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Dynamic foraging in Risso’s dolphins revealed in 4-dimensions

Kelly J. Benoit-Bird*, Brandon L. Southall, Mark A. Moline

*Corresponding author:

ABSTRACT: Quantifying details of an animal’s foraging strategy can provide insight into its energetic requirements, environmental constraints, and other pressures it must balance in behavioral decision-making. Data from ship-based echosounders provided 4,888 observations of Risso’s dolphins (Grampus griseus) in the context of heterogeneous prey fields while an autonomous underwater vehicle carrying similar instruments was used to describe 519 discrete prey patches containing dolphins. We integrated these data with prey net tows, dolphin surface observations, and results from a parallel tagging study that quantified detailed aspects of behavior of three individuals. These data provide complementary perspectives on the foraging strategies of Risso’s dolphins at a range of time scales from individual prey patch selection to diel cycles. Rather than being solely nocturnal teuthivores as previously suggested, we found Risso’s dolphins dove to depths exceeding 500m both day and night. Risso’s dolphins switched many times daily from being more generalist predators near the surface, to specializing on larger squid at depth, commonly within the course of a single 5-10 minute dive. Simple energy calculations suggest that shallow prey comprise relatively small contributions to individual energy gains, yet these prey played a strong role in determining the spatio-temporal habitat use of Risso’s dolphins. This underscores the importance of examining strategic foraging behavior in light of requirements to access both prey resources and oxygen. The novel integration of multiple, complementary sensing methods employed simultaneously on Risso’s dolphins provide remarkable insights on the behavioral ecology of individuals and the population.