Determination of the trophic levels of four fish species in Engraulidae and Clupeidae in Laizhou Bay based on stable carbon and nitrogen isotopes

Abstract

Long-term overfishing and environmental pollution has led to a high degree of change in Laizhou Bay fisheries resource structures. Since 1980, small pelagic fish have become the new dominant species in this area.These small pelagic fish play a connecting role in the food chain and are often one of the key groups in marine ecosystems. Research on the trophic level of Engraulidae and Clupeidae fish in Laizhou Bay based on stomach content analysis has provided useful but dated information. More up-to-date research is now necessary. Based on bottom trawl surveys in Laizhou Bay during summer and autumn, 2011, the stable isotope signatures of four Engraulidae and Clupeidae species revealed that the stable carbon isotope ratios ranged from 21.97‰ to 11.67‰,while stable nitrogen isotope ratios ranged from 8.54‰ to 15.95‰. ANOVA analysis revealed that δ13C and δ15N varied significantly among the four species(P0.05). Linear regression analysis between fork length and stable isotopic ratios showed that Sardinella zunasi and Konosirus punctatus had significant ontogenetic variation in terms of δ15N(P0.05), while Setipinna taty and Thrissa kammalensis did not(P0.05). The S. zunasi trophic level ranged from 2.80 to 4.88, with the average being 3.52±0.46; the K. punctatus trophic level ranged from 2.71to 4.29, with the average being 3.34±0.33; the T. kammalensis trophic level ranged from 3.38 to 4.42, with the average being 3.97±0.28; the S. taty trophic level ranged from 3.28 to 4.13, with the average being 3.76±0.23.According to the stable carbon and nitrogen isotope results, we can draw the following conclusions:(1) The stable carbon isotope values showed that K. punctatus trophic level range( 20.92 to 11.67) almost covers that of the remaining three species( 21.97 to 16.8), meaning that K. punctatus inhabits a wide depth range which is different to that of other pelagic fish;(2) The K. punctatus trophic level decreases with increasing fork length while that of S. zunasi increases with increasing fork length, no such changes were observed in the other two species;(3) The T. kammalensis trophic level is about 0.7 times larger than that reported from the traditional stomach content analysis, which may be related to increased feeding on shrimp. The trophic level of the other three species was slightly higher according to the stable isotope results, but not significantly so.