Distribution and community structure of coastal sharks in the northeastern Gulf of Mexico

Coastal shark abundance and community structure was quantified across 10 geographic areas in the northeastern Gulf of Mexico using fishery-independent gillnet data from 2003 to 2011. A total of 3,205 sets were made in which 14,244 carcharhiniform sharks, primarily juveniles, were caught comprising 11 species from three families. The three most abundant species, Atlantic sharpnose Rhizoprionodon terraenovae, bonnethead Sphyrna tiburo and blacktip sharks Carcharhinus limbatus, were consistently captured over all sampling sites regardless of environmental conditions; however, some species (e.g., bull C. leucas, blacknose C. acrontous, finetooth C. isodon, and sandbar sharks C. plumbeus) were restricted to a specific area or a range of areas. Two-way crossed analysis of similarity (ANOSIM) found geographic area to significantly influence shark species-life stage assemblages while season did not. Resemblance matrices between environmental data and shark community assemblage found the two were weakly but significantly correlated, with the combination of salinity and water clarity producing the highest Spearman rank correlation value. Species diversity varied by geographic area, but was generally highest in areas with the greatest amount of fresh and saltwater fluctuations. Our results suggest that estuarine conditions adjacent to river mouths may affect juvenile shark assemblages across similar latitudes and some areas of the northeastern Gulf of Mexico may be considered important nursery areas for select shark species. This study provides important insight into the habitat use of a variety of coastal shark species and can be used to better manage these species through the determination of critical habitat.