Cross-Validation of Trophic Level Estimates from a Mass-Balance Model of Prince William Sound Using 15N/14N Data

Trophic mass-balance models of ecosystems constructed using the Ecopath approach and software include the diet composition of functional groups as model inputs, and trophic level estimates for these same groups as a model outputs. The well-documented 0.34% enrichment of lSN/14N that occurs at each feeding step in food webs can be used to determine trophic level as well. This contribution is the first to ever examine the relation between trophic levels estimated by these two independent methods. This was achieved by using a published Ecopath model of Prince William Sound (PWS) as reference, I.e., estimating lSN/14N ratios for each of the model's functional groups. Re-expression of theses ratios as absolute estimate of trophic levels (TL) was done following calibration using the herbivorous copepod Neocalanus cristatus, for which TL = 2. The correlation between both sets of TL values (n = 7) was extremely high (r= 0.986), with the points evenly distributed about the 1: 1 line. Also, the magnitude of the standard errors of the TL estimates based on lSN/14N data was similar to those of the Ecopath estimates. Applying lSN/14N data from PWS to an Ecopath model of the Alaska Gyre System resulted in a reduced correlation (r = 0.755, for n = 16), suggesting that TL estimates may be transferred between ecosystems, though at the cost of reduced precision. These encouraging results warrant further exploration. Elmer E. Rasmuson Library Cataloging-in-Publication Data International Symposium on Fishery Stock Assessment Models for the 21 st Century (1997 : Anchorage, Alaska). Fishery stock assessment models: proceedings of the International Symposium on Fishery Stock Assessment Models for the 21st Century, October 8-11, 1997, Anchorage, Alaska. (Lowell Wakefield Fisheries Symposium) (University of Alaska Sea Grant College Program; AK-SG-98-01) 1. Fish stock assessment-Alaska-Congresses. 2. Fishery management-AlaskaCongresses. I. Alaska Sea Grant College Program. II. Title. 111. Series: Lowell Wakefield fisheries symposium series. 15th. IV. Series: Alaska Sea Grant College Program report: 98·01.