Sampling errors in anchovy egg abundance estimates using the PAIROVET net

Abstract

A replicate haul experiment was carried out using the vertically towed PAIROVET net in an area of high abundance of anchovy eggs in the Bay of Biscay. The main aim of the experiment was to compare the performance of ships of different size working in the same area. In addition, we tested the performance of the PAIROVET net and the deployment methodology used to sample anchovy eggs for biomass assessment purposes. We concluded that under the experimental conditions experienced in this cruise, different ships yield similar performances of the vertically towed PAIROVET net if the methodology is kept similar. The ANOVA suggests that the systematic central sampling (SCS) scheme currently in use for the Bay of Biscay anchovy biomass estimations (one station out at 3 miles) adequately represents the surrounding area along the transect line. The among-station component of variance within the unit sampling area of the current SCS turned out to be comparable and, in some cases, smaller than the within-station variance. Consequently, the spatial design of the SCS adopted during routine surveys appears to be a reliable strategy for sampling anchovy eggs in areas of high egg abundance, as in this experiment. In addition, it ensures fairly independent data units. In the area of the experiment, the most disperse egg stages (like 2- and 3-day-old eggs) show a larger withinstation component of variance than the spatial component of variance within the rectangles of the current SCS. This suggests that for these stages, the sampling can still be improved cheaply by increasing the precision of egg abundance estimates per station with larger sample sizes, without changing the spatial sampling scheme or the number of stations. However, the benefits of this strategy will hardly affect the precision of the 1-day-old egg estimates because of their patchy distribution.