A Comparison of Methods for Estimating Fish Trophic Positions in Estuarine Systems Under Small Scale Fishing Pressure: Choosing the Right Indicator for an EBFM

Ecosystem-based fishery management programs require a reliable estimation of the trophic position of aquatic resources. Both stable isotope analysis (SIA) and stomach content analysis (SCA) have been used to estimate the trophic position of aquatic systems, but few studies have compared results from both methods. We used SIA (d15N and d13C) and SCA (estimated with the TROPH routine) to determine the trophic position of 66 fish species in a subtropical estuarine system of the Gulf of California, to determine if these two methods produced similar results. SIA values ranged from 2.6 to 5.6, with 56% of the species having a SIA value above 4.5, and 14% of the species having a SIA value below 4. The SCA values ranged from 2.6 to 4.8, with 76% of the species having a SCA value of 3 to 4. In general, SCA results underestimated trophic position compared to the SIA, which yielded better results.  The TROPH routine has oversimplified assumptions, although if the SIA is not available the recommendation is to use SCA but with a knowledge of the trophic position of the most important prey items.