Abstract
Air-breathing marine animals face a complex set of physical challenges associated with diving that affect the decisions of how to optimize feeding. Baleen whales (Mysticeti) have evolved bulk-filter feeding mechanisms to efficiently feed on dense prey patches. Baleen whales are central place foragers where oxygen at the surface represents the central place and depth acts as the distance to prey. Although hypothesized that baleen whales will target the densest prey patches anywhere in the water column, how depth and density interact to influence foraging behaviour is poorly understood. We used multi-sensor archival tags and active acoustics to quantify Antarctic humpback whale foraging behaviour relative to prey. Our analyses reveal multi-stage foraging decisions driven by both krill depth and density. During daylight hours when whales did not feed, krill were found in deep high-density patches. As krill migrated vertically into larger and less dense patches near the surface, whales began to forage. During foraging bouts, we found that feeding rates (number of feeding lunges per hour) were greatest when prey was shallowest, and feeding rates decreased with increasing dive depth. This strategy is consistent with previous models of how air-breathing diving animals optimize foraging efficiency. Thus, humpback whales forage mainly when prey is more broadly distributed and shallower, presumably to minimize diving and searching costs and to increase feeding rates overall and thus foraging efficiency. Using direct measurements of feeding behaviour from animal-borne tags and prey availability from echosounders, our study demonstrates a multi-stage foraging process in a central place forager that we suggest acts to optimize overall efficiency by maximizing net energy gain over time. These data reveal a previously unrecognized level of complexity in predator–prey interactions and underscores the need to simultaneously measure prey distribution in marine central place forager studies.
Highlights
Animals have evolved multiple strategies to optimize searching for prey and maximize net energy gain with respect to costs associated with acquiring energy [1]
We deployed multi-sensor digital archival suction-cup tags [31] on humpback whales in Wilhelmina Bay, in the near shore waters of the Western Antarctic Peninsula (WAP) in May 2009 and 2010
All air-breathing diving animals face the conflicting demands of minimizing oxygen use during diving and the energetically costly manoeuvres associated with capturing prey at depth [47]
Summary
Animals have evolved multiple strategies to optimize searching for prey and maximize net energy gain with respect to costs associated with acquiring energy [1]. There are voluminous examples in the literature about optimal foraging in terrestrial predators and the factors that influence this strategy. Two pertinent classic examples for our study are (i) the concept of central place foraging [3,4,5], where foraging rates and energy gain are affected by the need to come and go to a centrally located place (e.g. nest) and (ii) foraging and conflicting demands, where the foraging behaviour of an animal may be influenced by factors not directly associated with energy gain (e.g. predator vigilance or competing needs for other resources) as shown by Martindale [6]
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