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Habitat Program

Variation in Quality of Steller Sea Lion Prey from the Aleutian Islands and Southeastern Alaska

Nutritional stress is one of the leading hypotheses explaining the decline in the western stock of Steller sea lions (Aleutian Islands region). Central to this hypothesis is the possibility that western stock sea lions encounter prey of significantly lower quality than sea lions from the eastern stock (southeastern Alaska). To continue our investigations of this hypothesis, we collected and analyzed more than 1,200 whole fish representing species identified as Steller sea lion prey items from the Aleutian Islands and southeastern Alaska, including species that reside in both regions. We performed proximate analyses on the fish and calculated mean energy densities based on the lipid and protein contents.

Comparison of the energy densities between the Aleutian Island and southeastern Alaska fish on a species basis revealed significant differences in energetic prey content in the species from the two regions. Overall, the mean energy density for 22 forage species from southeastern Alaska (1.62 +/-0.02 kcal/g on a wet weight basis) was greater than that of 15 species from the Aleutians (1.44 +/-0.03 kcal/g), but these variations could be attributed to size differences among the fish sampled from the two regions as well as species composition differences. For example, Pacific cod sampled from the Aleutian Islands were significantly larger (P < 0.001) than those from southeastern Alaska and had a higher energy density (P = 0.002). However, controlling for size revealed no difference between the two populations of cod (P > 0.5). Similarly accounting for size, no difference was found in the energy density of pollock or arrowtooth flounder from the two locations. In contrast, squid and sandfish from southeastern Alaska had higher energy densities (P < 0.001) than those from the Aleutian Islands, while Aleutian Islands rockfish had larger energy densities than those from southeastern Alaska (P = 0.003). These data reveal the importance of considering size when making energy density comparisons of the prey available to the western and eastern sea lion stocks.

By Lawrence Schaufler and Elizabeth Logerwell.

Seasonal Prey Availability Near Two Steller Sea Lion Haulouts in Southeastern Alaska

To better understand the declining abundance of Steller sea lions in western Alaska, we examined the seasonal availability of prey in southeastern Alaska where sea lion abundance is increasing. From 2001 to 2004, we identified prey in nearshore waters (<100 m deep) near two Steller sea lion haulouts, Benjamin Island and The Brothers Islands, in summer and winter.

Catch and number of prey species available to sea lions were greater in summer than in winter at both haulouts and greater at The Brothers Islands than at Benjamin Island. Total catch at both haulouts and for all sampling periods was 201,331 fish by seining and 559 fish by jigging; 58 species were identified in summer and 44 species in winter. Seine catches for both locations were dominated by young-of-the-year walleye pollock, Pacific herring, and Pacific sand lance in summer and salmon fry, armorhead sculpin, and rock sole in winter. Jig catches were dominated by armorhead sculpin, Pacific cod, and rockfish in summer and winter.

Most fish captured by seining were juveniles (median fork length < 80 mm) and likely too small to be targeted by sea lions, whereas most fish captured by jigging (median fork length > 249 mm) were large enough to be consumed by sea lions. Thirty-four species captured have been identified in sea lion scat collected at either haulout. Availability of prey close to Steller sea lion haulouts contributes to overall diet diversity and provides a source of food that may reduce foraging effort in summer. Less available prey in winter, however, may force sea lions to travel farther from haulouts to forage.

By John Thedinga

Organochlorine Levels in Steller Sea Lion Prey from the Aleutian Islands and Southeastern Alaska

The ubiquitous distribution of organochlorines in high-latitude food webs suggests organochlorines may have a role in preventing the recovery of the western stock of Steller sea lions. However, there are few data describing the bio-availability of these contaminants in the Aleutian Islands region where the western Steller sea lion stock occurs.

We measured concentrations of dioxin-like and other selected polychlorinated biphenyls (PCBs), DDTs (dichlorodiphenyltrichloroethane), and hexachlorobenzene (HCB) in whole bodies of walleye pollock to test the hypothesis that contaminant loads in western stock food webs would be higher than those of the eastern Steller sea lion stock found in southeastern Alaska. More than 110 fish were collected from six areas in the Bering Sea and southeastern Alaska: the western Bering Sea, the western Aleutians, the eastern Aleutians, the Pribolof Islands, the northern Bering Sea, and southeastern Alaska. Whole fish were examined for the presence of 15 coplanar PCB congeners, four DDTs, and HCB; we also estimated total concentration of PCBs. Pollock from southeastern Alaska were significantly more contaminated than Bering Sea pollock (P < 0.01), with length-corrected concentrations of 167 ng/g lipid, 173 ng/g lipid, and 44.0 ng/g lipid for PCBs, DDTs, and HCB, respectively. The southeastern Alaska levels are higher than those reported for fish at similar trophic levels in the the Beaufort Sea, suggesting a higher degree of contamination in southeastern Alaska food webs. This high level of contamination in southeastern Alaska is consistent with the characteristically large amounts of precipitation in the region. Eastern stock sea lion populations have been increasing while apparently consuming prey with relatively high organochlorine loads. Consequently, the presence of organochlorines in high-latitude food webs does not appear to be inhibiting the recovery of the western Steller sea lion stock.

By Ron Heintz

Marine Salmon Interactions

NMFS Scientists Attend Third International Otolith Symposium

The Third International Symposium on Fish Otolith Research and Application was held in Townsville, Queensland, Australia in July 2004. The symposium, hosted by the CRC Reef Research Center and James Cook University, provided a forum for wide-ranging discussions of new ideas, research, and clarification of methodology used in the field of otolith research. More than 300 scientists from 32 different countries attended the symposium, including 40 researchers from the United States representing universities, federal and state governments, and private organizations. The National Marine Fisheries Service (NMFS) was represented by 19 scientists. (See also REFM Divisionís Age and Growth Program report.)

The symposium included oral and poster presentations in key themes areas. Dean Courtney of the ABL presented a poster of his work on otolith micro-increment periodicity of juvenile sablefish. Don Mortensen from the ABL presented posters on using otolith elemental analysis to separate Bristol Bay sockeye salmon stocks, and on the evaluation of strontium and calcein for marking hatchery and wild juvenile chum salmon.

The symposium also included several otolith-related workshops and visits to research facilities in the northeast Queensland area, the Coral Sea, and the Great Barrier Reef. The otolith symposium scientific committee recommended that the Fourth International Symposium on Fish Otolith Research and Application be held on the west coast of the United States in 2009, probably in Monterey,California.

By Donald Mortensen


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