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Sea Lions of the World 117 Alaska Sea Grant College Program • AK-SG-06-01, 2006

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Sea Lions of the World 117 Alaska Sea Grant College Program • AK-SG-06-01, 2006 Sea Lions of the World 117 Alaska Sea Grant College Program • AK-SG-06-01, 2006 Geographical Variation in Steller Sea Lion Prey Quality in Alaska Lawrence Schaufler NOAA, NMFS, Alaska Fisheries Science Center, Juneau, Alaska Elizabeth Logerwell NOAA, NMFS, Alaska Fisheries Science Center, Seattle, Washington Johanna Vollenweider NOAA, NMFS, Alaska Fisheries Science Center, Juneau, Alaska Abstract Nutritional stress is one of the leading hypotheses explaining the de- cline in Steller sea lion populations of the western stock. Central to this hypothesis is the possibility that western stock sea lions encounter prey of significantly lower quality than those from the eastern stock. We collected and analyzed over 1,200 whole fish representing species identified as sea lion prey items from the Aleutian Islands and south- eastern Alaska, including species that reside in both regions. We present proximate composition and calculated mean energy densities based on the lipid and protein contents for the sampled fish. Initial comparisons of the proximate compositions and energy densities between the Aleu- tian Islands and southeastern Alaska fish on a species basis revealed significant differences in prey energetic content in the two regions for the sampled prey. Overall, the mean energy density for 22 forage species from southeastern Alaska (1.62 ± 0.02 kcal per g on a wet weight basis) was greater than that of 15 species from the Aleutians (1.44 ± 0.03 kcal per g), but these variations could be attributed to size differences among the fish sampled from the two regions as well as species composition and collection season differences. For example, Pacific cod sampled from the Aleutians were significantly larger p( < 0.001) than those from south- eastern Alaska and had a higher energy density (p < 0.001). However, controlling for size revealed no difference in energy density between the two populations of cod (p > 0.5). Similarly accounting for size, no differ- ence was found in the energy density of walleye pollock or arrowtooth 118 Schaufler et al.—Prey Variation in Alaska flounder from the two locations. In contrast, squid and sandfish from southeastern Alaska had higher energy densities (p < 0.01) while Aleu- tian rockfish had higher energy densities than those from southeastern Alaska (p < 0.001), though these may represent seasonal and species composition differences. These data reveal the importance of consider- ing size, season, and species when making energy density comparisons of the available prey between geographical regions. Introduction The nearly 80% population decline of the western stock of Steller sea lions over the last 30 years has been attributed to a number of factors, including the “junk food hypothesis.” This theory postulates that a shift in diet from higher energy forage fish such as Pacific herring, to a diet consisting mainly of lower energy fish such as walleye pollock led to a nutritional deficiency (Alverson 1992, Trites and Donnelly 2003). The lack of a significant decline in the eastern Steller sea lion stock could indicate a difference in the quality of available food between the Aleutian (west- ern) and southeastern Alaska (eastern) regions during the Steller sea lion population decline. Because pinnipeds rely heavily on a piscivorous prey base, prey quality becomes paramount during lactation, molting, and other periods of increased energetic need. Relevant prey quality issues include the amount of fat and protein, vitamin and essential fatty acid contents, and caloric value or energy density. In this study we focused on the fat and protein contents and estimated energy density aspects of prey quality. The prey items available to and consumed by the western and east- ern Steller sea lion stocks vary significantly both in species composition and average prey size (Merrick et al. 1997, Zeppelin et al. 2004). Atka mackerel (Pleurogrammus monopterygius) and walleye pollock (Theragra chalcogramma) dominate the diets of Aleutian Steller sea lions, while for- age fish such as Pacific herringClupea ( pallasii ), eulachon (Thaleichthys pacificus), and capelin (Mallotus villosus) as well as walleye pollock form the majority of the southeastern Alaska Steller sea lion diet (Winship and Trites 2003). Other species are common to both regions, such as Pacific cod (Gadus macrocephalus), arrowtooth flounder Atheresthes( stomias), and certain squid (Berryteuthis) and skates (Bathyraja), but compose different percentages of the western and eastern stock Steller sea lion diets. Differences in prey sizes have also been reported between the two regions, likely from adaptations to environmental conditions