Steller sea lions show diet-dependent changes in body composition during nutritional stress and recover more easily from mass loss in winter than in summer Tiphaine Jeanniard du Dot ⁎, David A.S. Rosen, Andrew W. Trites Marine Mammal Research Unit, Department of Zoology and Fisheries Center, University of British Columbia, 2202 Main Mall Vancouver, British Columbia, Canada V6T 1Z4 abstract article info Article history: Received 6 March 2008 Accepted 7 August 2008 Keywords: Body composition Compensatory growth Diet Nutritional stress Sea lion Controlled feeding experiments were undertaken with captive Steller sea lions (Eumetopias jubatus) to assess seasonal (winter vs. summer) physiological responses of individual animals to reduced quantities and qualities of food that are hypothesised to occur in the wild. Eight animals were randomly divided into two experimental groups fed isocaloric diets: Group H ate Pacific herring (Clupea pallasi) throughout the experiment while Group P was switched to walleye pollock (Theragra chalcogramma) during a 28-day food restriction (after a 28-day baseline) and back to herring during a 28-day controlled re-feeding. Diet type did not impact the rates of body mass lost when food was restricted, but did influence the type of internal energy reserve (protein vs. lipids) the sea lions predominantly used. In both summer and winter, Group H lost significantly more lipids and less lean mass than Group P that was fed pollock during the restriction phase. The response of Group H was consistent with the predicted pattern of nutritional stress physiology (i.e. protein sparing and utilization of lipid reserves). Group P lost a surprisingly high proportion of body protein while consuming restricted levels of pollock, which could lead to muscle impairment and vital organ failure on a long-term basis. When given increased amounts of herring during the controlled re-feeding phase, the capacity of both groups to compensate for the previous mass loss was found to depend on season and was independent of previous diet. All of the sea lions increased their rates of mass gain and returned to their pre- experimental weight during winter, but not during summer. Some intrinsic energetic plasticity related to seasonal adaptation to the environment may render winter an easier period than summer to recover from nutritional stress. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Animals routinely adjust their physiology and behaviour when faced with seasonally predictable or unpredictable shifts in nutrition and ecosystem conditions (Boyd, 2002; McNab, 2002). However, the range of adjustments an animal can make is often limited in scope and may be insufficient to compensate for significant declines in energy intake. Determining how animals respond physiologically to changes in quantity and quality of diet, and whether these responses differ between seasons when the energy budgets of animals are intrinsically different is essential to understanding how environmental changes affect the health and dynamics of populations. Nutritional stress has been a leading hypothesis to explain the decline of several populations of marine mammals and sea birds in the North Pacific Ocean (Trites and Donnelly, 2003; Jodice et al., 2006). A number of studies have endeavoured to link observed changes in diets and body condition of birds and mammals with population changes (Merrick et al., 1997; Rea et al., 2000; Rosen and Trites, 2000; Donnelly et al., 2003; Romano et al., 2006). Some studies have found that birds and mammals cannot always fulfill their daily energy requirements when eating only low energy density prey even when fed to satiation. For example, black-legged kittiwakes (Rissa tridactyla) and tufted puffins (Fratercula cirrhata) fed to satiation on walleye pollock (Theragra chalcogramma) had reduced growth in mass and wing length compared to chicks fed high energy density fish such as Pacific herring (Clupea pallasi) and sand lance (Ammodytes americanus) (Romano et al., 2006). Similarly, harp seals (Phoca groenlandica) and Steller sea lions (Eumetopias jubatus) also fed to satiation on a pollock diet experienced mass loss and depletion of body fat stores (Kirsch et al., 2000; Rosen and Trites, 2000). Changes in quantity and quality of prey have been shown to affect changes in amounts of lipids and proteins animals catabolize to meet energy demands, which in turn can affect survival and reproduction in marine mammals (Cherel et al., 1992; Boltnev et al., 1998; Pitcher et al., 1998; Donnelly et al., 2003; Rosen and Trites, 2005). How animals respond physiologically to periods of nutritional restriction appears to vary with the time of the year in which the deficit occurs (Kitts et al., 2004; Sigler et al., 2004). In the case of Steller sea lions, total energy requirements are thought to be higher in winter than summer, due primarily to higher foraging and thermoregulation costs associated Journal of Experimental Marine Biology and Ecology 367 (2008) 1–10 ⁎ Corresponding author. Tel.: +604 822 9150; fax: +604 822 8180. E-mail addresses: dudot@zoology.ubc.ca, tiphainejdd@yahoo.fr (T. Jeanniard du Dot). 0022-0981/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2008.08.005 Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe