JFM 2000 Quarterly Rpt. sidebar
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(Quarterly
Report for Jan-Feb-Mar 2000)
Groundfish
Assessment Program
The Effect of Trawl Speed on Footrope Contact
of a Survey Trawl
Groundfish and crab assessment surveys conducted by
the Groundfish Assessment Program standardize towing
speed to 3.0 knots speed over ground (SOG). However,
the catchability of our survey trawls may change
with towing speed because of its effect on trawl
dimensions and fish swimming speed. The
between-tow variability in catch per unit effort (CPUE)
caused by changes in trawl geometry might be reduced
if we were to instead standardize towing speed to
speed through water (STW). In an experiment
conducted off the coast of Washington in September
1999 (described in July-September 1999 Quarterly
Report), we examined whether changing towing speeds
(STW) had an effect on footrope contact with the
bottom, a situation which could potentially affect
fish escapement beneath the trawl.
The fishing vessel Sea Storm was chartered for 8
days to tow the RACE Division抯 standard Poly
Nor扙astern bottom trawl fitted with roller gear
at half-knot increments in speed, ranging from 2.0
to 5.0 knots SOG as determined by differential GPS.
In addition to a locally moored current meter
set 3 m from the bottom, two current meters were
mounted to the trawl to monitor STW. Bottom
contact was measured by means of a pivoting tilt
meter mounted to a sled that attached to the center
of the footrope permitting continuous contact with
the seabed, even when the centermost bobbins lifted.
Tilt measurements were later calibrated to
distance off bottom. A video camera was used
to verify whether the sled and tilt meter were
forced off bottom due to excessive water pressure
and to describe the varying levels of footrope
contact at the different towing speeds. Observations
were classified as: 5) best contact, center bobbins
rolling; 4) center bobbins not rolling but close
enough to create a mud cloud; 3) center bobbins off
bottom but tilt meter still in contact; 2) center
bobbins and tilt meter not in contact and the bottom
still in view; 1) bottom no longer in view.
Bottom contact and STW data were collected for 19
tows, each spanning a 2.5 - 5.0 knot range of towing
speeds. Door collapse occurred at several of
the 2.0 knot speed increments as evidenced by a
rapid decline in wing spread suggesting that
the lowest STWs used in the experiment were at the
operational limit of the trawl. The
relationship between distance off bottom (cm) and
trawl speed through water was found to be described
by the equation: distance = 5.60 - 5.90 STW +1.55
STW2. At speeds under 2.7 knots STW the
footrope made hard contact with the seafloor. At
3.0 knots STW, our survey target speed, the center
bobbins came off bottom 2 cm on average. At
4.0, 4.5, and 5.0 knots STW the centermost bobbins
lifted by 7, 10, and 15 cm, respectively. However,
at speeds greater than 4.5 knots the predicted
distances off bottom are likely underestimated
because the tilt meter was often observed losing
contact with the bottom, and in many cases the
bottom could no longer be seen.
The evidence that footrope contact and trawl fishing
dimensions vary over this range of towing speeds is
clear and makes a strong case for monitoring STW
during AFSC groundfish assessment surveys. What
is not certain, however, is whether fish escapement
beneath the footrope is any different when the
footrope is in contact with the seafloor, or is 2
cm, or even 7 cm off bottom. A study aimed at
quantifying the escapement underneath the trawl at
varying trawl speeds using an underbag is scheduled
for this summer.
In preparation for this summer抯 planned
catchability study, preliminary testing of the gear
took place during March 7-10 off Blaine, Washington,
aboard the fishing vessel Larkin.
By Ken Weinberg.
