Population consequences of disturbance by offshore oil and gas activity for endangered sperm whales (Physeter macrocephalus)

Abstract

Sperm whale (Physeter macrocephalus) populations are still recovering from massive population declines associated with commercial whaling operations. The species continues to face a suite of contemporary threats, including pollution, ship strikes, fisheries interactions, habitat loss and degradation, oil spills, and anthropogenic noise. The sperm whale stock in the northern Gulf of Mexico was exposed to oil from the Deepwater Horizon (DWH) oil spill and is exposed to high levels of anthropogenic noises generated by geological and geophysical (G&G) surveys for hydrocarbon deposits. Population impacts from oil and gas activities were predicted from models that incorporated two stressors: (i) oil exposure from DWH and (ii) noise from G&G surveys. Oil exposure was projected to reduce survival and reproductive success, causing a mean stock decline of 26% by 2025. Additionally, exposure to underwater noise can adversely impact whale hearing, communication, foraging efficiency, and disturb essential behaviors. Exposures to G&G survey noise were determined by simulating individual movements through three-dimensional sound fields generated by different survey methods. Behavioral disturbance was evaluated as reduced foraging opportunities under four dose-response functions. Bioenergetic models tracked the depletion of reserves in blubber, muscle, and viscera. All simulations suggested significant reductions in relative fitness of reproductive females were a likely consequence of persistent disturbances to foraging behaviors. Under a 160 dB SPL unweighted dose-response function, up to 4.4 ± 0.3% of the stock may reach terminal starvation due to behavioral disturbance associated with future G&G surveys, leading to abortions, calf abandonment, and up to 25% greater stock declines beyond those predicted from DWH oil exposure. Uncertainty in our results emphasizes a need for further controlled exposure experiments to generate behavioral disturbance dose-response curves and detailed evaluation of individual resilience following disturbance events. Given our focus on a limited suite of threats and need for field verification of these modeled impacts, precautionary management application of our results is recommended for this endangered species.