Assessing rockfish abundance in complex habitats using acoustics and cameras.

Abstract

Many species of rockfishes (Sebastes spp.) are difficult to assess using trawl surveys due to their propensity to aggregate near the seafloor in rocky high relief areas. A feasibility study was conducted during October 2009 in such an area south of Kodiak Island, AK, to evaluate the use of standard fisheries acoustic survey methods in conjunction with stereo‐video cameras for estimating the distribution and abundance of dusky and northern rockfishes. Uniformly spaced parallel transects were repeatedly surveyed using single beam echosounders over several days. A multibeam echosounder was used to characterize the seafloor as trawlable or untrawlable and these designations were corroborated by camera. Rockfish abundance was estimated using a combination of acoustic and camera measurements. At least 80% of the rockfish detections were observed in untrawlable habitat areas, and within 2.0 m of the seafloor. Over half of the rockfish seen by the camera were within the acoustic dead zone. Repeat passes exhibited high precision and there was no significant difference in fish abundance or height off bottom between night and day. Future work is planned during summer 2011 to evaluate the feasibility of using these methods in broader areas and for other rockfishes in the Gulf of Alaska.