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Auke Bay Laboratory (cont.)

Do Pacific Sleeper Sharks Prey on Steller Sea Lions?

As part of NMFS research to determine the decline of Steller sea lion abundance off Alaska, ABL scientists conducted studies to determine whether sleeper sharks prey on Steller sea lions, and if so, estimate the predation rate. Evidence of shark predation on Steller sea lions would identify a source of Steller sea lion mortality.

Analysis of 99 sleeper shark stomach contents collected during an initial cruise in August 2001 indicated no evidence that sleeper sharks actively prey on sea lions. A second and final cruise in the study was completed in May 2002 using the chartered fishing vessel Norska. Of 99 sleeper shark stomachs collected in the May cruise, 6 contained cetacean tissue and 4 contained unidentified mammal tissue. (The equal sample size from the August and May cruises was coincidental). Species identification of unknown mammal tissue and cetacean tissue by micro- satellite DNA analysis is pending.

During the May 2002 cruise, a combined total of 15 longline sets was completed in the central Gulf of Alaska near Steller sea lion rookeries at Marmot Island, Sugarloaf Island, Outer Pye Island, and Seal Rocks. It was important that the field operations occur when sea lions pups were most vulnerable to shark predation. May field operations were scheduled to occur when sea lion pups are weaned (April - May). In addition to the 99 sleeper sharks collected for stomach samples, 24 were released with archival satellite tags. Sleeper shark lengths ranged from 140 to 330 cm total length (TL), with most between 190 and 250 cm TL. Sleeper shark weight ranged from 15 to 210 kg, with most between 70 and 140 kg. The ratio of male to female sharks was 0.65:1.

Archival satellite tags are being used to collect data on the sharks' geographic and vertical movements. By comparing sleeper shark movements to those of Steller sea lions, we hope to determine if sleeper shark and sea lion habitats overlap in time and space in the central Gulf of Alaska.

Diet Composition

The 99 Pacific sleeper shark stomachs were analyzed from sharks sampled during the May 2002 cruise; 83 contained food items, and 16 were empty. Cephalopods (octopus and squid) were the dominant food category on a numerical basis (80.7%), while Osteichthyes (bony fishes) were the dominant food category by weight (41%), frequency of occurrence (76%). Mammal tissue was the second most important diet item by percent weight (34%). Giant Pacific octopus (Octopus dofleini), was the most important identified prey species, representing 25% by weight and 36% by frequency of occurrence. Other non-teleost prey included squid (Teuthoidea), and crabs (Decapoda), teleost prey included Pacific cod (Gadus macrocephalus), Pacific salmon (Onchorhynchus spp.), Pacific halibut (Hippoglossus stenolepis), arrowtooth flounder (Atheresthes stomias), rockfish (Sebastes spp.), and sablefish (Anoplopoma fimbria).

By Lee Hulbert.

Southeast Alaska Juvenile Sablefish Studies

Scientists from the ABL tagged and released 447 juvenile sablefish at St. John Baptist Bay, Alaska, using the NOAA ship John N. Cobb during 1-6 June 2002. Most fish were age 1+, though some were age 2+. ABL scientists have tagged and released juvenile sablefish in St. John Baptist Bay annually since 1985. A pilot study also was implemented during this year's cruise to learn more about the behavior and movements of juvenile sablefish in near shore habitats. Acoustic tags were surgically implanted in ten age-1+ sablefish on 3 June. The tags transmit data on depth, water temperature and location. All ten fish remained in the bay as of 6 June. On a return trip to on 1 July to relocate the fish with acoustic tags, nine of the ten fish were still in the bay. Preliminary results indicate the fish use the entire water column during their daily movements and inhabit only a small portion of this bay.

By Tom Rutecki.

Southeast Alaska Sea Lion Prey Study

Two cruises of the Southeast Alaska Steller Sea Lion Prey Study were conducted in the second quarter of 2002. The purpose of the study is to test the hypothesis that juvenile Steller sea lion prey diversity and seasonality are related to Steller sea lion population trends. It is a comparison study to a similar study being conducted near Kodiak Island by the University of Alaska. Prey abundance was measured using echo- integration and midwater trawl methods. Sea lion scat was collected to infer diet. Fish also were collected for proximate and free fatty acid analysis.

The fourth quarterly cruise of this study was conducted 27 March - 5 April 2002 using the chartered fishing vessel Solstice. Scientists participating in the cruise were Johanna Vollenweider and Dave Csepp of ABL and Susan Heaslip of the University of British Columbia. One notable observation was two groups of approximately 20 sea lions each cooperatively feeding in Lynn Canal associated with three concurrent layers of fish: diffuse, near-surface schools of capelin; dense, midwater schools of eulachon; and dense near-bottom schools of herring. The fifth quarterly cruise of this study was conducted 23 May - 4 June 2002 using the chartered fishing vessel Viking Storm. Scientists participating in the cruise were Michael Sigler, Johanna Vollenweider, and Dave Csepp of ABL and Andrew Trites of the University of British Columbia.

By Johanna Vollenweider and Michael Sigler.

Southeast Alaska Estuarine Habitat Survey

Surveys of Essential Fish Habitat (EFH) in Alaska estuaries continued in June 2002. Mitch Lorenz, Dean Courtney, Phil Rigby, and Wyatt Fournier of ABL boarded the NOAA ship John N. Cobb on 15 June to complete a 5-day fish and habitat survey along the northeast shore of Chichagof Island, Southeast Alaska. The purpose of the survey was to help develop a database for geographic information system (GIS) mapping of EFH in Alaska estuaries.

Eighteen sites in four estuaries were sampled for fish and fish habitat characteristics. Two of the estuaries are on a relatively exposed coastline along a deepwater inlet (Chatham Strait), and two are adjacent to a more protected bay type environment (Port Frederick). Salmonids were the predominant Fishery Management Plan (FMP) species at the exposed sites, and species diversity there was relatively low. Fish habitat in the more exposed estuaries was characterized by small deltaic formations that supported narrow bands of emergent marsh, eelgrass, and kelp. Flatfish were the most abundant FMP species in the more protected estuaries, and species diversity was higher than at the more exposed sites. Fish habitat at those sites was characterized by relatively complex deltaic formations with extensive emergent wetlands, eelgrass stands, and mudflats.

These data will be incorporated into a database that can be associated with estuarine maps interpreted from satellite imagery. The resulting GIS will be useful in assessing EFH distribution and relative productivity in Alaska's estuaries.

By Mitch Lorenz


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