Implementation of circle hook regulations in the Gulf of Mexico will impact the length structure and age structure of the snapper-grouper fishery catch as well as demographic data for stock assessments; therefore, an understanding of circle hook selectivity patterns is critical. Indirect selectivity analysis of the vertical-line catch of Red Snapper Lutjanus campechanus and Vermilion Snapper Rhomboplites aurorubens showed that for both species, there were significant differences in mean FL among hook sizes, broad length frequency distributions, and wide selectivity curves. Although the results suggest that hook size regulations could be a useful management strategy for targeting desired size-classes of these snappers, the broadly overlapping length distributions indicate that undersized catch would not be eliminated. Selectivity curves generated from the different families of distributions produced equally good fit to the data and provided a basis for evaluating various selectivity curves when the size structure of the sampled population is unknown. If the size structure of the population is known, then the use of direct selectivity methods is recommended. Right-skewed distributions generally fit the data best, suggesting that hook regulations are likely to be more effective if the desired goal is to reduce the amount of undersized catch by eliminating small hook sizes. Conversely, elimination of large hooks appears to be less likely to reduce the catch of larger size-classes because mouth gape is likely the primary limiting factor and small hooks can catch large fish. Catch rates were significantly different among hook sizes; thus, regulations based on hook size could impact fishing effort and change the dynamics of how the snapper-grouper fishery is prosecuted. Tradeoffs between moderate improvements in size-class targeting, changes in effort, and various components of fishing mortality (i.e., catch and regulatory discards) will require further investigation through simulation modeling or field experimentation.
