A Review and Evaluation of Methodology for Estimating Pacific Herring Egg Deposition

The history of Pacific herring egg deposition surveys in British Columbia is reviewed. Variability in components of the SCUBA egg deposition survey methodology used to assess the spawning biomass of herring stocks was evaluated by bootstrap simulations, thus providing confidence interval estimates for mean egg density, total area of deposition, and spawning-stock biomass. Variability in estimated spawning-stock biomass was larger than expected from variation in egg density alone. Bias was evident in the mean egg density estimated by a visual assessment model, apparently due to inter-diver variation in estimating model variables. Optimal sampling designs are robust and remain within current capabilities for sample collection. Alternative sampling approaches should be investigated to improve the accuracy of egg density and consequently spawning biomass estimates. Egg loss from spawning beds during the incubation period appears to be significant and may bias abundance estimates if assessment surveys occur late in the incubation period, Additional investigation will be required to develop coastwide adjustment factors. Aspects of the variability associated with surface spawn surveys are also discussed briefly.