NUTRITIONAL STRESS

The two leading hypotheses explaining the rapid decline of Steller sea lions in the Gulf of Alaska and Aleutian Islands are that the population was nutritionally stressed due to a reduction in overall prey abundance, or because of a change in the relative abundance of different types and quality of prey available. However, it has been difficult to test or evaluate these hypotheses.

In a recently published paper, Andrew Trites and Carolyn Donnelly reviewed the evidence for nutritional stress in Steller sea lions. One of the components of the hypotheses was the role that changes in quality rather than quantity of prey may have played in the population decline. They also briefly reviewed how mammals in general respond to nutritional deprivations.

Many studies have shown that pinnipeds and other mammals suffering from nutritional stress typically exhibit reduced body size, reduced productivity, high mortality of pups and juveniles, altered blood chemistry and specific behavioural modifications. Morphometric measurements of Steller sea lions through the 1970s and 1980s in Alaska indicate reduced body size. Reduced numbers of pups born and an apparent increase in juvenile mortality rates appear to also be nutritionally based. Blood chemistry analyses have further shown that Steller sea lions in the Gulf of Alaska and Aleutian Islands exhibited signs of an acute phase reaction, or immune reaction, in response to unidentified physical and/or environmental stress. Behavioural studies during the 1990s have not noted any changes that are indicative of an overall shortage in the quantity of prey available to lactating female sea lions.

Estimated numbers of Steller sea lions (all ages) in Alaska from 1956 to 2000 (from Trites & Larkin, 1996; A.W. Trites, unpublished data).

All told, the data collected in Alaska are consistent with the hypothesis that Steller sea lions in the declining regions were nutritionally compromised due to the relative quality of prey available (chronic nutritional stress), rather than due to the overall quantity of fish per se (acute nutritional stress). This is further supported by captive studies that indicate the overall quality of prey available to Steller sea lions in the declining population could compromise their health and hinder their recovery.

Full details and conclusions of the review are contained in:

The decline of Steller sea lions in Alaska: A review of the nutritional stress hypothesis. Trites, A.W. and L.P. Donnelly. 2003. Mammal Review 33:3-28

Also, see Research > Nutritional Stress for more.

28 April 2003