Diel vertical migration by yellowtail rockfish, Sebastes flavidus, and its impact on acoustic biomass estimation


Stanley, Richard D., Robert Keiser, Bruce M. Leaman, and Ken D. Cooke
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We used hydroacoustic techniques to obtain biomass estimates of yellowtail rockfish, Sebastes flavidus, in a 100-km2 area off the west coast of British Columbia, Canada. The purpose of our study was to estimate sampling variance and explore the effect of diel aggregation behavior on the precision of biomass estimates. A set of eight transects was sampled eight times: four at night, four during daylight. Although we observed a pronounced diel behavioral pattern of diurnal aggregation and nocturnal dispersion, we found no significant differences between nocturnal and diurnal estimates of mean biomass. Diurnal estimates showed a tendency towards higher variance, but the differences were not significant in most comparisons and were too small to influence survey design. The coefficient of variation of the eight observations for any individual transect ranged from 13 to 128%. The coefficient of variation in biomass for the whole survey area, based on repeating the set of eight transects eight times, was 13.9% and the estimate of mean biomass for the survey area was 1152 t. The observed diel behavioral patterns did not, in this study, produce different estimates of yellowtail rockfish biomass. Survey time might therefore be optimized without concern for this source of variance for this species.