Do Pacific Sleeper Sharks Prey on Steller Sea Lions?
Steller sea lion abundance from Prince William Sound (PWS) westward
has declined since at least 1965. The cause of the sea lion
decline is unknown. The purpose of our study is to determine whether
sleeper sharks, Somniosus pacificus, prey on Steller sea lions,
and if they do, estimate the predation rate.
Twenty-one long line sets were completed near four Steller sea lion
rookeries in northeast GOA of Alaska in August 2001. Ninety-nine sleeper
sharks were collected for sampling. Sleeper shark lengths ranged from
106- to 226-cm precaudal length (PCL), with most between 130- and 210-cm
PCL. Sleeper shark weight ranged from 15 to 216 kg, with most between 30
and 90 kg. Of the 99 shark stomachs analyzed; 13 were empty, and
86 contained food. Walleye pollock was the most dominant identified
prey, representing 16% by weight and 44% by frequency of occurrence.
Cetacean tissue was the most important diet item (30% by weight), and
octopus was the most frequently ocurring prey species (50%).
Unidentified Teleost fish was the most frequently occurring food item
(53%). Other Teleost prey included Pacific cod, Pacific salmon, Pacific
halibut, arrowtooth flounder, rockfish, and sablefish. Other non-Teleost
prey included squid and one spiny dogfish. No Steller sea lion parts
were found. A second cruise is scheduled for May 2002.
By Leland Hulbert.
The Role of Salmon Sharks in the Trophic Ecology of the Gulf of Alaska
The final Alaska salmon shark, Lamna ditropis, assessment
research cruise was completed in PWS in July 2001. Thirty-two salmon
sharks were caught with two handlines, weighted, and baited with a
single hook. Seventeen sharks were caught between Deer Cove and Bear
Cape at Hinchinbrook entrance; 14 sharks were caught at Port Gravina.
All were females ranging from 197 to 246 cm in total length and 92 to
162 kg. Of the 32 salmon sharks caught, 16 were sacrificed to collect
samples for analyses of hormones, free fatty acids, genetics, stable
isotopes, vertebrae (age determination), stomach contents (diet), and
A total of 18 salmon sharks in PWS have been released with satellite
transmitters attached for the study of large-scale movements of salmon
sharks in the northeast GOA. Movement data generally suggest a southeast
migration trend by late fall. One shark (A) remained, however, in the
northern GOA until early December before moving 800 km south from near
the north end of Kodiak Island on 6 December to the last transmitted
location on 14 December, an average rate of 100 km per day for 8 days.
Another shark was near Pigot Bay in northern PWS on 1 November. Data
from satellite transmitters are being received and analyzed on nearly a
daily basis, and other transmitters are scheduled to transmit archived
data in February and July 2002.
Of the 121 sharks captured and handled during the study period
(1999-2001), 119 were females that ranged in total length from 167 to
223 cm and averaged 198 cm. The two males measured 175 and 190 cm. Purse
seine gear was used to capture 89 sharks in 1999 and 2000 when the
sharks were near the surface. Hook and line gear was used in 2001 as the
sharks were rarely sighted at the surface. Approximately 93% of the
salmon sharks sampled in PWS were females that generally ranged from 2
to 2.5 m in length and averaged 135 kg in weight. Results of hormone and
reproductive tract analysis are pending. Principle prey of salmon sharks
during summer months in PWS is salmon, although they have a varied diet
that includes squid, walleye pollock, and rockfish.
Throughout the study period, 223 salmon sharks were also tagged with
conventional spaghetti tags in a cooperative effort with NMFS, the ADF&G,
and Virginia Institute of Marine Science. To date, two sharks have
been recaptured. The first shark was tagged in Port Gravina on 26 July
1999 and was recaptured by a commercial fisherman on southeast Prince of
Wales Island on 12 September 1999. Distance and elapsed time between the
point of tagging and recapture was 1,200 km and 48 days. The
second shark was tagged in Port Gravina on 29 August 2000 and recaptured
at Deer Cove on Hinchinbrook Island on 8 September 2001. Total elapsed
time from tagging to recapture was 375 days. Distance between the
point of tagging and recapture was only 50 km.
By Leland Hulbert.
Habitat Investigations Staff Participate in AFS Chapter Meeting
Three members of the Habitat Investigations presented talks at the
meeting of the Alaska Chapter of the American Fisheries Society in
Sitka, Alaska, 12-15 November. Ron Heintz chaired Environmental
Chemistry session and gave a talk entitled: “Quantitative diet
estimation using fatty acid composition data taken from northern fur
seals and their prey.” Also presenting talks were Dr. Stan Rice:
“Seasonal input of petroleum hydrocarbons into freshwater lakes from
recreational use places salmonid rearing habitat at risk,” and Bonita
Nelson: “Effect of supplementation with marine-derived nutrients on
the lipid class and fatty acid composition of juvenile coho salmon” as
well as “Research scientists and kids: making the connection with Sea
quarterly Oct-Dec 2001 sidebar
Auke Bay Lab