Comparison of Serum Retinol, Tocopherol and Lipid Levels in Free-Ranging Steller Sea Lion Pups from the Eastern and Western U.S. Stocks
The United States population of Steller sea lions (Eumetopias jubatus) is divided into two stocks. The eastern stock (east of 144°W) is listed as threatened but data show that it has actually increased two to three percent over the last three decades. The western stock, however, is listed as endangered because it has declined by more than 80 percent since the mid 1970's. Nutritional stress has been identified as a likely significant factor in the decline of the Steller sea lion in this part of its range (Marine Mammal Commission Annual Reports). Investigations into this hypothesis have focused on food availability (Merrick et al. 1997), caloric composition (Rosen and Trites 2000a) and various energetic concerns arising from both these issues (Rosen and Trites 1999, 2000b). The lower fat content and higher digestive costs of a pollock diet resulted in weight loss in captive animals, lending credence to the argument that dietary shifts necessitated by depleted stocks of energy-rich prey such as herring may be having population-wide effects on Steller sea lions. Though food quality is recognized as an important issue in the consideration of nutritional stress (Rosen and Trites, 2000a,c), little attention has been directed to the specific nutrients that might bear on health and productivity. This research effort focuses on vitamin A and E, which are both known to play a role in reproductive success. In view of the recognized differences in fat content between the historically preferred and present, more commonly available, diets of Steller sea lions, these fat soluble vitamins may be of particular importance. Our working hypothesis is that vitamin A and E levels in the declining western stock are lower than levels found in the stable eastern stock.
Blood samples were collected by researchers with the Alaska Department of Fish and Game (ADF&G) and the National Marine Mammal Lab (NMML) from Steller sea lion pups on rookeries in the Central and Eastern Aleutian Islands, Southeast Alaska and Gulf of Alaska. All pups were less than 2 months old. Serum samples (n=110) were analyzed for retinol, alpha-tocopherol, cholesterol and triglycerides. High performance liquid chromatography was used for vitamin analysis and commercially available test kits from Sigma Chemical were used to perform lipid analysis. Vitamin E in the blood is carried by lipoproteins and is affected by the intake of vitamin E and by blood lipid levels. For these reasons vitamin E is usually reported as E/lipid ratio. Statistix software was employed in all statistical analyses. Within the eastern stock, samples were collected from 2 different islands. Analysis of variance showed no significant differences in any of the parameters between the locations. Samples from the western stock were examined by area (central and eastern Aleutians, Gulf of Alaska) and also by island. Again there were no significant differences in any of the measured parameters. When data were analyzed by stock (eastern vs. western), there were a significant differences in tocopherol and triglyceride levels, with the western stock showing higher concentrations. Serum retinol (mean ± std. dev.; 0.46 ± 0.11 μg/ml) and alpha-tocopherol (19.82 ± 4.03 μg/ml) levels were within presumed 'normal' ranges compared to both captive and free-ranging levels for other pinniped species. There were also no correlations between vitamin E and either cholesterol or triglyceride levels suggesting that there is a true difference between vitamin E levels in the two populations that can not be explained by lipid levels. Correlations between vitamins and weight or length data were also absent.
The data presented here are from only a small portion of a much larger project. Data from free-ranging juveniles, adults as well as captive animals and prey species remain to be examined. The results seen here do not support our initial hypothesis. Thus we conclude that for this age group neither vitamin A or E status is a factor contributing to the declining population.
This work was supported by a grant from the National Marine Fisheries Service's Steller Sea Lion Research Initiative Program (Award # NA16FX1418). The authors wish to thank Tom Loughlin at National Marine Mammal Lab and Lorrie Rea and Tom Gelatt at Alaska Department of Fish and Game for their help in collecting samples from the Steller sea lion pups. For help with the vitamin and lipid analysis a big thank you to University of Connecticut Graduate students Kristen Gaffney and C. Diana Vasile.
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