JAS 2000 Quarterly Rpt. sidebar
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(Quarterly
Report for July-Aug-Sept 2000)
GROUNDFISH
ASSESSMENT
Eastern Bering Sea Crab-Groundfish Survey
The Bering Sea resource assessment group completed
the 30th annual survey of the eastern Bering Sea
shelf from 23 May to 20 July 2000. The early
surveys covered a much smaller area than our present
efforts and were primarily aimed at assessing stocks
of red king crab (Paralithodes camtschaticus).
As time passed, the surveyed area increased,
and sampling methodology became more consistent.
Since 1982, the standard survey has covered
over 463,000 km2, and the trawl gear and sampling
pattern have remained the same.
After a record cold year in 1999, water temperatures
returned to near the long-term mean in 2000 (2000
mean bottom temperature: 2.17oC, long-term mean:
2.42oC). Unlike last year, no ice was encountered
during the survey.
The biomass estimate for walleye pollock (Theragra
chalcogramma) was significantly higher in 2000
at over 5.1 million metric tons (t). Pollock
distribution was also more widespread in 2000,
unlike 1999 where the cold water in the middle
domain (waters 50 - 100 m) seemed to force the fish
to the outer shelf. The biomass estimate for
Pacific cod (Gadus macrocephalus) continued a
general decline shown since 1994 to an estimate of
528,000 t. Of the major flatfish species, only
Alaska plaice (Pleuronectes quadrituberculatus)
showed a decline from 1999. Northern rock sole
(Lepidopsetta polyxstra) showed the greatest
increase, up to 2.1 million t.
It has been known for some time that a portion of
the yellowfin sole (Limanda aspera) stock was
present in waters shallower and inshore of the
standard survey area. Yellowfin sole are
spawning during the survey time and are therefore
potentially missed and underestimated by the survey.
As part of a program to evaluate the magnitude
of the problem, 20 inshore stations were sampled in
2000 during the progression of the survey. These
stations were along the Alaska Peninsula, in Togiak
Bay and in Kuskokwim Bay. The geographic area
that these stations represent is presently being
evaluated, so it is difficult to estimate the actual
biomass for yellowfin sole in this area. However,
the raw catches were very high and amounted to
nearly 40% of the survey catches. It is
expected that these new stations, or possibly more,
will be sampled again next year to determine if they
should be added to the standard survey in the
future.
At the end of the standard survey another experiment
in the continuing program of investigating trawl
performance was performed. The experiment was
designed to determine how trawl speed and currents
affect the footrope’s contact with the bottom. The
results are presently being evaluated.
During the last 2 years the survey has started about
2 weeks earlier due to time constraints brought on
by changes in fishery seasons near 1 August. The
ramifications of this early start are not known;
however, one apparent effect is on the survey of red
king crab. Part of the crucial data collected
is the stage of eggs present on the females. In
past years the females have already molted and have
new eggs present. In 1999 and 2000 many of the
eggs were old eggs, not yet hatched. To gauge
the success of the hatch, we have had to return to
inner Bristol Bay at the end of the standard survey
and resurvey parts of that area after the red king
crab have molted. Whether this effect is due
strictly to timing, or is a result of the severe
cold anomaly of 1999 (since a large part of the
maturation of the 2000 hatching eggs occurred in
1999) is unknown. Further study of this
process, plus discussion of resetting the survey
timing is under way
By Gary Walters.
MIDWATER ASSESSMENT & CONSERVATION
ENGINEERING (MACE)
An underwater video camera is attatched
forward of the Aleutian wing trawl to observe
walleye pollock behavior inside the net during the
summer 2000 Bering Sea shelf echo integration-trawl
survey aboard the NOAA research ship Miller Freeman
Bering Sea Survey
The Midwater Assessment and Conservation Engineering
(MACE) program has monitored abundance of walleye
pollock in U.S. waters of the eastern Bering Sea for
more than two decades using echo integration-trawl
survey (EIT) techniques. Between 8 June and 3
August 2000, MACE scientists employed the NOAA ship
Miller Freeman to survey pollock on the eastern
Bering Sea shelf from Port Moller, Alaska, to the
U.S./Russia Convention Line (Figure
1). Principal objectives of the
research cruise were 1) to collect echo integration
and trawl data to estimate pollock abundance and
distribution and 2) to collect pollock target
strength information to validate the relationship
between pollock length and target strength–a
measure of acoustic reflectivity. This
midwater time series together with near-bottom
abundance and distribution data derived from annual
summer bottom trawl surveys, are used in stock
assessment models and ultimately assist the National
Marine Fisheries Service and North Pacific Fishery
Management Council in managing the commercial
fishery.
