CONICET | Buscador de Institutos y Recursos Humanos

The use of combinedconventional and stable isotope analyses to study the diet of seabirds hasbecome very frequent. Unfortunately species and tissue specific TrophicDiscrimination Factors (TDF), necessary to run stable isotope mixing models inthe most performing way, are lacking for a wide number of seabird species. Weassessed the process of inspecting and selecting the most adequate TDFs byrunning mixing region simulations with three different TDFs scenarios. This wascarried out in a combined dietary study of a widespread but poorly studied seabirdfrom the Americas: the Neotropic cormorant (Phalacrocoraxbrasilianus), at a breedingcolony from coastal Patagonia. The mixing models were run with the best fittingcarbon and nitrogen TDFs (whole blood), which were those obtained with Rpackage SIDER, a Bayesian inference-based model predicting TDFs of consumersconsidering their ecology and phylogenetic relatedness. We found thatcormorants fed on rather mobile fish of a fairly variable ecological spectrum,both benthic and pelagic, and showed low prey diversity. Our results coincidewith the overall generalist and opportunistic piscivorous habit found atdifferent coastal areas along its broad distribution range. Despite somedifferences found in the proportion estimates of the main prey between the twomethods, their combination through the incorporation of prior information intothe mixing models provides a comprehensive trophic profile. Finally, the use ofSIDER, alongside the inspection and comparison of different potential TDFvalues, offered a simple and effective framework to calculate and select themost adequate specific TDFs to be employed in stable isotope mixing models.

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