Compound Specific Isotope Analysis in Food Web Studies: The Need for Accurate Estimates of Trophic Enrichment Factors

Abstract

Ecosystem-based fisheries management strategies require knowledge of trophic relationships. Trophic position (TP) estimates from compound specific nitrogen isotopic analysis of amino acids (AA-CSIA), although promising, have not been thoroughly tested. TP estimates from AA-CSIA require knowing ß, the difference in δ15N values between glutamic acid and phenylalanine in primary producers and ∆, the 15N enrichment at each trophic step or trophic enrichment factor. Values of ß (3.4%o) and ∆ (7.6%o) have been suggested for aquatic environments, however recent observations indicate that ∆ may be variable particularly among elasmobranchs. We determined ∆ values for three species of sharks, sand tiger (Carcharias taurus), lemon (Negaprion brevirostris), and leopard sharks (Triakis semifasciata), and one teleost species, opakapaka (Pristipomoides filamentosus) grown on semi-controlled and well characterized diets for durations ranging from three (Triakis semifasciata) to over five years (Pristipomoides filamentosus). We found the ∆ values for both elasmobranchs and opakapaka were similar and were significantly lower than the ∆ value of 7.6 %o previously reported. These results do not support the hypothesis that urea retention lowers δ15N enrichment values in elasmobranchs. Rather, similar ∆ values may be due to carnivorous feeding strategies in elasmobranchs and teleosts.