STATUS OF STOCKS & MULTISPECIES ASSESSMENT PROGRAM
Susanne McDermott successfully defended her doctoral dissertation “Improving
abundance estimation of a patchily distributed fish, Atka mackerel (Pleurogrammus
monopterygius)” on 13 March 2003 at the School of Aquatic and
Fishery Sciences, University of Washington. Two separate survey
approaches were developed to estimate Atka mackerel local abundance. A
mark recapture experiment was conducted in Seguam Pass, Alaska, inside
Area 1 and outside Area 2 of the trawl exclusion zone. In 1999, 1,375
tagged fish were released. A biomass of 76,679 t was estimated in
Area 2 using the Peterson method. In 2000, 8,773 tagged fish were
released. An integrated tagging model was developed to estimate biomass
and movement rates. Biomass was estimated to be 117,900 t (standard
deviation (SD) 45,267) in Area 1 and 82,057 t (SD 13,311) in Area 2.
Movement rate after 107 days from Area 2 to Area 1 was 0.0056 (SD
0.0072) and from Area 1 to 2 was 0.81 and associated with high
uncertainty (SD 0.3). The feasibility of a demersal egg count survey was
also examined. Estimated fecundity for a 40-cm female was 30,256 eggs,
spawning a total of 4.6 batches per year, after adjusting for atresia. A
new approach to population estimation, the Annual Batch Production
method, estimates abundance from a survey of batches per area within the
spawning grounds using underwater cameras and scuba divers. A simulation
was conducted to examine the number of batches present in nests
throughout the spawning season for 11 different reproductive scenarios.
Optimal timing for a batch count survey was determined to be 1 month
after beginning of spawning, with a window of 34 to 78 days. Defining
the spawning area was identified as the biggest challenge followed by
the development of sampling platforms and tools to withstand the harsh
conditions in the Aleutian Islands.
By Libby Logerwell.
FIT Presentations at Marine Science Symposium
Members of the Fishery Interaction Team (FIT) from the Center’s REFM and RACE
Divisions presented results of their work at the symposium, “Marine
Science in the Northeast Pacific: Science for Resource Dependent
Communities,” held in Anchorage, Alaska in mid-January 2003. The
following papers and posters were presented during the sessions on
Steller Sea Lion Investigations.
- Elizabeth Conners, Peter Munro and Sandra Neidetcher: “AFSC Pacific
cod studies at Unimak Pass.” This presentation concerned the Fisheries
Interaction Team (FIT) field study around the intensive winter fishery
for Pacific cod in waters north of Unimak Pass in the eastern Aleutian
Islands. The experiment uses pot catch of cod as a measure of local
abundance, both in the intensively fished area and in an adjacent
control area within the Cape Sarichef no-trawl zone. The goal is to
compare the rate of seasonal change in abundance between sites in the
treatment and control zones and quantitatively test for presence or
absence of a fishing effect. This study has been in feasibility work
since 2001, and winter 2003 will be the first full trial of the
experiment. This presentation showed the results of feasibility studies
to assess the overall variability and spatial correlation structure of
pot catch data and presents briefly the final experimental design.
- Anne Hollowed, Chris Wilson, Phyllis Stabeno, and Sigrid Salo: “Effect
of ocean conditions on the cross-shelf distribution of walleye pollock (Theragra
chalcogramma) and capelin (Mallotus villosus).” The
presentation provided results of acoustic trawl surveys conducted in
2001 as part of a multiyear (2000-2002), multidisciplinary experiment
off the eastern coast of Kodiak Island in the Gulf of Alaska (see
Wilson, et al. presentation, below). The authors found that the spatial
distribution of walleye pollock and capelin differed and appeared to be
related to differences in habitat preferences. Capelin appeared to be
associated with cool slope water intrusions and topographic gradients,
while walleye pollock appeared to select habitat based on the availability of prey.
- James lanelli and Martin Dorn: “Using fishing vessels to collect
acoustic data for scientific purposes: preliminary results from midwater
trawlers in the eastern Bering Sea walleye pollock fishery (poster).”
This poster described a project to log acoustic backscatter data on
midwater trawlers fishing for walleye pollock in the eastern Bering Sea.
