach, mean stomach fullness index and diet compositions had significant monthly variation (P<0.05). The vacuity coefficient changed significantly in each group of FL and showed the upward trend (P<0.05). Mean stomach fullness index had significant monthly variation. With the group between 130 and 160 cm FL, Shannon-Weiner diversity index H¢ ranged between 0.05 to 1.18 and Pielou evenness index J varied from 0.10 to 0.32. The average trophic level of yellowfin tuna in the South China Sea was 3.54. In conclusion, the results improved the understanding of the yellowfin tuna biological characteristics and feeding habits, also provided some empirical data for marine ecosystem modeling in the South China Sea.