A cost-benefit analysis of prey selection in deep-diving pilot whales: choosing from a broad menu with kid options

Speaker: Natacha Aguilar de Soto, CREEM-SOI (CREEM – SOI)

Abstract

The survival of a social group depends on the foraging decisions of each individual and of a balance in sharing foraging resources among group members. Data from multisensor DTAGs attached to 80 short-finned pilot whales show that they perform different foraging tactics: their “cheetash of the deep sea” tactic: sprinting at depth to target few rewarding prey; and less energetic night-time hunting of more prey per dive in the shallow and deep scattering layers. The fact that pilot whales perform foraging dives during day and night to different depths and at different swimming speeds, results in high differences in the transport cost of the dives. Our hypothesys is that whales may target different prey types in these dives. This is because many species of deep water fauna perform daily vertical migrations, meaning that not all prey types are available 24 hrs at all depths. Also, we expect that the relative caloric value of  prey targeted in different dives is indicated by how much energy whales are prepared to invest in a dive to perform so many prey capture attempts.

Here, we explore how a Random Forest classification method supports  dividing the dives in classes that seem to correspond to broad prey types with variable cost-benefit trade-offs, and perform a cost-benefit analysis to estimate the relative caloric value of these prey, and the reliance of the whales on the different prey types. For this we use: i) independent estimates of relative hunting energetic costs per dive from indirect respirometry and from movement indicators, comparing the performance of different indicators such as Overall and Vectorial Dynamic Body Acceleration (ODBA and VeDBA), acceleration rate (jerk) and speed-dependent hydrodynamic drag to predict oxygen uptake after dives; ii) estimations of abundance and depth-distribution of prey derived from the echolocation activity of the whales (buzzes indicating prey capture attempts).

Results show that there are cheap and expensive foraging tactics and the post-dive oxygen uptake is well predicted by the depth and the speed reached during each dive. Diving capabilities are related to body mass, however, large adult males and smaller females/sub-adult males reach similar maximum depths and speeds. In contrast, juveniles perform “cheap” dives, foraging shallower/slower than adults, probably because of the higher mass-specific metabolic rate and lower oxygen stores of young. This apparent ontogenetic partial niche segregation and the broad diet-breadth of short-finned pilot whales may be essential to sustain the large and cohesive social groups of this top-predator in the deep ocean. Knowing about the foraging requirements of deep-water top-predators is essential to predict the potential impacts of expanding mesopelagic fisheries. Overfishing in coastal areas has affected local populations of birds and other taxa; it is timely to manage fisheries to prevent these effects on top-predators inhabiting the deep ocean. Also, learning about the energetic balance of the species informs transfer functions to predict effects of human disturbance, e.g. from anthropogenic noise.

This seminar reports findings of papers in preparation with the following coauthors: Mark Johnson (SMRU), Jacobo Marrero (Univ. La Laguna, Canary Islands), Peter Madsen (Univ. Aarhus, Denmark), Lucía Martín (SMRU), Jeanne Shearer (Univ. St. Andrews).