Using stable isotope analysis to validate effective trophic levels from Ecopath models of areas closed and open to shrimp trawling in Core Sound, NC, USA

Abstract

We used stable isotope ratios of nitrogen (15N/14N) as a validation tool for determining the accuracy of the effective trophic levels computed in network analysis models for Core Sound, North Carolina, USA. Ecological network models (using Ecopath) were assembled for two trawling management areas, closed areas where shrimp trawling has been restricted for 30 years, and open trawling areas within the estuary. Effective trophic levels (ETLs) were computed from the two Ecopath models and compared with δ15N values measured from the same species at the same time in the two areas. Comparisons of ETLs between species compartments of the two models were very similar, and ETLs were positively related to δ15N values. Linear regression of ETL as a predictor of δ15N value was highly significant (R2=0.50, p>0.00001), suggesting that δ15N values increase by 2.52‰ per trophic level. This increase in δ15N value agrees well with a recent global meta-analysis of nitrogen fractionation estimates. There was no significant difference in δ15N values between trawling zones over all species examined, although there were significantly higher δ15N values in the areas open to trawling for certain species of fish (pinfish, spot) and invertebrate (blue crabs). Atlantic menhaden, which feed on detritus and zooplankton, had much higher δ15N values than the Ecopath ETL and δ15N regression model predicted (large studentized residuals), suggesting that Ecopath incorrectly computes the trophic level of animals that consume dead material (in our model, detritus and shrimp trawl bycatch species). Shrimp trawl bycatch species are mostly pinfish, spot and blue crabs, with ETLs of 2.5 or higher, but are inappropriately assumed to be at trophic level 1 when dead in Ecopath. Despite this discrepancy for some species of detritivores and scavengers, we conclude that this Ecopath network model of Core Sound is reliable and predicts with good accuracy the shifts and changes in trophic level and diet as measured by stable isotopes and observed in diet studies. The Core Sound network model will be a useful approach when predicting impacts in the ecosystem due to changes in trawling or other fisheries management decisions.