AFSC/RACE/GAP/Palsson: Gulf of Alaska and Aleutian Islands Biennial Bottom Trawl Survey estimates of catch per unit effort, biomass, population at length, and associated tables

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NOAA’s Alaska Fisheries Science Center conducts the Alaska Biennial Bottom Trawl Survey of the Gulf of Alaska (GOA) and Aleutian Islands (AI) in alternate years. These surveys have been conducted since 1980 in the AI and since 1984 in the GOA and conform to NOAA Protocols for Groundfish Bottom Trawl Surveys of the Nation’s Fishery Resources (2004, G. Stauffer compiler, NOAA Technical Memorandum NMFS-SPO-65). The survey consists of three chartered commercial fishing vessels that tow a bottom trawl at randomly-selected stations stratified by region, depth, and in the case of the GOA, habitat.

For the GOA survey, the sampling frame is known trawlable or new sites throughout the survey area, stratified by management areas; by shelf, slope, or gully habitats; and by depth zones including 0-100 m, 101-200 m, 201-300 m, 301-500 m, 501-700 m, and 701-1000 m (for GOA survey methodology, see von Szalay et al. 2010 Data Report: 2009 Gulf of Alaska Bottom Trawl Survey, NOAA Technical Memorandum NMFS-AFSC-208).

For the AI survey, the sampling frame is past successfully towed stations, stratified by management region and the same GOA depth zones but only as deep as 500 m (for GOA survey methodology, see von Szalay et al. 2011. Data Report : 2010 Aleutian Islands Bottom Trawl Survey. NOAA Technical Memorandum NMFS-AFSC-215). At each station the research trawl is set for a targeted period of 15 min and if the tow is successful, the catch is processed. Catch processing includes identifying, counting, and weighing all living taxa, collecting length measurement for commercially and ecologically important groundfish and invertebrate species, and collecting other materials, such as age structure, for laboratory or other scientific analysis. For each tow, detailed data are collected about the characteristics of the sample including position, time, depth, surface and bottom temperatures, and net characteristics. Sensors on the net measure how the net is fishing, and we track the contact of the foot rope with the seafloor, the width of the net at the wing tips, and the height of the headrope.

Tow-by-tow data are entered into computer databases or acquired electronically. Once the survey is completed, the data are error checked and finalized into a database known as RACEBASE.

These data are further processed into estimates of abundance in terms of biomass and numbers of organisms.

Estimates of abundance are based upon the area-swept technique whereby the catch per unit of effort (cpue) is computed for each successful station as the weight or count of each species divided by the area that the net contacted the seafloor during the tow. These calculated cpues are then averaged among the stations in each stratum and then multiplied by the calculated area of the stratum (as measured from digitized nautical charts and a geographic information system).

The product of these terms is an estimate of the biomass or numerical abundance (see von Szalay et al. 2010 Data Report: 2009 Gulf of Alaska Bottom Trawl Survey, NOAA Technical Memorandum NMFS-AFSC-208 and von Szalay et al. 2011. Data Report : 2010 Aleutian Islands Bottom Trawl Survey. NOAA Technical Memorandum NMFS-AFSC-215 for more complete methods).

The abundance estimates for the strata are combined by area-depth categories for convenient analysis. In addition, the population-at-length is estimate by applying length-sex frequency observations to the population abundance estimates. As age structures are evaluated for age determination, the population-at-size estimates are used with an age-length key to estimate the population-at-age structure of selected species.