The objective of the project is to evaluate fishing impacts on
endangered Steller sea lions at fine spatial and temporal scales. Work
to date has focused on evaluating the spatial coverage of the data and
examining the general characteristics of cruise tracks and uncalibrated
backscatter data. Preliminary results show good correspondence with
survey data during the same period. However, technical problems related
to data processing need to be resolved.
- Libby Logerwell and Chris Wilson: “Discrimination of Steller sea lion
prey fish using frequency-dependent backscatter.” This work evaluates
the effect of commercial fishing activity on the prey availability to
Steller sea lions in the Gulf of Alaska (see Wilson, et al. presentation
below). The project employs acoustic methodology to assess the
distribution and abundance of sea lion prey, so one challenge is
identifying the species composition of mid-water scatterers,
predominantly juvenile pollock and capelin. The difference between mean
volume back-scattering strength at 120 and 38 kHz ()MVBS)
has been used by researchers to acoustically discriminate between
macrozooplankton species and between macrozoo-plankton and fish or small zooplankton.
The presentation was on an analysis to determine whether )MVBS
can be used to discriminate between aggregations of juvenile pollock and
capelin. Analysis of acoustic data at 38 kHz and 120 kHz from the east
coast of Kodiak Island was presented. The authors conclude that acoustic
differencing at the scale of the fish aggregation and at high
integration thresholds can be an effective technique to distinguish
between juvenile pollock and capelin.
- Libby Logerwell and Ruth Christiansen: “Energy density of Steller sea
lion prey in western Alaska: Species, regional, and seasonal differences
(poster).” The energy density of prey fish is a necessary component of
foraging models that show how changes in prey abundance or distribution
(natural or fishery-related) might impact the feeding success of Steller
sea lions. Although values of fish energy density can be found in the
literature, data are not available for many species that sea lions eat.
This is particularly true for specific geographic regions or seasons.
The goal of the FIT project described in the poster was to fill these
gaps by collecting fish that are common in sea lion diets, but for which
energy density data are unavailable during the seasons and in the
regions that sea lions eat them. The data were presented in tables so
that scientists could readily incorporate the necessary energy density
values into their foraging models. In addition to providing modelers
with data, the poster shows that energy density varies not only with
fish species but with region, season, and fish reproductive status. The
authors caution against building a foraging model with prey energy
density values that are not specific to the time and place the model
- Libby Logerwell and Susanne McDermott: “Interactions between
commercial fishing and Atka mackerel: a pilot study to resolve the
experimental design.” The goal of the research project was to
characterize the effects of commercial fishing on local abundance and
movement of Atka mackerel. The Fishery Interaction Team has been
conducting tag release-recovery studies of Atka mackerel in the Aleutian
Islands since 1999 (see McDermott and Logerwell presentation, below).
Results have provided valuable insight into the effectiveness of trawl
exclusion zones at preventing prey shortages for Steller sea lions
foraging on Atka mackerel. Tag recoveries in previous years have
occurred during and after the commercial fishery, so it is not possible
to assess the effect of the fishery on the resulting estimates of local
abundance and movement. The authors conducted a pilot study in summer
and fall 2002 in the area of Seguam Pass to assess the feasibility of
collecting tag recovery data both before and after the commercial
fishery in areas open and in areas closed to the fishery. They examined
these data to determine whether the number of tag releases and
recoveries would be sufficient to detect a fisheries effect. They also
evaluated whether the trawl exclusion zone is a suitable control for
variability in local abundance and movement due to causes other than
- Susanne McDermott and Libby Logerwell: “Estimates of Atka mackerel
movement and abundance based on tagging data: Are trawl exclusion zones
effective?” The FIT research project uses mark recapture methods to
estimate local abundance and small-scale movement of Atka mackerel
relative to trawl exclusion zones in the Aleutian Islands region. The
motivation for the project was to determine whether trawl exclusion
zones are effective at preventing prey shortages for Steller sea lions.
To do so, information is needed about 1) local abundance of fish, and 2)
movement rates of fish into and out of the zones. Model results to date
indicate that there is little movement of Atka mackerel from inside to
outside the Seguam Pass trawl exclusion zone. This suggests that the
trawl exclusion zone is effective at preventing fisheries outside from
impacting fish abundance inside.
- Kimberly Rand and Susanne McDermott: “Seasonal changes in Atka
mackerel sex ratios in Seguam and Tanaga Passes during 2002 (poster)”.
