Selectivity of Gillnets for Sampling Alligator Gar: Effects on Estimates of Growth, Size and Age Structure, and Survival

We fit size-selectivity functions for common multifilament gillnet meshes used for capturing Alligator Gar Atractosteus spatula (88.9, 101.6, 114.3, 127.0, and 139.7-mm bar measure) and used simulations to estimate the effects of sampling bias on commonly used metrics.  Selectivity was best described by a binormal distribution with deviations. We tested a net in which we fished each mesh size equally against a weighted-effort net design created to minimize bias.  Both designs performed well for fish older than 10.  For fish below 10, simulations indicated that either net design would underestimate the proportion of fish below 1,200 mm, leading to a biased length-frequency distribution that affected estimates of most vital statistics.   Estimates of vital statistics for fish older than age ten were less biased, likely because one length class can contain many ages.  Survival estimates were biased by less than 2%.  For fish below age ten, biologists should consider adjusting length frequencies to account for the biases or add smaller meshes.  However, bias adjustments for older fish are probably unnecessary.  Results of this study will allow researchers to either adjust for known size biases or develop bias-adjusted sampling designs that catch a more representative sample of Alligator Gar.