Evaluating the Performance of Vertical Longlines to Survey Reef Fish Populations in the Northern Gulf of Mexico

A common critique of many stock assessments is the lack of fishery-independent abundance indices and age composition data. Such data streams are essential in evaluating population trajectories that are derived largely from harvest and age composition of landings. For example, high scientific uncertainty in the most recent stock assessment of Gulf of Mexico red snapper Lutjanus campechanus resulted from a conflict between trends in fishery-dependent and fishery-independent data. Because sample sizes for the latter data were an order of magnitude lower, resolution of the conflicting trends was even more problematic. Recognizing the need for cost-effective expansion of fishery-independent data in the region, we evaluated the performance of vertical longline surveys for sampling reef fish within a large artificial reef zone in the northern Gulf of Mexico. Specifically, we (1) determined species composition and the length frequency of red snapper (the dominant species captured) as a function of hook size and bait type within our survey area during 2010; (2) evaluated the effect of different soak times on catch for various hook types (a combination of hook size and bait type); and (3) utilized our results to test the effect of artificial reef type on red snapper CPUE and mean size. During March-November 2010, we conducted 532 vertical longline sets, capturing 1,217 red snapper that ranged from 184 to 827 mm FL. Mean FL of red snapper differed among hook sizes, with 3/0 and 8/0 hooks sampling smaller fish than 11/0 hooks. Soak time trials revealed a significant effect of soak time on CPUE, with peak catch rates observed at 5 min. As habitat area increased, the mean size and CPUE of red snapper increased. We conclude that our vertical longline is an effective gear for sampling red snapper, and we recommend protocols to maximize its utility and standardize its use.