Trophic overlap between blue sharks (Prionace glauca) and shortfin makos (Isurus oxyrinchus): Trophic linkages between two shark species in the Eastern Pacific Ocean food web

Trophic interactions in food webs must be considered in order to assess the trophic role of different species in the ecological network structure as well as the relationship between resource use and interactions between different organisms. Such studies provide important information for ecosystem-based fisheries management. Thus, our goal was to analyze the trophic overlap between blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) off the west coast of Baja California Sur, Mexico, in order to infer their diet preferences and spatial distribution. We sampled the dorsal white muscle of 39 P. glauca and 35 I. oxyrinchus in December 2009 and March 2011. No significant differences between sexes or maturity stage were identified for P. glauca from either sampling location, indicating similar foraging preferences. For I. oxyrinchus, there was a significant difference in the δ13C values for males and females, suggesting different foraging preferences or movement patterns. The trophic positions and the SIBER analysis assigned to both tertiary predators in this study indicate that P. glauca is a specialist, while I. oxyrinchus is a generalist. Significant differences were also identified in the δ15N and δ13C values for P. glauca and I. oxyrinchus, indicating low overlap between these two shark species. These patterns suggest that these two sharks have different trophic preferences and migratory histories; thus, each species plays a unique trophic role off the west coast of Baja California Sur, Mexico.