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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

 map of Oct.'97 locations for vessels Peggy Jo & Resolution during NMFS/ADF&G trawl comparison study. (5593 bytes)
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 Center’s (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.


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 Nor’eastern 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.


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 Nor’eastern 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 captain’s 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 captain’s 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 captain’s 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 Nor’eastern 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 Nor’eastern trawl with roller gear and the demersal distribution of flatfish and semipelagic distribution of pollock.


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.