OND99 Quarterly Rpt. sidebar
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National Marine Fisheries Service/Alaska Department of Fish and Game Trawl Comparison Study
(Quarterly Report for Jan-Feb-March 1998)
by Eric Brown and Harold Zenger
Figure 1. Station locations of the fishing
vessel Peggy Jo
and the
research vessel Resolution during the
October 1997
NMFS/ADF&G trawl comparison study.
Scientists from the Alaska Fisheries Science
Centers (AFSC) Resource Assessment and Conservation Engineering (RACE) Division in
cooperation with the Alaska Department of Fish and Game (ADF&G) completed a 12-day
fishing gear calibration study in waters off the east side of Kodiak Island, Alaska,
(Figure 1 above) on 24 October 1997 (see Quarterly Report,
Oct-Dec 1997). The purpose of the study was to detect and measure
fishing power differences between the net and vessel configuration used by the National
Marine Fisheries Service (NMFS) during their Gulf of Alaska (GOA) triennial groundfish
surveys and the net and vessel configuration used by the ADF&G during their annual
crab/groundfish survey of the western and central GOA. The results will allow both
NMFS and the ADF&G to augment each of their surveys by allowing direct comparisons of
their respective databases for the dominant species encountered during the experiment.
Three vessels, the state of Alaska
research vessels Resolution and Pandalus along with the NMFS chartered
fishing vessel Peggy Jo participated in the experiment. After
completing only nine tows, the Pandalus was forced to drop out of the experiment
due to mechanical problems. The results of the trawl comparision experiment are
reported only for the activities conducted aboard the Peggy Jo and Resolution.
METHODS AND GEAR
Each vessel employed trawling methods standard to
their respective surveys. The Peggy Jo conducted 15-minute tows at 5.56
km/hour (3 nautical miles/hour) while the ADF&G vessels conducted tows of
approximately 1.85 km (1 nmi) at a towing speed of 3.70 km/hour (2 nmi/hour). Aboard the
Peggy Jo, bottom trawling was conducted with a NMFS four seam, high-opening polyethylene
Noreastern trawl equipped with 14-inch rubber bobbin roller gear. This trawl
is capable of sampling moderately rough and irregular bottom types typically
encountered throughout much of the GOA. The standard ADF&G survey gear used
aboard the Resolution is the 400-mesh Eastern otter trawl. This net has a 21-m
headrope and 29-m footrope which lacks roller gear making it well suited for sampling
relatively smooth and soft bottom types. On each vessel, the net width and height was
constantly monitored using a system of headrope and wing sensors (SCANMAR) that was
linked acoustically to the vessel. Actual distance fished was determined using a
combination of global positioning system (GPS), footrope-mounted bottom contact sensors
(BCS), and headrope-mounted micro-bathythermograph units (MBT), which recorded depth along
with temperature. Catches on each vessel were sorted, weighed and enumerated by
species using methods common to each survey. Length-frequency measurements were collected
for selected species.
RESULTS
A total of 33 paired tows were completed by the
two vessels. Tows made by the Peggy Jo with the NMFS survey gear averaged 0.27 hour
in duration and 1.45 km in length, while those made by the Resolution averaged 0.41
hour and 1.74 km. The mean towing speed of the Peggy Jo was 5.28 km/hour
compared to 4.23 km/hour for the Resolution. Trawl measurements obtained with
the SCANMAR system indicated mean net widths of the NMFS and ADF&G nets were virtually
identical at 13.8 m while the vertical openings were significantly different at 6.9 m for
the Noreastern trawl and 1.9 m for the Eastern trawl. Mensuration data also
indicated that on some tows, the ADF&G net had a tendency to narrow somewhat as the
tow progressed, probably related to catch accumulation in the codend. Occasionally the net
width of the 400-mesh Eastern trawl decreased and then increased by several meters in a
pulsating pattern, as if the doors were losing contact with the bottom and then regaining
contact, or perhaps as if the footrope was running into sand or mud waves. The NMFS
net with its larger doors and roller gear rarely showed a similar pattern.
For each vessel, the distance fished
and mean net width of each tow was determined from the NMFS-supplied instrumentation. For
the NMFS tows, the fishing effort is generally based solely on an analysis of these
observations. On board the Resolution, two measures of fishing effort were
obtained. The first method is based on the standard methods employed during the
ADF&G annual crab/groundfish survey which uses the captains estimate of distance
fished along with a constant 12.2 m estimate of trawl width. During this study a
second estimate of fishing effort was measured based on an analysis of the instrumentation
data collected aboard the Resolution during each tow.
