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Groundfish Assessment Program

Gulf of Alaska Continental Shelf and Slope Survey

The AFSC conducts comprehensive bottom trawl surveys in the Gulf of Alaska (GOA) designed principally to monitor trends in abundance and distribution of groundfish populations. The survey area includes the continental shelf and upper continental slope (out to 1,000 m depth) in the Gulf of Alaska and extends from the Islands of Four Mountains 2,300 km east to Dixon Entrance.

The survey began in 1984 and was conducted triennially and then biennially since 1999. During a typical survey, three commercial trawlers are chartered for 75 days each, during late May - early August, sampling the standard 320,000 km2 survey area with approximately 820 survey stations.

Gulf of Alaska Survey Map. Click image to enlarge.
Map of the Gulf of Alaska bottom trawl survey area. (Click on image to enlarge)

The catch data result in observations of catch per unit area which are averaged and expanded by survey area to estimate the relative abundance of important groundfish species. These estimates are provided to stock assessment scientists who use the estimates and biological information from the surveys to determine Allowable Biological Catch (ABC) and Total Allowable Catch (TAC) for the North Pacific Fishery Management Council. Other information collected during the survey is used to improve understanding of life history of the fish and invertebrate species and the ecological and physical factors affecting their distribution and abundance.

The GOA Bottom Trawl Survey is a multi-species survey based upon a stratified-random design and the area-swept method of estimating abundance. The GOA shelf and upper slope to 1000 m are divided into strata based upon management area, sub-region, and depth and overlain with a grid consisting of 5 km by 5 km cells. A station is a successfully sampled trawl deployment or tow within a cell. Stations are selected at random within a stratum, except that stations that are too rocky for a successful tow are excluded. Stations are allocated among stratabased upon an optimal allocation scheme using past estimates of abundance and variance, strata areas, and the economic importance of key groundfish species. The Alaska Fisheries Science Center contracts with commercial fishers and vessels to conduct the survey. The skipper is given up to 2 hours to search the station for a suitable, smooth position to deploy the trawl. If the station is untrawlable, then the vessel moves on to the next adjacent station within the stratum and repeats the process. The net is deployed from the vessel and allowed to sink to the seafloor while the vessel steams ahead at 3 knots. Acoustic net sensors provide real-time depth and opening width measurements from the net. Once on the seafloor, the net is towed for 15 minutes after which the net is retrieved, all the time maintaining the target speed of 3 knots.

The catch is then processed by the scientific crew who identifies all living organisms, weighs and counts them, and takes biological samples from key groundfish species or other species of interest. CPUE is calculated by dividing the catch weights by the area swept by the net (distance fished by the net times the average net width). The CPUEs are averaged among stations in the stratum and then multiplied by the stratum area to estimate biomass for each stratum. Standard errors are estimated as the simple standard deviation divided by the square root of the sample size within the stratum. Population biomasses are compared among years to establish relative trends or are used directly with other parameters to set ABCs. The net contacts the sea floor for approximately 1 to 2 km during a 15 minute tow. The net width is 9 to 20 m but the doors maybe up to 50 m apart. The trawl doors, bridles, and net may disrupt the seafloor by furrowing troughs in the sediment, overturning rocks, smoothing hummocks and mounds, and damaging sessile or slow moving invertebrates. The vessel path brackets the location of the actual tow by 0.5 to 2 km depending on depth in order to deploy and retrieve the net. With door spread, each tow impacts approximately 0.0695 km2 or 57 km2 for all tows during a survey.

Pinnipeds, baleen whales, dolphins, porpoises, and sea birds are commonly observed during transiting between stations in the Gulf of Alaska. When occupying trawl stations, the RACE marine mammal avoidance procedures are followed. The Field Party Chief or their designee scans the nearby water for the presence of protected species. Direct vessel encounters are avoided, and if any species are present at a station, the FPC waits until the animals pass. If the protected species remain, then alternate stations are selected to minimize the chance of encounters by the fishing gear. Direct interactions and station relocation are noted and reported to administrative staff at the Alaska Fisheries Science Center. Only once during the past five years did a northern fur sea become caught and then die in the net. Stations have been relocated when sea otters were present at a nearshore stations and the station was moved to the next alternate to avoid encounters

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