Direct Estimates of Gillnet Selectivity to Walleye (Stizostedion vitreum vitreum)

Abstract

The selectivity of gillnets is usually estimated "indirectly" from comparisons of catches by two or more mesh sizes, assuming that the selectivity curves for all meshes have the same shape and amplitude. We estimated selectivity "directly" by fishing marked walleye (Stizostedion vitreum vitreum) with gillnets of [Formula: see text] (3.81–11.43-cm) mesh. The selectivity curves were bimodal but each could be resolved into two unimodal components representing fish caught by wedging and tangling, respectively. We fitted the selectivity of each mesh to a pair of Pearson’s Type I curves. As the mesh size increased, amplitude of the selectivity curves increased rapidly and tangling occurred over a progressively narrower range of (fish girth)/(mesh perimeter) ratio. Our estimates agreed with data from four out of seven other gillnetting studies, indicating a wider applicability but also a need to consider additional variables or to standardize fishing methods. Because the assumption of equal amplitudes is not realistic, the indirect methods overestimate selectivity curves on the left and underestimate them on the right.