Comparing the two measures of
fishing effort obtained aboard the Resolution indicates the effort based on the
data collected by the NMFS electronic instrumentation was 17% higher than the effort based
on the captains estimate of distance fished and a constant net width of 12.2 m.
A major factor in this difference is the net staying on bottom and continuing to
fish after initiating trawl retrieval resulting in a mean distance fished of 1.85 km
compared to the captains bridge estimate of 1.74 km. Another factor
contributing to a higher fishing effort is an observed mean net width 1.6 m wider than
the standard estimate of 12.2 m.
Four groundfish species dominated
the catches. Flathead sole (Hippoglossoides elassodon) was the most
abundant species accounting for over 30,000 kg of total catch between both vessels.
The second most dominant species was walleye pollock (Theragra
chalcogramma)
with 26,000 kg followed by arrowtooth flounder (Atheresthes stomias)17,000
kg, and Pacific cod (Gadus macrocephalus)2,000 kg. Other species or
species groups such as snow (Tanner) crabs, scallops, sculpins, skates, and other flatfish
were taken throughout the comparison study but represent far lower abundance levels and
far fewer occurrences in catches.
In general, the NMFS trawl
configuration with higher vertical opening and roller gear was more efficient in capturing
the two roundfish species while the lower opening ADF&G trawl without roller gear was
most efficient for the two flatfish species. Mean CPUEs based on the standard methods of
calculating fishing effort common to each survey indicated the greatest relative
difference in catch rates was for walleye pollock where the Noreastern trawl was 3.1
times more efficient than the 400-mesh Eastern trawl. The NMFS trawl was also most
efficient at capturing Pacific cod by a factor of 1.59. Conversely, the ADF&G
configuration was most efficient for the two dominant flatfish species, flathead sole and
arrowtooth flounder, with relative differences of 1.61 and 1.49, respectively.
While the Resolution
generally captured considerably more flathead sole and arrowtooth flounder than the
Peggy
Jo did, the respective size compositions of those species were quite similar between
the two vessels. On the other hand, the Peggy Jo captured far more
walleye pollock of all sizes than the Resolution did. The size compositions
of pollock captured by each vessel were quite similar for fish 30 cm or larger, but
the absence of age-0+ and age-1+ pollock from catches by the Resolution was very
notable. These observations are not surprising given the lower vertical opening of
the 400-mesh Eastern otter trawl, which is designed for maximum bottom contact, compared
to the high opening Noreastern trawl with roller gear and the demersal distribution
of flatfish and semipelagic distribution of pollock.
DISCUSSION
While the experiment was successful
in determining the relative differences between the two surveys in catchability of walleye
pollock, Pacific cod, flathead sole, and arrowtooth flounder, the results indicate a large
difference in the relative availability of pollock to each of the two survey
configurations. Incorporating the ADF&G pollock data into the NMFS triennial
survey database would require expanding each ADF&G pollock catch by a factor of over
three. Compounding the large difference in catch rates between the two gear types is
an apparent size selectivity difference suggesting that juvenile pollock are not readily
available to the lower opening 400-mesh Eastern trawl. Incorporating the pollock
data collected by the two gear types into stock assessments for management purposes will
require careful consideration.
The cooperative nature of the
experiment represents a commitment by both NMFS and ADF&G to assist and support each
other on problems of mutual interest. Plans are already well under way for future
cooperative efforts. Researchers from the AFSC are making plans to participate in an
ADF&G seasonality study of Marmot Bay, Alaska, planned for 1998 and 1999. The
AFSC has also committed supplemental resources to the ADF&G annual crab/groundfish
survey in an effort to provide stock assessment scientists with additional biological
information for selected groundfish species.
The AFSC would like to acknowledge
the cooperation and teamwork demonstrated by the captains, crews, and scientists aboard
each of the three vessels. Their commitment and enthusiasm were instrumental in
meeting the study objectives. Particular thanks go to Captain Ron Kutchick of the
Resolution
whose intimate knowledge of the study area resulted in choosing tow sites that provided
consistent catches for the major target species. Thanks also to Captain Brian Beaver
and crew of the Peggy Jo and Captain Paul Desjardin and crew of the Pandalus
for their skill and patience in executing the research plan. ADF&G biologists
Dan Urban of the Resolution and Bill Bechtol of the Pandalus willingly
shared their expertise and provided valuable leadership throughout the experiment.
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