Incorporating experimentally derived estimates of survey trawl efficiency into the stock assessment process: a discussion

The efficiency (Q) of survey trawls can now be estimated using a variety of experimental techniques. In this paper, we review these techniques, then consider how the estimates of Q might best be used in stock assessment. Although catch-at-age models such as tuned VPA and Stock Synthesis can estimate the catchability (q) of a survey trawl when the required time series of catch and survey catch per unit effort (c.p.u.e.) data are sufficiently informative, if the time series are short, q estimates may be quite imprecise. In such cases, q in a catch-at-age model might be fixed at a value derived from an experimental estimate of Q. Furthermore, data series are sometimes so deficient that catch-at-age methods can not be used and fishery managers must turn to swept-area estimates of biomass. In these cases, experimentally estimated values of Q may allow less biased estimates of biomass than those now derived under the assumption of Q=1.0.