Stable isotope analysis versus TROPH: a comparison of methods for estimating fish trophic positions in a subtropical estuarine system

Ecosystem-based fishery management programs require a reliable estimate of the trophic positions of aquatic resources. Both stable isotope analysis (SIA) and stomach content analysis (SCA) have been used to estimate the trophic positions (TP) of aquatic systems, but few studies have compared results from both methods. To determine whether the two methods produced similar results, we used SIA (delta N-15 and delta C-13), SCA (estimated with the TROPH routine) and data from FishBase to estimate the TP of 66 fish species in a subtropical estuarine system of the Gulf of California. 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 3-4. Overall, SCA results underestimated TP (including FishBase data), while SIA yielded better results, particularly if both delta N-15 and delta C-13 are used. The TROPH routine has oversimplified assumptions such as the same TP for all organisms in the same taxon, but if SIA is unavailable, SCA could be used, accompanied by knowledge of the TP of the most important prey items.