COS 3-4 - Assessing potential shifts in habitat use and foraging ecology of sperm whales after the Deepwater Horizon oil spill

Abstract: The 2010 Deepwater Horizon explosion discharged millions of barrels of oil and chemical dispersants in the Gulf of Mexico (GoM). Yet, the long-term consequences on sperm whales (Physeter macrocephalus) that live in the vicinity of the spill area remain uncertain. Here, we combine bulk-skin isotopic data obtained from free-ranging whales before the oil spill with isotope data from whales sampled after the spill to assess potential dietary and habitat use shifts. The δ¹³C values were marginally significant between periods (pANOVA, P = 0.05) and between sexes (pANOVA, P = 0.09), suggesting that, at fine scales, males and females changed their foraging habitats after the oil spill. In contrast, the ẟ¹⁵N values did not vary between sexes (pANOVA, P > 0.1) but varied significantly between periods (pANOVA, P < 0.01) suggesting that both males and females shifted towards higher trophic-level prey after the spill. Bayesian stable isotope mixing models estimating prey consumption based on values reported in the literature showed that Histioteuthidae had a consistent mean contribution of ~ 40% to the diet of females across both periods. Conversely, males had a more diverse contribution of prey from different families, including Vampyroteuthidae, Histioteuthidae, Octopodidae and Chiropodidae. Although this diet variation between sexes is expected given their differences in movements and habitat use, it is noteworthy that the mean contribution of Vampyroteuthidae to the diet of both males (from 30% to 10%) and females (from 24% to 11%) drastically decreased after the oil spill. The next step in our research is to combine these bulk-isotope data with compound-specific stable isotope analysis, as well as satellite tracking and diving data to help understanding how sperm whales respond to stressful events, thereby furthering our understanding of behavioral/physiological changes to environmental disturbances in the GoM. Doing so can help sharpen current conservation actions and inform the mitigation and recovery efforts for current and future acute and chronic stressors, related directly to sperm whales and indirectly to the components of their food web, thereby safeguarding their ecological role for years to come.