Steller sea lions (Eumetopias jubatus) have greater blood volumes, higher diving metabolic rates and a longer aerobic dive limit when nutritionally stressed

Abstract

Marine mammal foraging behaviour inherently depends on diving ability. Declining populations of Steller sea lions may be facing nutritional stress that could affect their diving ability through changes in body composition or metabolism. Our objective was to determine whether nutritional stress (restricted food intake resulting in a 10% decrease in body mass) altered the calculated aerobic dive limit (cADL) of four captive sea lions diving in the open ocean, and how this related to changes in observed dive behaviour. We measured diving metabolic rate (DMR), blood O 2 stores, body composition and dive behaviour prior to and while under nutritional restriction. We found that nutritionally stressed sea lions increased the duration of their single long dives, and the proportion of time they spent at the surface during a cycle of four dives. Nutritionally stressed sea lions lost both lipid and lean mass, resulting in potentially lower muscle O 2 stores. However, total body O 2 stores increased due to rises in blood O 2 stores associated with having higher blood volumes. Nutritionally stressed sea lions also had higher mass-specific metabolic rates. The greater rise in O 2 stores relative to the increase in mass-specific DMR resulted in the sea lions having a longer cADL when nutritionally stressed. We conclude that there was no negative effect of nutritional stress on the diving ability of sea lions. However, nutritional stress did lower foraging efficiency and require more foraging time to meet energy requirements due to increases in diving metabolic rates and surface recovery times.

Body condition impacts blood and muscle oxygen storage capacity of free-living beluga whales (Delphinapterus leucas)

“…Second, beluga whales with reduced body condition presumably also have elevated diving metabolic rates that could potentially further compromise dive duration. For example, mass-specific diving metabolic rates of nutritionally stressed Steller sea lions (Eumetopias jubatus) were significantly (>10%) higher than pre-trial controls, which corresponded to an overall decrease in foraging efficiency during a dive bout (Gerlinsky et al, 2014). Finally, a recent study on captive beluga whales suggests that use of interspecific allometric equations obtained on nearshore marine mammals may overestimate values for total lung capacity in deep-diving beluga whales (Fahlman et al, 2019a).…”

“…More precise physiological measurements on beluga whales may show similar variation in end-expired O 2 content that is associated with dive duration and allow more precise estimates of maximal usable lung O 2 stores. A potential caveat is that nutritional stress was also accompanied by increased blood volume in Steller sea lions (Gerlinsky et al, 2014); if this also occurs in beluga, it would act to counter the above aspects.…”

Scaling matters: incorporating body composition into Weddell seal seasonal oxygen store comparisons reveals maintenance of aerobic capacities

“…In addition to changes in O 2 stores, changes in DMR have the potential to greatly alter the ADL both on an individual dive basis (Castellini et al 1992b;Williams et al 2004;Fahlman et al 2008), as well as across the year (Gerlinsky et al 2014). For example, cetaceans tend to have lower TBO 2 stores than phocid seals, but their large size is associated with low DMRs, extending their aerobic capacities (Noren and Williams 2000;Croll et al 2001).…”

“…In multiple sea lion species, variation in blood and muscle O 2 stores emerged in response to changes in food availability and/or nutritional plane (Villegas-Amtmann Villegas-Amtmann et al 2012;Gerlinsky et al 2014). In contrast to otariids, the Phocidae family appears to follow a different pattern.…”

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“…Air-breathing mammals, such as fur seals, that forage on aquatic prey have challenging constraints when finding energy. They dive repeatedly, may travel long distances to find their food, an energetically intensive strategy (Staniland et al 2007), and must return to the surface to breathe, only diving for as long as their oxygen stores allow (Gerlinsky et al 2014). As relatively small marine mammals, thermoregulatory costs at sea are high, as water conducts heat 25 times faster than air (Hind & Gurney 1997).…”