Abstract
Quantification of costs associated with foraging contributes to understanding the energetic impact that changes in prey availability have on the energy balance of an animal and the fitness of populations. However, estimating the costs of foraging is difficult for breath-hold divers, such as Steller sea lions, that feed underwater. We developed models parameterized with data from free-diving captive Steller sea lions to estimate the costs incurred by wild animals while foraging. We measured diving metabolic rate of trained sea lions performing four types of dives to 10 and 40 m in the open ocean and estimated the separate costs of different dive components: surface time; bottom time; and transiting to and from depth. We found that the sea lions' diving metabolic rates were higher while transiting (20.5 ± 13.0 ml O2 min(-1) kg(-1)) than while swimming at depth (13.5 ± 4.1 ml O2 min(-1) kg(-1)), and both were higher than metabolism at the surface (9.2 ± 1.6 ml O2 min(-1) kg(-1)). These values were incorporated into an energetic model that accurately predicted oxygen consumption for dives only (within 9.5%) and dive cycles (within 7.7%), although it consistently overestimated costs by 5.9% for dives and 21.8% for dive cycles. Differences in the costs of individual components of dives also explained differences in the efficiency of different dive strategies. Single dives were energetically less costly than bout dives; however, sea lions were more efficient at replenishing oxygen stores after bout dives and could therefore spend a greater portion of their time foraging than when undertaking single dives. The metabolic rates we measured for the different behavioural components of diving can be applied to time-depth recordings from wild Steller sea lions to estimate the energy expended while foraging. In turn, this can be used to understand how changes in prey availability affect energy balance and the health of individuals in declining populations.
Highlights
The ability to determine accurately the costs of underwater activity for breath-hold divers is essential for understanding the energetics of diving and associated foraging strategies
We found that the sea lions’ diving metabolic rates were higher while transiting (20.5 ± 13.0 ml O2 min−1 kg−1) than while swimming at depth (13.5 ± 4.1 ml O2 min−1 kg−1), and both were higher than metabolism at the surface (9.2 ± 1.6 ml O2 min−1 kg−1)
Diving metabolic rate (DMR) was not affected by depth in any of the dive types (LRT = 0.060, P = 0.81), so depths were combined for each dive type
Summary
The ability to determine accurately the costs of underwater activity for breath-hold divers is essential for understanding the energetics of diving and associated foraging strategies. Steller sea lion (Eumetopias jubatus) populations in Alaska have declined by ∼80% since the 1970s, possibly because of changes in prey availability, abundance or nutritional quality (Loughlin et al, 1998; Trites and Donnelly, 2003). Any of these factors could, in theory, affect foraging behaviour, the. Conservation Physiology Volume 3 2015 associated foraging costs, and the animal’s overall energy balance. Several studies have examined how reductions in the quantity or quality of food affect the physiology of Steller sea lions (Rosen et al, 2000; Rosen and Trites, 2000, 2004; Atkinson et al, 2008; Jeanniard du Dot et al, 2009; Gerlinsky et al, 2014); few studies have looked at how foraging costs may be affected by reductions in the quantity and quality of prey
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