Developing a commercial-vessel-based stock assessment survey methodology for monitoring the U.S. west coast widow rockfish (Sebastes entomelas) stock

The widow rockfish (Sebastes entomelas) inhabits the continental shelf and upper slope of the west coast of North America. The U.S. west coast widow rockfish stock has declined since the mid-1980s, leading to implementation of increasingly stringent management restrictions upon the commercial fishery. The low numbers, patchy distribution, and preference for rocky habitats of the widow rockfish make this stock difficult to monitor using standard bottom trawl survey techniques and commercial landings data; currently, there is no reliable index of abundance. An ad hoc government-industry working group began meeting in 2004 to devise a new stock assessment survey strategy. The proposed strategy utilized local fishermen's knowledge of widow rockfish distribution and behavior, chartered commercial fishing vessels, and a combination of fisheries acoustics and underwater video sampling techniques. Results are presented from fieldwork conducted at three study sites off the central coast of Oregon, USA, in March 2005. Acoustic backscatter at 38kHz between 50m from the surface to 15m off the bottom was attributed primarily to widow rockfish. This classification was based on historical widow rockfish distribution at these sites, fish school appearance on acoustic echograms, and deployments of an underwater camera sled. Repeated acoustic sampling along predetermined transects spaced at 0.3nmi (0.56km) at two of the study sites documented the temporal and spatial variability of widow rockfish schools over the course of a single day and week-to-week. The CVs of average vertically integrated acoustic backscatter measured in repeated sampling passes at these two sites were 0.31 and 0.36, which are not unreasonable values for a fisheries survey time series. Fine-scale acoustic sampling at a total of three study sites showed that the spatial scale (horizontal dimensions) of the groups of widow rockfish schools observed ranged from 0.2 to 0.8nmi (0.37–1.48km). This study demonstrates that sampling of a coastwide suite of study sites selected using local fishermen's knowledge with a standard 38kHz scientific echosounder, supported by underwater video and limited midwater trawling, may be a promising way of monitoring U.S. west coast widow rockfish abundance. These techniques may also be relevant to monitoring other difficult-to-assess semi-dermersal species inhabiting rocky, untrawlable areas.