Respiratory behaviors in sympatric rorqual whales: the influence of prey depth and implications for temporal access to prey

Abstract

Energetically costly lunge feeding at depth causes the respiratory patterns and feeding performance of rorqual whales (Family Balaenopteridae) to hinge in part upon prey patch depth. This contingency has the potential to precipitate differences in prey preference and habitat suitability for sympatric species and may be a factor in competitive interactions, but comparative respiration studies are a necessary first step in assessing this hypothesis. We concurrently sampled dive behavior in sympatric, euphausivorous humpback (Megaptera novaeangliae) and fin whales (Balaenoptera physalus), as well as prey depth distribution within a British Columbia fjord system over the course of 2 summers. Ventilation and dive patterns differed significantly between species, including differential respiratory response to increasing prey depth, despite their foraging upon a common prey resource. Thanks to longer dives and shorter surface recoveries, fin whales spent a greater proportion of their time on dives. This behavior, coupled with faster swim speeds during descent and ascent reported in previous studies, afford fin whales greater periods of time at the depth of their prey. These interspecific discrepancies in dive behavior determine the whales' relative temporal access to prey. Simulations based on our observations indicate that the fin whale's relative advantage in this fjord system increases with increasing prey depth when all other prey parameters are held constant. Simulation results emphasize the importance of swim speed in rorqual foraging strategy. Small differences in prey access per dive can have important implications over the course of a foraging season, which may precipitate differences in habitat suitability. Our findings, when coupled with the body of knowledge from tagging studies, highlight this link and point to its potential role in the habitat preferences of foraging whales.