Quantifying costs and rewards in optimal foraging models of the marine environment. Using bulk feeding mysticete whales as an example

Abstract

Animals feed to maintain body condition and maximise fitness. Feeding is a multi-stage behaviour that encapsulates aspects of an animals' sensory and foraging ecology, and that can be divided into the following broad functional categories; prey detection, capture, and handling (assimilation of acquired energy stores). The evolutionary consequences of foraging have the potential to shape predator-prey dynamics, influence community and ecosystem structure; and to drive the evolution of physiological and morphological adaptations that minimise energetic costs and maximise energy uptake. Excess energy can be converted to fat (lipids) and stored to enhance reproductive potential or to protect against temporary (voluntary) starvation associated often with long distance migration or changes in prey distribution. Understanding how an animal balances these conflicting forces, and makes key foraging decisions is an important aspect to understanding their ecology. Simple questions such as how, when, where, why and what an animal chooses to feed on are difficult to answer without the development and application of new technologies. The development of new tools and analytical approaches is particularly important in the marine environment where these key life history decisions occur offshore and or at depth. Bulk feeding rorqual whales are amongst the largest species to have lived on Earth, and are major consumers of schooling krill and forage fish. Because of their size and ability to consume significant quantities of fish and krill they have an important role in structuring ecological communities. Lunge feeding is an energetically costly bulk feeding behaviour that is practised by all members of this super-family. To understand how energy fluxes pass through the consumer to be expressed as body condition it is first necessary to have reliable proxies of the energetic costs and gains associated with feeding. In this thesis methods for estimating these energy fluxes were explored using data from summer feeding (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in Canada, and winter feeding humpback whales in northern Norway. Calibrated speed measurements were used as a proxy for the energetic costs associated with bulk feeding (lunge) and transport in baleen whales (chapter 2). Speed measurements were used to develop a lunge detector (chapter 3). Lunge speed and other kinematic variables associated with bulk feeding were used to identify species-specific signatures in feeding behaviour as a potential method for identifying prey type, and thus their caloric value as a proxy for energy uptake (chapter 4)). The feasibility of using new sensors, (i) sonar to measure prey density and dynamics from the perspective of the feeding whale (chapter 5), and (ii) an ultrasound to measure blubber depth in wild marine mammals as a proxy of body condition (chapter 6) were tested.