Meeting Abstract

7.3  Jan. 4  Efficiency of protein incorporation and diet quality determine trophic isotope spacing in fish CARLETON, Scott A*; AAMICK, Lori; University of Wyoming scarlet@uwyo.edu

Ecologists use the enrichment in ¹⁵N of an animal�s tissues relative to its diet to determine trophic position. They assume that isotopic spacing between trophic levels is constant and approximately equal to 3.4�. We will present a model that questions the constancy of this value with data supporting the predictions of this model. Briefly, the model predicts that ¹⁵N isotopic spacing should depend on the efficiency of protein incorporation and on diet quality. We tested this model with Nile tilapia (Oreochromis niloticus). Tilalpia were placed on three diets differing in protein content. Tilapia within these three treatments were fed 4%, 8%, or 16% of their body mass. We weighed Tilapia weekly to adjust feed rations for a total of 58 days until fish in all treatments had tripled in mass. Fish were individually dried to a constant mass, homogenized, defatted, and analyzed for ¹³C and ¹⁵N. We found that 1) ¹⁵N declined in fish�s tissues with increased efficiency of protein incorporation 2) fish on lower protein diets had higher ¹⁵N isotopic spacing than fish on higher protein diets, 3) fish on higher protein diets had larger ¹³C isotopic spacing than fish on lower protein diets, and 4) that ¹⁵N spacing is not dependent on fractional growth rate. Our results suggest that the ¹⁵N trophic spacing between tissue and diet is not constant, nor equal to 3.4�. Studies that rely on stable isotopes to diagnose trophic position must take into account the effect of both the efficiency of N incorporation and diet quality on the isotopic enrichment between trophic levels.