Square mesh panels in North Sea demersal trawls: Separate estimates of panel and cod-end selectivity

We report on trials to compare the selectivity of a trawl with no square mesh panel and trawls with a 90 mm square mesh panel inserted 3–6, 6–9 and 9–12 m from the cod-line. We use smoothers to show that, for haddock and whiting, all three trawls with square mesh panels are more selective than the net with no panel. For whiting, the nets with a panel in the 6–9 and 9–12 m positions are more selective than the net with a panel in the 3–6 m position. The same result is suggested for haddock, but only at a marginal level of significance. For haddock, we also fit a six-parameter model to a subset of the data and hence find separate estimates of the selectivity of the panel and the cod-end. This model reflects the dual selection process of cod-end and panel and is a better fit to these data than the standard logistic curve.