Using experimental data and a model to predict size selection based on morphological data, we investigated size selection of redfish (Sebastes spp.) in the grid-based selection systems used in Northeast Atlantic trawls. We found that not all redfish make physical contact with the spacing between the bars in the sorting grids while they pass in the direction of the codend, and therefore the escapement of some undersized redfish depends on size selection in the codend. We estimate that most of the escapement in the combined selection system (consisting of a 55-mm grid and a diamond mesh codend) happens through the grid. We demonstrated that for one of the two grid systems investigated the increase in size selection obtained experimentally by increasing grid-bar spacing is well in line with what could be expected based on the morphology of redfish. However, the size selection observed experimentally was significantly lower than the size-selective potential of the grids estimated based on the morphology of redfish. By computer simulations, we show that a possible explanation for this difference could be that not all redfish that attempt to escape through the grid make their attempt with an optimal angle of attack.

A method is presented to estimate and diagnose the sources of uncertainty in acoustic survey measures of fish density. The method involves simultaneous Monte Carlo simulation of density and uncertainty resulting from imprecision in all terms in acoustic analyses. Known bias can also be considered. Results are presented from theoretical simulations based on assumed distributions of fish density and acoustic parameters at sample sizes from 10–1000, and on survey data for Atlantic cod and redfish. Uncertainty can be reduced by either increasing the sample rate or by decreasing the error in input variables (system and ocean parameters, backscatter, target strength, species identification, detectability). Uncertainty in inshore cod surveys in Newfoundland waters can be attributed for the most part to heterogeneous fish distribution and detectability variance (total R2 = 0.64). Uncertainty in offshore redfish surveys is attributed to heterogeneous fish distribution, and variance in target strength and species identification (total R2 = 0.82). Uncertainty can be reduced by survey design, not only by the classical methods of achieving less diverse measures of backscatter, but also by increasing precision in the input parameters to the density estimate, in particular, target strength, detectability, and species identification.