COS 158-3 - Scat Signatures Indicate Proactive Responses of Cape Fur Seals to White Shark Predation Risk

Abstract: Predators often have strong, non-consumptive effects on prey behavior that impart energetic costs. Prey can proactively alter their behavior to reduce predation risk, which requires that this risk is predictable, or they can respond reactively. In South Africa, the risk of predation by white shark Carcharodon carcharias varies across Cape fur seal Arctocephalus pusillus colonies and between seasons, providing a unique opportunity for studying non-consumptive effects of predators on prey in a natural setting. Previous studies show that Cape fur seals proactively alter the timing and duration of their foraging trips during the high-risk season, but that they also exhibit a strong reactive stress response to uncontrollable daily risk of white shark attack. Here, we used stable isotope analysis and diet composition of Cape fur seal scat from five colonies to further examine non-consumptive risk effects in this system across sites and seasons. We hypothesized that: 1) predictable seasonal variation in predation risk would drive proactive changes in the foraging behavior and, as a result, diet of Cape fur seals, 2) these proactive changes in foraging behavior would reduce the need for a reactive response at the individual level.

Our analysis of scats collected from 2012-2015 revealed little spatial or temporal variation in the diet composition of Cape fur seals, which had a strong dietary preference for anchovy. However, stable isotope results showed depleted δ¹³C signatures under high-risk scenarios. Shark attack rates are highest near inshore sites, so this shift may indicate that seals use increased offshore foraging as a proactive approach to reducing predation risk. Shifts in δ¹³C signatures were not associated with a reduction in reactive stress responses measured using fecal glucocorticoid stress hormones (fGCM). Instead, Cape fur seals exhibiting the highest fecal stress concentrations, an indication of increased white shark attack rates, had more depleted δ¹³C signatures, suggesting increased offshore foraging. Foraging offshore to avoid predation risk is likely to incur energetic costs on seals in terms of time and distance traveled. Given a recently documented decline in white shark numbers at several South African sites and associated predation stress release, consequent alterations to seal foraging locations are likely to occur, which may have cascading ecological impacts.