Pelagic marine ecosystems in the sub-Arctic have been changing during the past 30 years, indicated by declining or shifting abundances of fish, seabirds and marine mammals. Some of these changes have been alarming, such as the well-publicized 50-85% population decline of Steller sea lions. Harbor seal populations have also declined by 85% in the western Gulf of Alaska, and trend count surveys in Prince William Sound have indicated a decline of over 40% between 1984 and 1988. This population was further impacted by the Exxon Valdez Oil spill, and seal populations in oil-impacted areas have not yet recovered. In contrast, southeast Alaska populations of both harbor seals and Steller sea lions have remained relatively stable during this period.
Gulf of Alaska harbor seals have been the subject of an aggressive interdisciplinary research program headed by the Alaska Department of Fish and Game (ADFG) in an attempt to identify causes of this decline and of the non-recovery of Prince William Sound populations. The premise of my research is that if changes in prey availability occurred throughout the past 30 years, then this may be reflected by changes in harbor seal body condition. Body condition can be indexed as changes in growth rate, or in the amount of fat stores (blubber) carried by the seal. Changes in prey abundance or distribution directly impact the energetics of seals by altering foraging costs. However, to perform interannual comparisons of body condition, other effects such as sex, age, season and location of capture must be quantified to avoid introducing bias. Similarly nutri ional, stress or disease related effects may be found by examining blood chemistries if normal ranges are known. Our laboratory has been involved with capturing, measuring and sampling harbor seals from the Kodiak archipelago, Price William Sound, and southeast Alaska since 1992, and some data exist back to 1972 for comparison. This project has received funding from the Center for Global Change and Arctic System Research, Exxon Valdez Oil Spill Trustee Council, Rasmuson Fisheries Research Center, ADFG and the National Marine Fisheries Service.
Currently we have defined reference ranges for 30 plasma and hematologic values from up to 140 seals, and have found effects of sex, age and handling technique on several of these parameters. Once these have been fully quantified, we will perform categorical comparisons to examine potential interannual or interregional differences. We have also generated preliminary models relating blood values to body fat. The most satisfactory model utilizes blood urea nitrogen, hematocrit, albumin and capture location as significant predictors of body fat. We have found that in general there is little effect of age or size on the blubber content of seals, but that these have significant effects on commonly used morphometric indices of blubber content. Large sex-related seasonal differences in the usage of core and blubber compartments of the seals were found (Figure 1). Males exhibited little seasonal variation in total body mass, yet relative proportions of core and blubber changed by 10-15%. Conversely, females exhibited large seasonal fluctuations in both total and core mass, corresponding in magnitude to the birth weight of seal pups. In a preliminary comparison of the scaling of core and blubber components with body mass, seals from the 1970s were approximately 28% blubber, while seals from the 1990s seem to be approximately 11% blubber. We are continuing to refine this model, which if true has considerable energetic and thermoregulatory ramifications for the seals.
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