Midwater
Assessment and Conservation Engineering Program
Echo Integration/Trawl Surveys of
Prespawning Pollock in the Eastern Bering Sea Shelf,
Bogoslof Island, and Shelikof Strait
Scientists from the Midwater Assessment and
Conservation Engineering (MACE) Program completed
echo integration-trawl (EIT) surveys of walleye
pollock (Theragra chalcogramma) on the
southeastern Bering Sea shelf and in the Aleutian
Basin near Bogoslof Island aboard the NOAA ship
Miller Freeman between 27 February and 13 March
2000. Following this cruise, a survey of the
distribution and abundance of spawning walleye
pollock within the Shelikof Strait area of the Gulf
of Alaska was conducted between Chirikof Island and
Cape Chiniak between 15 and 28 March 2000. The
objectives of the surveys were to obtain echo
integration and trawl data for determination of the
distribution, biomass, and biological composition of
spawning pollock.
The Bering Sea shelf survey was the sixth such
winter survey conducted since 1989. Originally
scheduled to last 2 weeks and cover an area between
Cape Rozhnof on the Alaska Peninsula and St. George
Island, the survey was scaled down to 4 days because
of heavy January sea ice cover and forecasts of
continued severe ice conditions. Data
collected during the survey will be used to estimate
the abundance of pollock inhabiting the eastern
portion of the area designated as Steller sea lion
Conservation Area (SCA). Surveys of the
Bogoslof area have been conducted annually (except
1990 and 1999) since 1988. They are designed
to monitor pollock spawning over deep water in the
southeastern Aleutian Basin. The biomass
estimate for pollock inside U.S. management area 518
obtained during these surveys provides an index of
Aleutian Basin pollock abundance for each year抯
Central Bering Sea Convention meeting. The
Shelikof Strait survey was the eighteenth spawning
stock survey of walleye pollock in this area area
since 1980 (surveys were not conducted in 1982 and
1999).
A calibrated, scientific-quality echo sounder/echo
integrator (Simrad EK500/BI500) operating at 38 and
120 kHz was used to collect acoustic data during
these surveys. Two series of parallel
transects were surveyed on the Bering Sea shelf and
in the Bogoslof area (Figure
1). Transect spacing was 12.5 nmi on
the Bering Sea shelf and transects were oriented
east-west. The survey covered about 500 nmi of
trackline. In the Aleutian Basin-Bogoslof
Island area, 10-, 5-, or 2.5-nmi spaced transects
(depending on fish density) were oriented
north-south. The survey began in the east at
about 166癢 and proceeded westward to about 170�W,
covering 2,000 nmi of trackline. Northern
boundaries for transects 1-8 were about 54�N, and
about 54癗 for transects 9-16. About 1,100
nmi of transect trackline and 31 trawl hauls were
completed during the survey of Shelikof Strait.
Opportunistic trawl hauls targeting pollock and
other fish echosign were made with an Aleutian wing
trawl (AWT) and a Poly Nor扙astern bottom trawl
with roller gear. Species composition, and for
pollock, sex composition, length frequencies, whole
fish and ovary weights, maturities, and otoliths
were collected from each haul. Pollock tissue
samples were taken from selected hauls for fecundity
and genetic studies. Video recordings of pollock
behavior within the midwater trawl were collected
during the Shelikof Strait survey and will be used
to evaluate sampling gear performance.
On the southeastern Bering Sea shelf, pollock were
observed from near the start of transect 101 to near
the end of transect 108. On the first several
transects, pollock formed dense, near-bottom,
aggregations between 95- and 100-m bottom depths.
These aggregations often extended for several miles.
Dense pollock schools were found adjacent to
Unimak Island beginning at about 50-m bottom depth;
some continued westward to >150-m bottom depths.
Highest densities were observed on transects 103,
104, and 106 (Figure
1). Fork lengths (FL) of pollock from
eight trawl hauls ranged between 30 and 73 cm and
averaged 44 cm. Smaller pollock were
encountered in the final shelf-area trawl haul that
targeted isolated, dense, schools over bottom depths
of around 160 m. Most males and females were
found in prespawning (74% and 48%, respectively)
condition, although 43% of the females had small
ovaries and were categorized as developing. The
preliminary estimate of biomass for the eastern
Bering Sea shelf is 816,000 metric tons (t).
