Transitioning to the North American standard gill net: Size selectivity corrections and the effects of net design on CPUE, size structure, and site selection

The Oklahoma Department of Wildlife Conservation (ODWC) has used Standardized Sampling Procedures (SSP) to monitor fish populations in Oklahoma waters since 1977. The gill net configuration suggested by Miranda and Boxrucker (2009) for the entire southern USA was recently adopted by ODWC. This change in standard sampling gear warranted the comparison of catch rates, variability, and length frequency distributions of the old and new net configurations. Fixed site sampling has been the standard for ODWC gill netting although random sites are thought to be less biased (Wilde and Fisher 1996). Therefore, comparisons were also made between fixed and random sites using the new net configuration. Gill nets are known to be size selective, but this bias can be corrected with the use of selectivity curves. I used the SELECT method to find the best-fit selectivity models to adjust the gill net catch for channel catfish, white crappie, white bass, hybrid striped bass, saugeye, and walleye. I then examined corrected length frequencies and size indices to assess the magnitude of these corrections.;There were no significant differences in catch rates between the old and new net configuration for four of the six target species. The catch rate variability of the new nets at fixed sites was lower or similar to the old nets, except for hybrid striped bass, which had higher variability in the new nets. Length-frequency distributions differed between the two net types for all lakes where channel catfish were sampled and one third of the lakes where hybrid striped bass were sampled. Only white bass had a significant difference in catch rate between fixed and random sites. Length frequencies were typically unaffected by fixed or random sampling. The bimodal model was the best fit selectivity model for all species. One third of the hybrid striped bass length distributions and two thirds of the white bass length distributions differed after adjustments were made using selectivity correction curves. Roughly one third of PSDs showed meaningful changes after selectivity adjustments were made. By correcting for contact selectivity the data are always improved (even if only subtly), and at times the adjustments can be large enough to alter management decisions. Therefore, I suggest that selectivity adjustments should become a part of routine data analysis for the North American standardized gill net design as they improve data for fisheries management.