The Fisheries Interaction Team has been conducting tag release-recovery
studies on Atka mackerel in the Aleutian Islands since 1999. To
incorporate data into the best possible tagging model, it is also
important to understand whether there are differences in abundance or
movement by sex. This poster described data on Atka mackerel length and
sex frequency from three research charters conducted by the AFSC during
the 2002 field season. Using these data in a GIS map, seasonal changes
in the sex ratio were observed in both Seguam and Tanaga Passes. The
authors also examined differences between sexes relative to depth. In
observing the differences in movement between the sexes, we suggest that
it is important to construct a sex-specific tagging model.
- Chris Wilson, Anne Hollowed, Michiyo Shima, Paul Walline, and Sarah
Stienessen: “Fishery interaction study: Interactions between
commercial fishing and walleye pollock.” The talk summarized the
results from the first 2 years of a multiyear fishery interaction study
near Kodiak Island. Acoustic surveys were conducted during August 2000
and 2001. The goal of the surveys is to provide information to assess
whether the abundance and spatial patterns of walleye pollock are
impacted by commercial fishing activities over short spatio-temporal
scales. Results presented do not suggest a significant link between
fishing activities and changes in estimates of juvenile and adult
pollock geographical distribution, biomass, and vertical distribution.
The authors note that it will be important, however, to evaluate whether
these trends persist during subsequent years.
By Libby Logerwell.
Cod Pot Studies Summary
Design and feasibility studies for the FIT Pacific cod pot studies and local
abundance experiment were completed in 2001-2002, and the project was
presented to the North Pacific Fishery Management Council (NPFMC) in
October. In order to conduct the experiment without gear conflicts, NMFS
asked the Council for a 2-week special closure of the Cape Sarichef
study area in March of 2003 -2006. While the request drew some concern
from industry, a compromise solution was worked out and the modified
closure was approved by the NPFMC.
Field operations in fall 2002 and winter 2003 were a mixture of success and
frustration. Final gear feasibility trials in September - October
2002 ran smoothly. Newly constructed experimental pots and modular
trigger assemblies performed well and fished consistently in comparison
with standard industry pots. However, the 28 December - 12 January
cruise was delayed by mechanical and equipment problems. The weather in
the study area started out badly and rapidly deteriorated. After only 3
days of fishing, conditions were so rough that continued operations were
deemed unsafe, and fishing was suspended. The ship and crew then spent
several days in Akutan Harbor sitting out gale winds up to 100 knots.
When the storms finally blew over, problems with one of the boat’s two
main engines forced a return to Dutch Harbor which effectively ended the
A second cruise leg was conducted 4-17 February , after the close of the
snow crab (Chionoecetes opilio) season. The main purpose of this
cruise was to tag and release Pacific cod in the vicinity of the study
area in order to learn more about seasonal migration patterns and local
movement of cod during the spawning season. The cruise also went
equipped to collect local abundance data, in the hope that some of the
data collection missed by the January cruise could be collected. However,
pollock trawling was running around-the-clock in part of the experimental region outside the Cape Sarichef
no-trawl zone, so fishing experimental pots in the area was not
possible. Samples were collected at experimental stations within the
no-trawl zone. Fish were collected and tagged at three locations: within
the Cape Sarichef no-trawl zone, west of the study area within the Akun
no-trawl zone, and eastward along the 100-m contour toward Amak Island.
Over 2,000 cod with hot-pink spaghetti tags were released. Tag shedding
and mortality studies were also conducted.
At the time of this writing, field crews are at sea on the March leg of the
local abundance experiment. Due to bad weather conditions at the start,
both scientific and ship crews were stranded in Anchorage for nearly a
week before the cruise could get under way. So far, however, mechanical
difficulties have been minor, and an initial data set for the entire
study area has been obtained. The cruise provides a unique platform for
collecting tissue samples for genetic studies of cod stock structure;
the objective is to collect tissue from approximately 1,000 mature cod
over the course of the cruise.
Despite the difficulties encountered, we expect the 2003 field season to provide
valuable information on cod movements and the behavior of pot catch data
as an experimental variable. With a limited sample size, quantitative
tests for the presence or absence of a fishery effect may be
inconclusive. The data collected, however, should be sufficient to
validate the performance and power of the experimental design.
By Elizabeth Conners.
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