Conducting Visual Surveys of Rockfishes in Untrawlable Habitats Using Manned Submersibles

Many fish stocks have strong affinities to specific habitats, resulting in patchy spatial distributions in abundance. Sample stratification or otherwise explicitly incorporating habitats into survey design can increase precision and accuracy of estimated densities of these stocks. Several economically valuable rockfish species off Alaska and the West Coast of North America occur in rugged rocky terrain, making them impossible to accurately survey using such conventional methods as bottom-trawl gear. We have developed direct-count, habitat-specific methods to improve stock assessments of a number of these species in the Gulf of Alaska and California. Seafloor maps of substratum type and bathymetry are used to identify and quantify rockfish habitats on a large spatial scale, providing the frame within which to distribute sampling effort. Fish surveys, distributed by habitat, are conducted from a human-occupied research submersible. Abundance and biomass are estimated from fish density, size composition, and area of the habitat. These habitat-specific visual survey methods not only contribute to improved assessments of rockfish stocks, but also are necessary for an ecosystem approach to the management of diverse communities on rocky areas of shelf and slope. Additionally, we are using these methods to characterize fish and habitat associations to improve identification of essential fish habitats, to design and monitor marine protected areas, and to understand the significance of deep-sea coral habitats.