Resilience of the endangered sperm whale Physeter macrocephalus to foraging disturbance in the Gulf of Mexico, USA: a bioenergetic approach

The endangered sperm whale Physeter macrocephalus spends the majority of its time foraging, relying upon echolocation to locate and consume several 100 kg of prey per day. In the northern Gulf of Mexico, sperm whales are exposed to a variety of anthropogenic stressors, including ship strikes, fisheries interactions, habitat loss and degradation due to oil and gas development, and chemical and noise pollution. In particular, they are exposed to high levels of anthropogenic noises related to geological and geophysical surveys for hydrocarbon deposits. The sounds produced by these surveys could reduce sensory volume, increase search effort required to locate resources, and interfere with auditory signal processing critical to foraging success. We developed a stochastic life-stage structured bioenergetic model to evaluate the consequences of reduced foraging efficiency on carbohydrate, lipid, and protein reserves in the blubber, muscle, and viscera. The model indicates that individual resilience to foraging disruptions is primarily a function of size (i.e. reserve capacity) and daily energetic demands. Mothers are the most vulnerable life stage due to the high energy demands associated with pregnancy and lactation. Continuous disruption has a greater impact than intermittent disruption; even minor foraging disruptions may lead to terminal starvation if the whales have no opportunity to replenish reduced reserves. Infrequent, minor disruptions in foraging are unlikely to be fatal, but may result in reduced body reserves that may be associated with reduced reproductive success. Our model provides a bioenergetic framework for evaluating the level, frequency, and consequences of foraging disruptions associated with anthropogenic stressors.