Preliminary cruise results show that pollock were
present on all but the second transect of the survey
(Figure 1).
East of the Pribilof Islands, the highest
pollock concentrations were observed along transects
7-8, north of Unimak Island. Pollock abundance
was lower between Unimak and the Pribilofs. West
of the Pribilof Islands, pollock increased and
peaked in abundance on transects 20 and 21 southwest
of St. Matthew Island. In the far west,
pollock were heavily concentrated in a few spots
along the U.S./Russia border (transects 26-28, Figure
2). In general, water
temperatures were warmer in 2000 than in 1999 and,
on most transects, pollock were distributed somewhat
farther north in 2000 than in 1999. Overall
pollock biomass was similar to that observed in
1999.
One of several additional research projects
conducted during the cruise involved mounting an
underwater video camera on the midwater trawl to
observe pollock behavior in front of the the codend
as part of ongoing research on fish behavior in
relation to trawl gear (see photo above).
By Taina Honkalehto.
Gulf of Alaska Survey
Figures 4a, 4b. Echo integration-trawl (EIT)
methods require combining acoustic data, which are
represented in the echogram of adult walleye pollock
(top), with associated trawl samples (bottom) to
estimateabundance and distribution patterns for the
targeted species. Illustrative data are from
Barnabus gully during the August 2000 EIT survey.
The trawl catch of fish being sorted by scientists
in the photograph are primarily adult walleye
pollock.
A feasibility study
was conducted off the east side of Kodiak Island
between 8-20 August 2000 as a collaborative effort
between scientists of the Center’s RACE and REFM
Divisions. The purpose of the study was to evaluate
the suitability of this location for conducting a
multiyear summertime field experiment to evaluate
the effect of commercial fishing activity on the
availability of walleye pollock to Steller sea lions
(Eumetopias jubatus).
Principal objectives for the feasibility study were:
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to use standard EIT survey
methodologies to describe the abundance and
distribution patterns of walleye pollock within
Barnabas and Chiniak gullies
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to determine whether acoustic
back-scattering from nontargetted species would
prohibit meeting the first objective
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to determine the spatio-temporal
variability in pollock abundance and
distribution patterns within and between the two
gullies over the duration of the study.
The researchers will
use this information to assess the proposed
experimental design for more comprehensive field
work in subsequent years. The more comprehensive
work will require that the gullies serve as
treatment and control sites where commercial fishing
would be allowed in one location and prohibited in
another.
The EIT survey operations included the collection of
38 and 120 kHz acoustic data, as well as net catch
data from large midwater and bottom trawls (Figures
4a, 4b above). These data were collected along
a series of uniformly spaced (i.e., 3 nmi) parallel
transects during the 2-week survey. Two
complete survey passes were conducted in both
Barnabas and Chiniak gullies.
Preliminary survey
results indicated that it was possible to use EIT
survey methods to assess the summer distribution of
pollock within the study area (Figure
5). Substantial back-scattering was
attributed to “adult” pollock as well as
“small fishes” which included age-0 pollock,
age-1 pollock, and capelin (Mallotus villosus).
It was difficult, however, to determine the
relative contribution of capelin and age-0 pollock
to the “small fishes” back-scattering. Additional
analyses of the data are in progress to quantify the
spatio-temporal distribution patterns and abundance
of pollock and capelin. Interpretation of
these results will provide information to determine
whether these two submarine gullies off of the east
side of Kodiak Island are appropriate sites for
conducting a more comprehensive field experiment
next summer.
By Chris Wilson.
Halibut Excluder for Cod Trawlers
Figure 6. Full halibut excluder,
including the skate deflector (solid panel),
which was made of 20 x 20 cm (8 x 8 inch)
square meshes. (the top and near-side panels
of the trawl are not shown).
