Experimental evaluation of stable isotope fractionation in fish muscle and otoliths

Abstract

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 13C and 15N fractionation between diet and muscle. We also examined fractionation of 13C between otolith and both diet and muscle. Both 13C and 15N 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 13C and 15N by approximately 1‰. Lipid extraction also increased variation in treatment means for 15N, but not 13C. Otoliths were enriched in 13C compared to both diet and fish muscle. Bulk otolith 13C values were strongly correlated with muscle tissue, and reflected the same total change in 13C observed among diet treatments. It was tempting to conclude that otoliths were accurately recording 13C values of the diet. However, more information is required on the effects of diet, metabolic rate, and 13C of ambient dissolved inorganic carbon on otolith 13C before these structures can be used to reconstruct diet histories of individual fish.