Surveys of Rockfish Abundance in Untrawlable Habitat Using a Combination Acoustic-Optic-Trawl Survey

Rockfish are difficult to assess using standard trawl surveys due to their aggregation in high-relief habitats. Therefore, alternative methods are needed to assess rockfish biomass in these habitats. We conducted two experimental acoustic surveys for rockfish in untrawlable habitats in Alaska. One survey was conducted at the Snakehead bank in the Gulf of Alaska which is an important offshore bank for the commercial fishery. The other survey was conducted at Zhemchug ridges in the eastern Bering Sea shelf, an area which has seen little commercial rockfish fishing in the last two decades. The habitat at each site was designated as trawlable or untrawlable using video observations of the seafloor. Calibrated acoustic data was collected along parallel survey tracks in both surveys to estimate fish biomass. In total, three methods were used to groundtruth species identification and length composition of acoustic targets; a remote operated vehicle (ROV), a stereo-video drop-camera, and a modified bottom trawl. All were deployed at sites where fish sign was observed. Camera tows indicated rockfish species occurring pelagically at both sites consisted of primarily dusky rockfish and northern rockfish. These species also occurred on the seafloor with a mix of juvenile Pacific ocean perch, harlequin rockfish and other species. The species and size compositions were similar among the trawl, ROV and stereo drop camera, but different between trawlable and untrawlable areas and between the two survey areas. Acoustic data at both sites indicated that the fish biomass in the water column was consistent over multiple passes along the survey grid. The biomass estimates were much different between the two sites and probably reflected the history of commercial fishing. In the unfished site rockfish occurred pelagically to heights of > 40 m off the seafloor over untrawlable ridges during the daytime, but were found on or near the bottom during nighttime. While at the fished site, rockfish occurred in both trawlable and untrawlable habitats and most of the observed fish sign occurred near the seafloor (< 2 m off bottom) during both the night and day. The different behavioral patterns probably reflect the differences in species composition and feeding conditions. In both cases estimating the height of the acoustic dead zone above the seafloor was critical to estimating fish biomass. The methods developed during this study will be important to guiding future work estimating rockfish biomass in untrawlable habitats throughout Alaska.