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.
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
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
Quarterly April-June 2002 sidebar
April - June 2002
Auke Bay Lab