Prey density and distribution drive the three-dimensional foraging strategies of the largest filter feeder

Abstract

Despite their importance in determining the rate of both energy gain and expenditure, how the fine-scale kinematics of foraging are modified in response to changes in prey abundance and distribution remain poorly understood in many animal ecosystems. In the marine environment, bulk-filter feeders rely on dense aggregations of prey for energetically efficient foraging. Rorqual whales (Balaenopteridae) exhibit a unique form of filter feeding called lunge feeding, a process whereby discrete volumes of prey-laden water are intermittently engulfed and filtered. In many large rorqual species the size of engulfed water mass is commensurate with the whale's body size, yet is engulfed in just a few seconds. This filter-feeding mode thus requires precise coordination of the body and enlarged engulfment apparatus to maximize capture efficiency. Previous studies from whale-borne tags revealed that many rorqual species perform rolling behaviours when foraging. It has been hypothesized that such acrobatic manoeuvres may be required for efficient prey capture when prey manifest in small discrete patches, but to date there has been no comprehensive analysis of prey patch characteristics during lunge feeding events. We developed a null hypothesis that blue whale kinematics are independent of prey patch characteristics. To test this hypothesis, we investigated the foraging performance of blue whales, the largest filter-feeding predator and their functional response to variability in their sole prey source, krill using a generalized additive mixed model framework. We used a combination of animal-borne movement sensors and hydroacoustic prey mapping to simultaneously quantify the three-dimensional foraging kinematics of blue whales (Balaenoptera musculus) and the characteristics of targeted krill patches. Our analyses rejected our null hypothesis, showing that blue whales performed more acrobatic manoeuvres, including 180° and 360° rolling lunges, when foraging on low-density krill patches. In contrast, whales targeting high-density krill patches involved less manoeuvring during lunges and higher lunge feeding rates. These data demonstrate that blue whales exhibit a range of adaptive foraging strategies that maximize prey capture in different ecological contexts. Because first principles indicate that manoeuvres require more energy compared with straight trajectories, our data reveal a previously unrecognized level of complexity in predator-prey interactions that are not accounted for in optimal foraging and energetic efficiency models.