Experimental evaluation of stable isotope fractionation in fish muscle and otoliths

Stable isotope analyses (SIA) of carbon and nitrogen are used routinely in food-web studies to determine diet and trophic position. We tested several common assumptions of SIA by rearing juvenile mummichog Fundulus heteroclitus on 5 isotopically distinct diets under controlled laboratory conditions. We determined the effect of diet type and lipid extraction on delta C-13 and delta N-15 fractionation between diet and muscle. We also examined fractionation of delta C-13 between otolith and both diet and muscle. Both C-13 and N-15 were enriched from diet to fish muscle, but the degree of fractionation differed among diets. Carbon isotope fractionation from diet to fish muscle exceeded assumed values of < 1% and ranged from 1.2 to 3.9%, while nitrogen fractionation ranged from 2.7 to 7.8 %. Extracting lipids from fish muscle increased both delta C-13 and delta N-15 by approximately 1%. Lipid extraction also increased variation in treatment means for delta N-15, but not delta C-13. Otoliths were enriched in C-13 compared to both diet and fish muscle. Bulk otolith delta C-13 values were strongly correlated with muscle tissue, and reflected the same total change in delta C-13 observed among diet treatments. It was tempting to conclude that otoliths were accurately recording delta C-13 values of the diet. However, more information is required on the effects of diet, metabolic rate, and delta 13C of ambient dissolved inorganic carbon on otolith delta C-13 before these structures can be used to reconstruct diet histories of individual fish.