Length Selectivity and Size-Bias Correction for the North American Standard Gill Net

Gill nets are inherently size selective, but selectivity curves can correct this bias. We sampled eight reservoirs with the North American standard gill net to develop a large length-specific data set for six species: Channel Catfish Ictalurus punctatus, hybrid Striped Bass (White Bass Morone chrysops x Striped Bass M. saxatilis), saugeye (Sauger Sander canadensis x Walleye S. vitreus), Walleye, White Bass, and White Crappie Pomoxis annularis. We then used the SELECT (share each lengthclass's catch total) method to find the best-fit selectivity model to adjust the gill-net catch for contact selectivity. To determine the magnitude of these selectivity corrections, we compared adjusted and unadjusted length frequencies and size indices for each species at each reservoir. The bimodal model was the best fit selectivity model for all species. When selectivity-adjusted length-frequency data were compared with the original data, one-third of hybrid Striped Bass length frequencies and two-thirds of White Bass length frequencies were significantly different (unadjusted distributions underestimated smaller length classes). Roughly one-third of the proportional size distributions (PSDs) from all species analyzed showed meaningful changes (>= 5 PSD units) after selectivity adjustments were made (unadjusted PSDs were too large). By correcting for contact selectivity the data are improved, and at times the adjustments can be large enough to alter management decisions. Therefore, we recommend that selectivity adjustments should become a part of routine data analysis for the design of the North American standardized gill net as this will improve data for fisheries management.