The advantages of diving deep: Fin whales quadruple their energy intake when targeting deep krill patches

How predators maximize energetic gains while minimizing the costs associated with exploiting heterogeneous prey remains a difficult ecological principle to test in natural systems. Deep-diving, air-breathing predators face conflicting demands of oxygen conservation to extend dive time and oxygen usage from the exercise required to find and capture prey. How predators balance these opposing factors is additionally complicated by prey patches that are heterogeneous spatially, temporally and in quality. Tags deployed on foraging fin whales revealed that deeper dives consisted of higher feeding rates (lunges/hr), as generally predicted by optimal foraging theory. By simultaneously measuring prey density and distribution in the local environment, we show that whales increased their dive depths in order to forage on the densest prey patches. Despite the increased travel time needed to find deeper prey during a breath-hold dive, the increase in feeding rates of fin whales and modelled prey consumption quadrupled compared to shallow foraging. Because the cost of transport is low at this extreme in body size, we posit that feeding on the deep prey patches significantly increases the energetic efficiency of foraging. Given the increasing recognition that anthropogenic disturbance can curtail deep foraging dives in many cetacean species, endangered fin whales may be susceptible to significant energetic losses that may impact individual fitness and population health in some areas. A free Plain Language Summary can be found within the Supporting Information of this article.