Indirect and Direct Estimates of Gill-Net Size Selectivity for Northern Pike

Size biases in gill-net sampling of populations of northern pike Esox lucius were described with indirect and direct estimates of gill-net selectivity. In deriving indirect estimates of selectivity, length distributions of northern pike retained in gill nets of five different mesh sizes were compared, and swimming performance relative to body length was used as a measure of the rate at which northern pike encounter gill nets. Selectivity was estimated directly by trapping and marking fish, and then recapturing the fish by gill netting. Indirect estimates showed that gill nets were most effective for retaining northern pike when fish length/mesh perimeter ratio was between 3.5 and 3.7. A generalized gamma function was used to model retention in the gill nets. The length structure of the northern pike population in a small lake was closely predicted when our indirect measure of selectivity was used to correct gill-net catches. Direct estimates of selectivity for all five meshes combined showed increasing selectivity with increasing fish length. Both direct and indirect methods projected increases in efficiency for catching northern pike up to 500 mm in total length. However, the indirect method tended to project higher efficiencies for catching small fish. Results from this study should be useful for interpreting northern pike catches in relation to size selection by gill nets.