The influence of lipid-extraction on the ¹³C of mesopelagic and demersal fish in the South China Sea: modification and application of lipid normalization models

Mesopelagic fish, the most important daily vertically migrating community in the oceans, are characterized by high lipid content which may obscure the interpretation of stable isotopes analysis. Demersal fish, which are important consumers in the food web dominated by mesopelagic fish, also have a high lipid content. Here we collected 127 fish samples from the South China Sea and evaluated the effect of lipid contents on delta C-13 of mesopelagic and demersal fish. In lipid-extracted mesopelagic fish, the C/N content ratio (< 5.5) shows a clear correlation with delta delta C-13 (the offset of bulk and lipid-extracted delta C-13 values), especially in non-migratory and semi-migratory species; these values were less correlation in demersal fish. Based on our results, we suggest that mesopelagic and demersal fish in different regions of the South China Sea should be studied separately using appropriate correction models and less fit for the traditional model. Moreover, the C/N content ratio should be used cautiously for establishing the lipid normalization model, especially for the fish in migratory mesopelagic fish and demersal fish. Our results also reveal that mesopelagic fish across nearby regions could be analyzed together. The new models described here can be applied in future studies of mesopelagic and demersal fish in the South China Sea.