Estimation of a widow rockfish (Sebastes entomelas) shoal off British Columbia, Canada as a joint exercise between stock assessment staff and the fishing industry

We conducted an acoustic study of widow rockfish (Sebastes entomelas) to test the feasibility of using acoustic methods to estimate the biomass of near-bottom shoals of rockfish, and to estimate the biomass of a particular mid-winter shoal, which the fishing industry suggested might be large enough to change current government perceptions of stock biomass. We repeated the acoustic survey of the study site 20 times. The total area backscattering coefficient (Sa) per survey ranged from 808-452 m(2) n.mi(-2). Total biomass estimates for the 21-28 km(2) area were 1000-2630 t. Trawl catches indicated that the species composition was approximately 88% widow rockfish. The variances in mean Sa for each of the micro-surveys were estimated with spatial analysis using either a unique or a global variogram. The coefficient of variation (CV) ranged from 4.8-17.8% when using individual variograms and from 9.6-29.5% when using a global variogram. The observed CV among the 20 estimates was 31%, almost three times the average CV based on the unique variograms, and almost two times the average CV derived From a global variogram. Although echograms indicated a diel movement, from near bottom during the day, to off-bottom at night, we observed no strong correlation of acoustic fish density with time of day or state of the diel tide. During the cruise, biomass estimates were derived within 24 hours of the completion of each micro-survey so that industry participants could review results and request changes to survey design. Although the estimates failed to indicate that current harvest recommendations were too conservative and were therefore a disappointment to industry, accommodating their scepticism during the cruise allowed us to provide scientifically credible estimates. The estimates now provide a shared reference point between fishers and stock assessment biologists for discussion about anecdotal sounder observations. The overall methodology can be implemented using portable equipment on commercial fishing vessels and will be useful for either scheduled or "ship of opportunity" surveys of specific shoals. Application to larger scale biomass estimation of widow rockfish stocks remains problematic, owing to problems of near-bottom detection, species resolution, and unpredictability of fish distribution. (C) 2000 international Council for the Exploration of the Sea.