Evaluating Smooth Sheet Bathymetry for Determining Trawlable and Untrawlable Habitats

Principal Investigator: Wayne Palsson
Co-Principal Investigators: Mark Zimmerman

Biennial bottom trawl surveys in the Gulf of Alaska (GOA) and Aleutian Islands (AI) provide fishery independent estimates of catch per unit effort, abundance, and biological parameters used in stock assessments for managed fisheries and species in the North Pacific. Not all bottom types or oceanographic conditions accommodate the bottom trawl survey method. The bottom trawls can only be towed on smooth and unconsolidated seafloors, so whether trawl stations are “trawlable” or “untrawlable” becomes a major factor limiting the sampling frame of the survey. In the AI, much of the seafloor consists of rock, pinnacles, and steep drop-offs and stations are resampled from a limited pool of previously sampled stations. In GOA, about half of all stations have been visited and about 20% were found to be untrawlable. Having prior and complete knowledge of trawlable areas would clearly define the survey frame for bottom trawl surveys and could become the basis for defining a survey of untrawlable habitats with acoustic or visual survey tools.

We wanted to evaluate whether hydrographic smooth sheets which are charts contains original soundings and seafloor observations could be used to predict whether areas the survey have not visited are trawlable or untrawlable. These charts are electronically available from the National Ocean Service through the National Geophysical Data Center. They contain many more observations than are found on nautical charts, but they require extensive validation for analytical use (Figure 1, http://www.afsc.noaa.gov/RACE/groundfish/bathymetry/). The reprocessed sheets may offer a data source to identify untrawlable habitats from criteria and approaches from focused seafloor studies. We wanted to identify criteria to predict untrawlable habitat from smooth sheet data, test these criteria with a predictive model with known areas of rocky habitat and unsuccessful bottom trawls, and assemble and interpret existing smooth sheet data into a map of untrawlable habitat that can be evaluated in future surveys and studies.

Arc GIS raster surface layers of RACE bottom trawl survey strata

Preliminary segmented regression fit to determine profiles (solid line) and confidence intervals (dashed line)

Preliminary predictive maps for trawlable and untrawlable habitat

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Alaska Fisheries Science Center (AFSC)


Annual Report - Year 1

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