Individual diet specialization in a pelagic predator

Background/Question/Methods

Individual diet specialization is well described across a range of taxa and has broad ecological, evolutionary, and conservation implications; diet preferences can affect resource partitioning, initiate sympatric speciation, and impact resilience in the face of harvest or fishing pressure.  We analyzed individual data from 42 striped marlin (Kajikia audax) from the Eastern Pacific near Baja California to assess diet consistency over time in this highly mobile pelagic predator. Studies of individual diet specialization have traditionally relied on stomach content analysis and/or stable isotope analysis.  In this study, we compared diet information from three complementary approaches reflecting diet over different time scales: stomach contents (immediate snapshot) and two chemical tracers in muscle tissue: fatty acid profiles (1- to 3-month turnover) and stable isotope ratios (3- to 12-month turnover). We used a new Bayesian fatty acid/stable isotope mixing model (fastinR) to estimate individual diet proportions based on fatty acids and stable isotopes of striped marlin and their prey, including fish, crustaceans, and cephalopods.  Comparing estimated diet proportions to stomach contents allowed us to assess diet consistency over time. 

Results/Conclusions

We did not find consistent diet preferences within individual fish over time but did find significant diet differences among fish caught in different seasons. These results suggest that striped marlin are opportunistic predators whose feeding patterns may be mediated more strongly by seasonal migration patterns and prey abundance than individual preferences.  This work also underscores the role of mobile pelagic animals as trophic links between ecosystems.