USING STABLE ISOTOPES TO ANALYZE FEEDING HABITS AND TROPHIC POSITION OF HAIRTAIL (TRICHIURUS LEPTURUS) FROM THE BEIBU GULF, SOUTH CHINA SEA
应用碳、氮稳定同位素研究北部湾带鱼(Trichiurus lepturus)食性及营养级

The data obtained from monthly sampling in the main fishing ports of the Beibu Gulf from October 2008 to September 2009 and the data obtained from the fishery-dependent trawl survey in February 2010, in the Beibu Gulf, South China Sea were using stable isotope analysis and stomach contents analysis to study the prey category, trophic position and ontogenetic feeding habit shifts of Trichiurus lepturus. The results showed that small pelagic fish, Decapterus maruadsi, Sardinella jussieu, Stolephorus heteroloba, benthic Bregmaceros rarisquamosus, and cephalopoda, Loligo chinensis, were relatively important in the diet of T. lepturus, with the weight percentages of 26.19%, 10.21%, 9.94%, 7.20% and 6.07%, respectively, whereas the other prey species were presence in lower percentages. The δ13C signatures went from relatively low (depleted) values for pelagic species to high values for benthic species. As expected, D. maruadsi displayed the lowest value (-17.830‰) while benthic Leiognathus lineolatus displayed the highest value (-14.925‰). The absence of a significant linear correlation between the preanal length and δ13C leads to the conclusion that T. lepturus are feeding from all the water layers during the majority part of their biological life cycle. Both the average trophic level and its annual mean value were calculated by prey weight percentages and δ15N were 3.7. The trophic level of T. lepturus varied not significantly with the preanal length, despite the fact that T. margarites and T. minor increased notably in the same sea area. Two samples of dehydration were undertaken prior to the analytical determination of stable isotope ratios were tested and the results showed that vacuum freeze-drying and drying with oven at constant temperature had significant variance in δ15N but not in δ13C. Undoubtedly, SIA will play an import role in evaluating the fish feeding habits and trophic positions. At the same time, the following three key points should be taken into account: (1) SIA must be combined with the stomach contents analysis; (2) an appropriate baseline organism has to be selected and its exact trophic position should also be deter- mined; (3) attention needs to be paid to the uncertainty of SIA application in the ontogenetic diet shifts and trophic position variations.