|
Resource Ecology &
|
Prey Name | % FO | %W |
Gastropod (snail) |
9.09 |
0.49 |
Fusitriton spp. (snail) |
9.09 |
0.19 |
Cephalopod (squid and octopus) |
27.27 |
0.17 |
Teuthoidea (squid) |
36.36 |
0.62 |
Octopus dofleini (octopus) |
72.73 |
4.63 |
Crangonidae (shrimp) |
9.09 |
0.01 |
Pagurid (hermit crab) |
9.09 |
0.01 |
Teleostei (unidentified fish) |
45.45 |
0.33 |
Oncorhynchus spp. (salmon) |
9.09 |
4.49 |
Gadidae (gadid fish) |
9.09 |
0.49 |
Theragra chalcogramma (walleye pollock) |
9.09 |
5.22 |
Atheresthes stomias (arrowtooth flounder) |
63.64 |
67.21 |
Sebastes spp. (rockfish) |
9.09 |
2.06 |
Pleuronectid (unknown flatfish) |
18.18 |
0.86 |
Hippoglossoides elassodon (flathead sole) |
9.09 |
0.98 |
Fishery offal |
9.09 |
12.27 |
Total prey weight |
24,017 g |
|
Number of stomachs with food |
11 |
|
Number of empty stomachs |
2 |
In response to
concerns about the possible linkage between increasing sleeper shark populations and the
decline in Steller sea lion populations, a study was performed to examine the diet of
sleeper sharks in the Gulf of Alaska to determine if there was evidence of predation on
juvenile Steller sea lions. Stomach contents were examined of 13 sleeper sharks collected
June-August 1996 from longline and bottom trawl vessels fishing at bottom depths of 86-267
m . The stomach contents consisted primarily of bottom-dwelling fish and
invertebrates such as arrowtooth flounder and octopus. Fish processing offal also
contributed largely to the diet composition by weight (Table 1 above).
The sleeper sharks collected during
this study were of intermediate size (218-295 cm total length) and were collected in
areas close to Steller sea lion rookeries and haulouts. There was no evidence of
predation on juvenile Steller sea lions. The sleeper sharks appeared to feed
primarily on benthic prey that was likely encountered on or near the ocean bottom.
By Patricia Livingston.
STATUS OF
STOCKS & MULTISPECIES
ASSESSMENTS: Improving Trawl-based Assessments of Shortraker and Rougheye Rockfishes
A long-term research project was
recently completed which focused on ways of improving the trawl-based assessments of
shortraker, Sebastes borealis, and rougheye rockfishes, S.
Aleutianus,
in the Gulf of Alaska. First, a retrospective analysis of the available survey
and fishery information was conducted to identify areas of high shortraker and rougheye
rockfish densities. Using geographic information system (GIS) mapping software and
statistical approaches, catch per unit of effort (CPUE) data were compared and contrasted
by depth strata, bathymetric features, and broad-scale geographic areas. The results
indicated that both species prefer the continental break and slope areas, usually
preferring depths between 300 and 500 m. Rougheye rockfish, however, were found to
occur over a much wider, shallower depth range than shortraker rockfish. In terms of
coastwide distributional patterns, both species appear to have a preference for the
eastern portion of the Gulf of Alaska. This information will prove useful in
establishing more refined and efficient survey stratification and sampling allocation
schemes in the future.
A major factor that contributes to
the success of any type of trawl survey is the ability of the gear to effectively sample
the habitats of the target species. Shortraker and rougheye rockfishes are known to
inhabit areas of rough bottom. Unfortunately, many of these areas are inaccessible by the
current survey trawl. As such, research also focused on the design, construction,
and evaluation of an alternative footrope to sample shortraker and rougheye rockfish
populations that inhabit the rugged continental slope regions. At-sea gear trials
were conducted to assess the performance of this alternative sampler using a net
mensuration system and an in situ video camera assembly. The video system was used
successfully to document trawl and fish behavior, as well as provide a quantitative
estimate of capture efficiency. The results showed that the alternative footrope
performed well in shortraker and rougheye habitats and that the capture efficiency in the
center section of the footrope was on the order of 80 percent for the
shortraker/rougheye
complex.
To assess the overall efficiency of
the alternative footrope, a two-phase depletion experiment was implemented. During
Phase I, repeated trawling in a very narrowly defined area was conducted during a 48-hour
period. This area was then revisited after 7 days and again trawled for another 48
hours, representing Phase II of the experiment. A simple analytic model was
developed to estimate initial population size and trawl efficiency within the depletion
zone. Detailed hydroacoustic information was also collected concurrently with a
scientific echo sounder during all depletion tows. The resulting efficiency estimates were
comparable with the video-based efficiency estimates. Furthermore, the hydroacoustic
indices were strongly correlated with the depletion catches, suggesting that hydroacoustic
approaches may hold promise for eventually providing a cost-effective, noninvasive means
of measuring absolute or relative abundance of shortraker and rougheye rockfish stocks in
waters off Alaska.
(This research was completed by
Daniel Ito and was published as a Ph.D. dissertation at the University of
Washington.)
By Daniel Ito.
AFSC/ADF&G
Research Planning Meeting
AFSC scientists attended the first
AFSC/ADF&G Joint Research Planning meeting on 10-11 May in Juneau. The purpose
of the meeting was to improve communication and identify opportunities for joint research
between the two institutions. Discussion topics covered a range of subjects
including trawl and echo intergration survey plans for the eastern Bering Sea, Gulf of
Alaska, and inside waters of Southeast Alaska; rockfish research; stock identification
studies; sea lion research projects; and an overview of the current stock assessment and
allocation issues. The meeting was well received by both research entities and will likely
lead to future collaborative activities.
By Anne Hollowed.
Application
of Age-structured Stock Assessment Model to Southern Minke Whale Stocks
James Ianelli attended the
International Whaling Commission (IWC) scientific meetings in May (see NMML report in this
issue). He participated as an invited participant to comment specifically on the
assessments for southern minke whale stocks and for the Bering Sea bowhead whale stock.
James has worked closely with scientists on alternative analyses for the bowhead whale
assessment which was presented at the 1998 IWC meeting.
The issue regarding minke whales
dealt with the application of an age-structured model to this stock. The
interpretation in the change of recruitment, which apparently increased after
commercial
fishing began, is linked with alternative hypotheses about how the fishery catchability
may have changed during the course of the commercial fishery. This assessment is
particularly important since it is linked to the continuation of limited scientific
harvests by Japanese vessels.
In collaboration with Professor Doug
Butterworth, University of Cape Town, South Africa, Ianelli developed a statistical
age-structured model. The model has time-series effects as part of the trend between
age- and year-specific fishing mortality rates. The model allows for the inclusion
of age-specific tag-recapture data in addition to a variety of survey and CPUE indices.
The model was applied to southern bluefin tuna data and is intended to contrast with the
VPA-type model that is currently used for management. Ianelli presented this paper at the
50th International Tuna Conference held at the University of California at Los
Angeless Lake Arrowhead Conference Center in May.
By James Ianelli.