Abstract: In order to investigate the effects of different diet protein sources on carbon and nitrogen isotope fractionation of juvenile cobia Rachycentron canadum L., three isonitrogenous and isocaloric diets were formulated. The protein source in D1 was fish meal; beer yeast meal and corn gluten meal protein replaced 10% fish meal in D2 and D3, respectively. The diets were fed to juvenile cobia for 24 days. The results showed that, when 10% fish meal was replaced, the weight gain rate (WGR) of cobia decreased significantly. The carbon isotope ratio δ¹³C of cobia increased in time and nitrogen isotope ratio δ¹⁵N decreased in time in all groups. Although the change speeds of δ¹⁵N were different, the whole fish and muscle of cobia fed with different diets reached isotope equilibrium states with their diets when the feeding experiment was accomplished. When 10% fish meal was replaced by beer yeast meal (corn gluten meal) protein, the carbon isotope fractionation Δ¹³C decreased while nitrogen isotope fractionation Δ¹⁵N increased in the whole fish and muscle of cobia; Δ¹³C of the whole fish decreased from 4.19‰ to 3.94‰ and 3.63‰, Δ¹³C of the muscle decreased from 4.46‰ to 3.98‰ (to 3.67‰), and Δ¹⁵N of the whole fish increased from 0.18‰ to 0.88‰ (to 0.94‰), Δ¹⁵N of the muscle increased from 0.18‰ to 0.74‰ (to 0.87‰). When the three diets with different protein sources were fed, the variation trends of Δ¹³C and Δ¹⁵N were similar between whole fish and muscle of cobia, but the change speed of whole fish δ¹⁵N was slower than that of muscle. These results indicated that cobia muscle can represent the δ¹³C and δ¹⁵N characters of whole fish in trophic level study of ecosystem (long time scale), but cannot represent the change process of whole fish δ¹⁵N in metabolic physiology study (short time scale).

Key words: juvenile cobia, beer yeast meal, corn gluten meal, &, #x003b4, 13C and &, #x003b4, 15N, isotope fractionation