Evidence for secondary predation in blue marlin ( Makaira nigricans ) differs between stomach content analysis and DNA metabarcoding

Résumé fourni par la source

Abstract Current methods for assessing predator diets struggle to distinguish between primary and secondary predation, obscuring the true breadth of targeted prey. Secondary predation occurs when a predator ingests another organism whose prey remain in the predator’s stomach after the organism itself has been digested. Although often acknowledged as a potential source of bias, secondary predation has rarely been quantified. Blue marlin (Makaira nigricans) swallow prey whole, making them ideal for assessing potential secondary predation and the magnitude of this bias. In this study, we document secondary predation in blue marlin by examining prey stomachs using a ‘Russian Doll’ approach to address the limited dietary information available for marlin prey species, although this step may not be necessary in systems where prey diets are already well characterized. We combined stomach content analysis (SCA) and DNA metabarcoding targeting mitochondrial COI and 16S rRNA markers to analyse the stomach contents of blue marlin and any intact prey they contained. This approach enabled the identification of taxa likely to represent secondary prey. Taxa were classified as secondarily predated when they; (1) occurred in both marlin and prey stomachs; and (2) were more frequently detected in marlin stomachs containing digested prey. Metabarcoding detected a greater diversity of soft-bodied taxa, whereas SCA more effectively identified hard-bodied taxa. SCA revealed seven prey families, which occurred in both marlin and prey stomachs (primarily small crustaceans and squid) and were significantly more common in stomachs containing digested prey, potentially indicating secondary predation, while metabarcoding did not reveal significant differences. These results highlight the potential of linking taxa shared between predator and prey with prey decomposition state to identify likely cases of secondary predation.

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