Abstract: Blue mackerel scad (Decapterus maruadsi) is one of the main economic fishes in the northern part of South China Sea (NSCS). In this study, stable isotope analysis was used to analyze the changes of trophic niche of D. maruadsi in the northern part of NSCS from September to October 2019. The results showed that the trophic levels of D. maruadsi ranged from 2.77 to 4.19, with an average of 3.81. The δ¹⁵N values varied from 7.29‰ to 12.10‰, with an average of 10.83‰. The δ¹³C values ranged from -19.32‰ to -16.10‰, with an average of-17.94‰. There was a positive correlation between the values of δ¹⁵N and δ¹³C. The values of δ¹⁵N and δ¹³C increased with ontogeny, and peaked at the body length near 100 mm, and then decreased slightly. The C/N value decreased with body length and increased slightly when body length reached about 100 mm. There was no significant difference of C/N values among different sampling depths (P>0.05). When body length was ≥140 mm, the δ¹⁵N value of female D. maruadsi was significantly higher than that of male (P<0.05), but there was no significant difference in δ¹³C and C/N values between males and females (P>0.05).

Key words: Decapterus maruadsi, stable isotope analysis, trophic level.