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Resource Assessment &
Conservation Engineering

(Quarterly Report for April-May-June 1999)

GROUNDFISH ASSESSMENT: Eastern Bering Sea Crab/Groundfish Bottom Trawl Survey

The chartered vessels fishing vessels Aldebaran and Arcturus departed Dutch Harbor, Alaska, on 19 May  to conduct the Alaska Fisheries Science Center (AFSC) annual bottom trawl assessment survey of groundfish and crab resources of the Bering Sea shelf.  Upon completion of the standard bottom trawl survey, the Arcturus will conduct the annual shrimp survey of Pavlof Bay. The Arcturus and Aldebaran will complete the charters in Dutch Harbor on 24  July 1999.

The primary objective of the survey is to continue the annual series of assessment surveys of crab and groundfish of the eastern Bering Sea shelf to provide information for:

  1. The North Pacific Fishery Management Council and Alaska Department of Fish and Game on the distribution, abundance, and biological condition of important groundfish and crab resources.

  2. The U.S. fishing industry on catch-per-unit effort and size composition of groundfish and crab resources.

  3. The support of ongoing studies of the biology, behavior, and ecology of groundfish and crab.

The standard survey consists of approximately 380 sampling stations positioned on a 20 nmi x 20 nmi grid pattern .  The stations are located at the center of the grids except in areas where additional stations are allocated to obtain more precise estimates of blue king crab abundance near the Pribilof Islands and St. Matthew Island.  Each vessel will occupy stations on alternate columns of the sampling pattern beginning in Bristol Bay and working westward.  Thirty-minute trawl hauls will be made at each station and the catches will be sorted, weighed, and enumerated by species.  Biological information (fish length, crab carapace length or width, maturity, age composition, weight, and stomach contents) will be collected from survey target species.  The 83-112 Eastern bottom trawl will be used to sample all Bering Sea stations.  Water column temperatures will be determined at all stations with a micro-bathythermograph system.  Survey results will be available in fall 1999.

By Russ Nelson.

Triennial Bottom Trawl Survey of Gulf of Alaska Groundfish Resource

The sixth triennial bottom trawl survey of Gulf of Alaska groundfish resources began in Dutch Harbor on 10 May 1999. The chartered fishing vessels Dominator, Morning Star, and Vesteraalen will be used to survey the continental shelf and slope waters of the Gulf of Alaska from the Islands of Four Mountains (170BW  long.) to Dixon Entrance (134B40’W long.). The major objective of the survey is to continue the triennial time series begun in 1984 to monitor the trends in the distribution and abundance of important groundfish species and to describe and measure various biological and environmental parameters.  The survey will end in Ketchikan, Alaska, on 23 July.

The survey area is divided into 59 strata based on depth, major geographic features, and International North Pacific Fisheries Commission (INPFC) statistical areas.  Strata range from shallow near-shore areas to 1,000-m depths of the upper continental slope and include a total of 776 stations which were designated through a stratified random sampling scheme.  Each of the survey vessels will conduct 15-minute trawl hauls at each preassigned station. Catches will be sorted, weighed, and enumerated by species. Biological information (fish length, crab carapace length or width, maturity, age composition, weight, and stomach contents) will be collected from survey target species.  Temperature and depth will be recorded by a micro-bathythermograph attached to the headrope of the trawl at each station.  A four seam, high-opening polyethylene Nor’eastern bottom trawl equipped with rubber bobbin roller gear will be used at all standard survey stations.  In addition to conducting the standard survey, areas designated during previous triennial surveys as untrawlable with the standard survey trawl will be sampled with two specially reinforced nets with tire gear footropes.  Due to differences in the fishing efficiencies of the standard survey trawl and the trawl used with tire gear, these experimental tows will be analyzed separately from the standard survey data.  Survey results will be available in fall 999.

By Russ Nelson.

Midwater Assessment & Conservation Engineering (MACE)

From 17 to 29 May, scientists from the Conservation Engineering group of the RACE Division conducted experiments with a flexible grate system to reduce the bycatch of undersized walleye pollock in midwater trawls.  This cruise continued a project to facilitate the development of more selective midwater trawls to reduce the catch of undersized pollock. The selectivity of fishing gear results from the interactions between fish and the stimuli presented by the fishing gear.  Underwater video systems with infrared illumination were used to observe pollock reactions to a flexible grate installed in the extension section of a midwater trawl. An auxiliary net was used to capture the fish which exited the trawl through the grate, thus allowing quantitative analysis of the resulting size selection.

Research trawling was conducted from the fishing vessel Hickory Wind off the eastern shore of Kodiak Island, Alaska.  Trawl equipment, procedures, and locations were similar to those used in the commercial fishery for walleye pollock.

The flexible grate consisted of 25 fiberglass rods 234 cm (7.7 ft) long and 11 mm (0.5 in) in diameter, held together by 1.9-cm (.75 inch) inside diameter, synthetic rubber, automotive heater hose with holes at constant intervals to regulate the width of the gaps between the bars. The hose divided the length of the bars into six sections, each about 36 cm (14 in) long.  Thicker fiberglass rods (1.6 cm diam, 5/8 in) were affixed across the forward and aft ends of the grate to keep it spread. Fiberglass and rubber were used in the design and construction to allow the grate to sufficiently bend to wrap onto a net storage reel and then return to a consistent configuration when the net was again set.

The grate was installed at an angle in the extension of a midwater trawl.  Fish that did not go through the grate were herded downward through a small opening between the aft (lower) edge of the grate and the lower panel of the extension.

The grate was very effective at separating pollock by size.  The pollock encountered were nearly all of commercial size, so a wide bar spacing (53 mm) was used that released half of the pollock smaller than 40 cm while retaining nearly all of the pollock longer than 50 cm (Figure below). Where more small individual are encountered, a smaller spacing could be used to exclude them while retaining more of the medium sized pollock.

selsize.jpg (30703 bytes)

By Craig Rose.