Multi-Scale Investigation of the Factors Affecting the Distribution of Sharks in the Gulf of Mexico

Successful management of marine populations requires an alignment between ecological and managerial scales. This is particularly true, yet difficult, for large, highly migratory fish such as sharks. Catch data from two fishery-independent bottom longline surveys, as well as multiple ancillary datasets representing primary and secondary productivity, physiochemical parameters and forage fish abundance were examined to better understand the mechanisms driving the distribution of sharks in the Gulf of Mexico (GOM). For this analysis, we considered basin-wide (hundreds of kilometers) as well as regional (tens of kilometers) scales, centered on the 88^th W parallel of the Gulf of Mexico. Multivariate statistical tools were used to examine the relationship between shark community structure and location across the GOM (using centered PCA), and primary/secondary productivity and location (using normed PCA). These matrices were then related using a co-inertia analysis. Results from our basin-scale analysis indicated that Atlantic sharpnose sharks (Rhizoprionodon terraenovae) were associated with high crustacean biomass, while species such as spinner (Carcharhinus brevipinna) and blacktip (C. limbatus) sharks were correlated to fish biomass and ChlA concentrations. Sandbar (C. plumbeus), tiger (Galeocerdo cuvier) and bull (C. leucas) shark distributions were best explained by high DO levels. The same series of analyses were then repeated on a regional scale to examine how scale affects the factors influencing the distribution of these predators in the GOM. Our data suggest the importance of combining and analyzing data at different scales and spanning multiple trophic levels, and have implications for the scales upon which future elasmobranch management plans should be constructed.