Demersal groundfish densities in trawlable and untrawlable habitats off Washington: implications for the estimation of habitat bias in trawl surveys

摘要

Demersal groundfish densities were estimated by conducting audvisual strip-transect survey via manned submersible on the continental shelf off Cape Flattery, Washington. The purposeudof this study was to evaluate the statistical sampling power of the submersible survey as a tool to discriminate density differences between trawlable and untrawlable habitats.ududA geophysical map of the study area was prepared with side-scan sonar imagery, multibeam bathymetry data, and known locations of historical NMFS trawl survey events. Submersible transects were completed at randomly selected dive sites located in each habitat type. Significant differences in density between habitats were observed for lingcod (Ophiodon elongatus), yelloweye rockfish (Sebastes ruberrimus), and tiger rockfish (S. nigrocinctus)udindividually, and for “all rockfish” and “all flatfish” in the aggregate. Flatfish were more than ten times as abundantudin the trawlable habitat samples than in the untrawlable samples, whereas rockfish as a group were over three times as abundant in the untrawlable habitat samples.ududGuidelines for sample sizes and implications for the estimation of the continental shelf trawl-survey habitat-bias are considered. We demonstrate an approach that can be used to establish sample size guidelines for future workudby illustrating the interplay between statistical sampling power and 1) habitat specific-density differences, 2)udvariance of density differences, and 3) the proportion of untrawlable area in a habitat.