Comparing trophic structure of a subtropical bay as estimated from mass-balance food web model and stable isotope analysis

Abstract

The trophic structure of a subtropical bay in Taiwan Strait was analyzed by using two methods: mass balance modeling (Ecopath) and stable isotopic analysis (SIA). Trophic levels (TLs) of main functional groups estimated from the two methods were compared. The Ecopath model was built based on the fishery resource survey in Xiamen Bay in 2009. Specifically, data on species composition, biomass, mortality rates, diet composition and fisheries catches were obtained from the survey in and around the bay. The model consisted of 26 functional groups, including plankton, benthos, fish, cephalopods, shrimps, crabs and marine mammals. TLs of the main functional groups were estimated to be between 2.89 (cephalopods) and 3.94 (congers), with an average of 3.11. Trophic transfer efficiencies from levels II to V were 12.8%, 19.2%, 19.7% and 12.1%, respectively. Catfish (Tachysurus sinensis and Netuma thalassina) and the fisheries have major trophic impacts on most functional groups in the Xiamen Bay ecosystem. Total system throughput was estimated to be 411tkm−2year−1. TLs derived from isotopic analysis were highly correlated with those estimated from Ecopath (Linear regression: R2=0.696, n=23, p<0.001). On an average, Ecopath underestimated TLs of the functional groups by about 12.2% compared to those estimated from SIA, with TLs from Ecopath being slightly higher at low TLs and lower at high TLs. This studies support value of using both stable isotopes and Ecopath methods to analyze this food web.