Individual Diet Specialization in a Pelagic Predator

Individual diet specialization has both ecological and management implications; diet preference can change resource partitioning, and resilience in the face of fishing pressure on prey species. We analyzed data from 42 striped marlin (STM, Kajikia audax) from Baja California to assess diet over time in this pelagic predator. Studies of diet specialization have traditionally relied on stomach content and/or stable isotope analysis. In this study, we compared information from three complementary approaches reflecting diet over different time scales: stomach contents (immediate snapshot) and two chemical tracers in muscle tissue: fatty acid (FA) profiles (1­ to 3­month turnover) and stable isotope (SI) ratios (3­ to 12­month turnover). We used a new Bayesian mixing model to estimate individual diet proportions based on FA and SI of STM and their prey, and compared those estimated proportions to stomach contents to assess diet over time.  We did not find consistent diet preferences within individual fish over time but did find significant diet differences among fish in different seasons. These results suggest STM are opportunistic predators whose feeding patterns may be mediated more by seasonal migration and prey abundance than individual preferences, and underscore the role of mobile pelagic animals as trophic links between ecosystems.