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

We used hydroacoustic techniques to obtain biomass estimates of yellowtail rockfish, Sebastes flavidus, in a 100-km(2) area off the west coast of Brit ish Columbia, Canada. The purpose of our study was to estimate sampling var iance and explore the effect of diel aggregation behavior on the precision of biomass estimates. A set of eight transects was sampled eight times: fou r at night, four during daylight. Although we observed a pronounced diel be havioral 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 t he differences were not significant in most comparisons and were too small to influence survey design. The coefficient of variation of the eight obser vations 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 biom ass for the survey area was 1152 t. The observed diel behavioral patterns d id not, in this study, produce different estimates of yellowtail rockfish b iomass. Survey time might therefore be optimized without concern for this s ource of variance for this species.