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Figure 7. Detail of ring grate (rings 22
cm (8.5 inch) diameter, one panel in aluminum
hoop and three panels behind it) and slot
sections (slots 35 x 7 cm [14 x 2.75 inch]).
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During cruises in June and August, RACE scientists
developed and tested a halibut excluder for trawl
fisheries which target Pacific cod. The resulting
design was then tested in September during a fishery
conducted under an exempted fishery permit sponsored
by two trawl industry associations, the Groundfish
Forum and the At Sea Processors Association (ASPA).
The most effective configuration of the device
released 80% of the Pacific halibut encountered,
while losing only 15% of the Pacific cod catch.
The initial cruise, conducted on the fishing vessel Hickory
Wind, during 18-28 June, used an excluder based
on designs previously tested for halibut exclusion
in sole fisheries. The device had a grate in the
section ahead of the codend to guide halibut to an
escape hole, while allowing the sole to pass through
to the codend. Instead of square grate openings used
in previous designs, the grates were constructed
with rigid circular holes, “ring grates,” to
better allow cod passage while excluding halibut.
An auxiliary net attached to the trawl with a
second codend was used to recapture the escaping
fish to allow tow-by-tow estimates of escape rates.
Ring grates with 22-cm (8.5-inch) diameter openings
effectively excluded large halibut while allowing
nearly all of the cod to pass through to the codend.
However, most of the halibut encountered were
less than 70 cm in length. Reducing the ring
diameter to 19 cm (7.5 inches) excluded some of the
smaller halibut but greatly increased the loss of
cod. Skates were encountered frequently during
the cruise, and underwater video showed that they
often blocked large portions of the excluder grate.
During the second cruise, conducted on the fishing
vessel Hazel Lorraine during 12-16 August,
additional modifications were installed to exclude
small halibut and to prevent skates from blocking
the grates. Two vertical panels with
horizontal slots 9 cm (3.5 inch) high were installed
behind the ring (22-cm, 8.5-inch diameter) grates to
form a narrowing chute. Animals passing
through the slots in the vertical panels escaped.
The purpose of these vertical panels with
narrow slots was to allow smaller halibut to escape,
while retaining all but the smallest cod. To
keep skates from blocking the grates, a downward
sloping mesh panel with large openings (20 x 20 cm
(8 x 8 inch) square mesh) was installed ahead of the
grate section. Nearly all fish except for
large skates were able to pass through this mesh.
The skate excluder panel was effective at preventing
the grate from becoming blocked by large skates.
The combination of slots and rings allowed 75%
- 80% of the halibut to escape across all of the
sizes present. However, medium- and
small-sized cod (up to 70 cm length) were also
escaping at rates from 30 to 70%. Underwater video
showed that while only the smallest cod could pass
through the slots in an upright position, when
crowding induced strong escape efforts, the cod
would turn on their sides, flattening their gill
plates and thus narrowing the widths of their heads
to pass through. Measurements of the compressed
widths of cod and halibut heads and their lengths
were taken to better optimize the slot openings.
The excluder system
tested during the Groundfish Forum/ASPA exempted
fishery, conducted 17 - 23 September the vessel Legacy,
was similar to that used on the previous cruise
except that the slots were narrowed to 7 cm (2.75
inch) in width (Figures 6 & 7 above). Initial
tows excluded 77% of the halibut, but also allowed
46% of the cod to escape, with escape rates
consistent across cod sizes. Restricting
access to the escape hole below the ring grate in
two steps reduced the cod loss to 31% and then to
15%, while the halibut escape rate remained
consistent at 80%. A final test with the ring
section removed retained all but 2% of the cod, but
only excluded 25% of the halibut.
This series of cruises developed and demonstrated an
effective halibut excluder for trawl fisheries that
harvest cod. The resulting system was actually
three excluders in sequence, a mesh panel to deflect
skates, a grate with circular holes to direct large
halibut to an escape opening, and vertical panels
with long, narrow slots to exclude small halibut.
Depending on the mix nd sizes of halibut, cod,
and skates encountered, one or more of these
components might not be needed. Further
development of the durability and ease of handling
of this system will improve its usefulness to all
cod trawlers. Field work to accomplish this
should occur within the next year.
By Craig Rose.
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