such as water temperature, available food sources, and the particular variety of predators in each region. (Shuter and Post 1990, Tollit et al. 2004, Zeppelin et al. 2004). This study examines prey quality (i.e., nutritional value of prey) available to Steller sea lion in the Aleutian Islands and southeastern Sea Lions of the World 119 Lynn Canal Frederick Sound Attu Buldir Sitka Sound Amchitka Akun Adak Western Region Eastern Region (Aleutians) (Southeast AK) Figure 1. Prey collection sites in the Aleutian Islands and southeastern Alaska. Alaska. Proximate analysis was used to determine protein, lipid, and moisture content, along with calculated energy densities based on these values for prey collected in the two regions. We present comparisons of proximate composition and energy density values between species com- mon to the regions and energy density values for prey found primarily in only one of the two regions. We report energy density on a wet weight basis since we are considering the nutritional value of a whole fish as it is consumed by a predator such as a marine mammal. Methods Sample collection and storage Opportunistic sample collections were performed on various National Oceanic and Atmospheric Administration (NOAA) cruises and charter vessels in the Aleutian Islands and southeastern Alaska. Specimens were collected in the proximity of (Aleutians) Adak, Akun, Amchitka, Attu, and Buldir Islands; (southeastern Alaska) Berners Bay, Frederick Sound, Lynn Canal, and Sitka Sound (Fig. 1). Aleutian trawls occurred primarily during summer months (April through July), while southeast Alaska trawls were performed year-round, mostly on a quarterly basis 120 Schaufler et al.—Prey Variation in Alaska (March, May, September, and December). Typical trawls were mid-water, approximately 20 minutes in duration, performed during daylight hours using a 164 Nordic rope trawl with 1.5 m2 alloy doors, 7 m × 17 m (height × width) with a 19 mm mesh codend liner. Trawling depths ranged from approximately 75 to 225 meters. Samples were frozen whole immediately after morphometric mea- surements and gender were recorded. Gender was determined by direct examination of the gonad, and all gut contents were vacuum-sealed along with the fish in individual bags. When practical due to fish size, specimens were quick-frozen in liquid nitrogen. When this protocol could not be implemented, fish were vacuum-sealed after gender determination and placed in single layers in a commercial-grade –20ºC freezer. Upon returning to the laboratory, samples were stored in a –20ºC freezer for short-term (0-3 months) or a –80ºC freezer for longer-term (4+ months) storage. Proximate analysis Entire frozen fish were cut into cross-sections with a Bizerba FK23 in- dustrial meat saw, then homogenized in a Fleetwood M12S meat grinder using a 4.5 mm die. Three to five gram subsamples of the homogenate were randomly chosen for analysis and further liquified using an Oster 4134 blender with food processor attachment. Lipid content was deter- mined gravimetrically after a modified Folch extraction employing 0.1% BHT as an antioxidant (Christie 2003, Vollenweider 2004) using a Dionex 200 accelerated solvent extractor (ASE). Protein content was determined with the Dumas method on a Leco FP-528 nitrogen analyzer and a 6.25 ni- trogen-to-protein conversion factor was used (AOAC 1995). Moisture and ash contents were measured gravimetrically using a Leco TGA-601 ther- mogravimetric analyzer, heating at 135ºC for 2-3 hours and then 600ºC for 3-4 hours for moisture and ash, respectively. Carbohydrate contents were estimated by subtraction. All proximate contents (lipid, protein, moisture, and ash) are reported as percentages of total wet weight. All analyses implemented quality control procedures, including a sample replicate, a method blank with no sample material, and a refer- ence standard with each group of 15-20 fish. For lipid analysis, an in- house herring composite reference sample was used to ensure sample group comparability. National Institute of Standards and Technology (NIST) Standard Reference Materials (SRM) 1946 and 2974 were analyzed for protein, moisture, and ash content to verify analytical accuracy. Protein analyses were performed on dried material, in duplicate, with samples reanalyzed if their deviation was more than 1.5 standard devia- tions from the mean. Sea Lions of the World 121 Data analysis For mean comparisons, Levene’s test was first performed to confirm homogeneity of variance, then a two-sample, two-tailed Student’s t-test was applied to determine significance if the variances were equal. In cases where the variances for the two data sets were not equal, Welch’s approximate t was determined, with the calculated degrees of freedom (DF) indicated. A general linear model was employed to identify covari- ates, and significance levels ofα = 0.05
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