The Impacts of Sex-Specific Diets of a Marine Predator on Ecosystem Models

Ecosystem modeling is an increasingly popular method to understand how organisms within ecosystems interact, relying on robust data incorporating important inter- and intraspecies interactions to predict ecosystem changes. However, no study has included sex-specific intrapopulation variation in an ecosystem model. In the well-studied Salish Sea, harbor seals (Phoca vitulina) are an important marine mammal that have significant sex-specific diet variability, which I hypothesized would have indirect effects on other functional groups in the region. Male harbor seals consume a higher diet proportion of salmon, while female harbor seals consume a higher proportion of herring and small demersal fish. I created an ecosystem model of the Salish Sea using the Ecopath framework and calculated predictions of the overall mixed trophic impact that male and female harbor seals each exert on other functional groups. To assess the importance of the sex-specific diets on the indirect impacts, I varied the sex ratio of the harbor seals to simulate the range of sex ratios present spatiotemporally in the Salish Sea. Changing sex ratios also allows me to assess how mixed trophic impacts respond to changing predation pressure from each sex. Male harbor seals were predicted to have a strong negative impact on raptors and a strong positive impact on piscivorous seabirds, neither of which are part of the harbor seal diets, while female harbor seals had a very low impact on these groups. There was a negligible difference in impact on herring despite having the largest difference in diet contribution between male and female harbor seals. Male harbor seals consistently exerted a stronger negative impact on Pacific salmon than females, even when females were predicted to consume a greater proportion of Pacific salmon production. The results suggest that indirect trophic cascades contribute to harbor seal sex-specific impacts on other groups, rather than predation alone. These sex-specific impacts may be lost in models that do not account for sex-specific diet variation within the harbor seal population in the Salish Sea.