In the Bogoslof area, pollock were observed in the
first 2 nmi of transect 1 in relatively shallow
water (Figure 1).
Farther to the west, pollock
aggregations were sparse. Between 1 and
2 nmi of pollock echo sign were observed on
transects 3 and 5. On the south end of
transect 7, a large pollock school was encountered
in nearly the same location northeast of Umnak
Island as in previous years. Very few pollock
were observed between transects 8 and 11.5
(168E-169oW). Much of the remainder of the cruise
was spent surveying and trawling between 169o-170oW,
north of Samalga Pass and east of the Islands of
Four Mountains, where relatively large pollock
spawning aggregations were observed. During
the Bogoslof survey, ten trawls were conducted and
catches contained pollock of 31-68 cm FL. On
average, fish were largest in the Samalga Pass area.
Percent female ranged from 35 to 79, with more
females caught overall. The vast majority of
fish were in prespawning (95% and 94% among males
and females, respectively) condition. Numbers
and biomass for the Bogoslof area appeared to be
somewhat lower than observed in 1998 and the 1999
Japanese survey. The preliminary estimate of biomass
for the Bogoslof area is 321,000 t.
In the Gulf of Alaska most of the mature pollock
were distributed along the western side of Shelikof
Strait, with the greatest densities occurring from
Cape Kekurnoi to Cape Nuskhak; a similar pattern of
distribution has been observed in previous years.
Fish were most abundant within 50-150 m of the
bottom. The size distributions of pollock from
hauls within the strait generally exhibited dominant
modes around 10-14 cm, 20-24 cm, 30-36 cm, and 43-57
cm FL. Seventy percent of the females greater
than 34 cm FL were mature, with 67% in a prespawning
condition, 1% in spawning condition, and 2% spent.
Pollock from the 1999 year class (mode 10-14
cm FL) formed a strong, well-defined midwater layer
(150-200 m depth) from about Chirikof Island to
Sitkinak Strait and off Cape Kekurnoi. The amount of
eulachon, Thaleichthys pacificus, in trawl
catches in the 2000 survey was substantially higher
than in recent years. Eulachon is considered to be a
contaminate of acoustic returns from pollock.
Analysis of the effect of increased eulachon
abundance on pollock abundance estimates is
currently in progress.
By Taina Honkalehto and Michael Guttormsen.
Fisheries
Oceanography Coordinated Investigations
Kevin Bailey and Susan Picquelle presented
the poster 揕arval distribution patterns of
offshore spawning flatfish in the Gulf of Alaska:
Sea valleys as transport pathways and enhanced
inshore transport during ENSO events� at the PICES
symposium, 揃eyond El Ni駉,� in San Diego,
California, during March 2000.
Offshore and deep-water, spawning flatfish species
in the Gulf of Alaska, such as arrowtooth flounder
and Pacific halibut, have juvenile nurseries that
are inshore, in bays, or at the mouths of bays.
Larvae must emigrate from their spawning areas
along the continental slope and outer shelf towards
inshore bays, in a direction normal to the
prevailing Alaskan Stream. Using a 20-year
time series of data from ichthyoplankton surveys in
the Gulf of Alaska, we examined patterns of
variability in larval halibut and flounder
distributions that may reflect processes resulting
in successful recruitment to nursery areas. Several
patterns can be observed in these data. Eggs
and the smallest sized larvae are located along the
outer shelf and slope. Larger larvae tend to be
located farther inshore over the continental shelf.
Larger larvae are also associated with
deep-sea valleys and troughs that penetrate the
shelf. Larvae of these flatfish species live
in deep water, and literature reports indicate that
bottom water flows up the sea valleys. Thus,
these topographic features may serve as transport
pathways to juvenile nursery grounds. ENSO-conditions
and warm year anomalies are linked to recruitment
strength of Pacific halibut. Variability in
larval transport as related to ENSO and other events
that enhance onshore advection, may play an
important role in recruitment of flatfishes to their
nursery grounds.
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