Relative Abundance of Juvenile Sharks in the Eastern Gulf of Mexico: Combining Multiple Fishery-Independent Gillnet Surveys to Get a Better Big Picture

Several small scale fishery-independent gillnet surveys for coastal shark populations have been conducted in the Gulf of Mexico since 1995.  These surveys were designed to determine shark nursery habitat and provide relative abundance indices as inputs to stock assessment models. Individually the relative abundance trends were limited both spatially and temporally and demonstrated considerable annual variation that could not be explained. As survey designs were similar, data were combined and analyzed using a meta-analysis in a generalized linear mixed model framework to increase the spatial and temporal scope of the abundance index. Factors considered in the model were year, month, location, depth, set time, and effort with a weighing factor (area surveyed) to account for differences in areas sampled. Factors most likely to influence abundance were evaluated in a stepwise forward manner, adding one independent variable and ranking factors from greatest to least reduction in deviance per degree of freedom when compared to a null model.  When analyzed as a combined data set, the final relative abundance index was more parsimonious than survey-specific time series’ and did not suffer from random outliers. This study demonstrates the utility of combining smaller scale surveys into a single time series, providing survey methods are analogous.