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Groundfish Assessment Program

Aleutian Islands Shelf and Slope Survey

survey map
Map of the Aleutian Islands bottom trawl survey area. (Click on image to enlarge)

The AFSC conducts comprehensive bottom trawl surveys in the Aleutian Islands (AI) designed principally to monitor trends in abundance and distribution of groundfish populations.

The survey area covers the continental shelf and upper continental slope (out to 500 m depth) in the Aleutian Islands from Islands of Four Mountains west to Stalemate Bank. The survey has been conducted since 1980, first at approximately three year intervals and then biennially since 2000. NOAA charters two commercial fishing vessels for 70 days each, during June - August, sampling the standard 64,400 km2 survey area with approximately 420 survey stations.

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 AI Bottom Trawl Survey is a multi-species survey based upon a stratified-random design of previously successful stations. The AI shelf and upper slope are divided into strata based upon management area, sub-region, and depth zones (0-100 m, 101-200 m, 201-300 m and 301-500 m). Stations are allocated among strata based upon an optimal allocation scheme using past estimates of abundance and variance, strata areas, and the economic importance of key groundfish species. Additionally, a grid of 5 km x 5 km is overlain over the survey area and stations are targeted to fit within the preselected grid of the station. 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 allowable catches.

NEW! Check out Alaska Fisheries Science Center Dispatches from the Field 2016: Surveying the depths of remote Aleutian Islands to read about our survey and to see more photos!

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