Stable isotopes analysis vs. TROPH: A comparison of methods for estimating trophic positions of fish in a subtropical estuarine system

An ecosystem-based management of fisheries requires a proper estimation of the trophic position (TP) of each component of the food web. Stable isotope analysis (SIA) and stomach contents analysis (SCA) are used to estimate TP of aquatic systems, but few studies have compared the results obtained from both methods. We used SIA (d¹⁵N) and SCA (TrophLab) to estimate the trophic position (TP) of 75 fish species from an estuarine system in the Gulf of California, and to compare TP estimates between methods. TP_SIA ranged from 2.6 to 5.6, and most of the analyzed species showed TP from 4.5 to 5. TP_SCA ranged from 2.5 to 4.5 and most of the species showed TP from 3.5 to 3.75. It was determined that TrophLab underestimates the TP, mainly in piscivorous fishes because the TP default for the consumed fish preys is always 3.5. However, it was observed that when the TROPH of a species was estimated from a known TP of its prey, other than the TrophLab defaults, the yielded results of the TP_SCA were similar to the results of TP_SIA. TP_SIA gives more accurate results, but the TP_SCA can give adequate results if the TP of the prey items is known.