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Abstract: In this paper, the paired-gear and covered cod-end methods for estimating the selectivity of trawl cod-ends are compared. A modified version of the cod-end selectivity simulator PRESEMO is used to simulate the data that would be collected from a paired-gear experiment where the test cod-end also had a small mesh cover. Thus, estimates of the selectivity parameters of the test cod-end can be made using both the paired-gear method and the covered cod-end method. These estimates are compared and, as it is assumed that the covered cod-end method is objective, we conclude that the paired-gear method is biased. We demonstrate that extreme parameter estimates as well as discrepancies between the paired-gear and covered cod-end experiments do not necessarily reflect physical or biological mechanisms. We believe that this phenomenon may help explain cases in the literature where the covered cod-end and paired-gear methods produce different estimates of cod-end selectivity. [Copyright &y& Elsevier]

The selectivity of fishing gear can vary considerably from set to set even when deployment is replicated under controlled conditions. Analysis of data from size-selectivity experiments will be misleading if this between-deployment variability is not taken into consideration. Here, two approaches are presented for including the effect of between-haul variability when performing size-selectivity experiments for trawls. The first is a simple ad hoc approach that provides a “catch-all” estimate of variability that also takes into account the effect of subsampling the catches. The second is a formal model of random between-haul variability that is fitted via maximum likelihood. The two approaches are applied to both covered-codend and twin-trawl selectivity experiments. The first method is seen to be more intuitive and interpretable, especially when selectivity may depend on catch size, but is limited in its ability to cope with complex experimental designs. [Copyright &y& Elsevier]