Seasonality and relative abundance within an elasmobranch assemblage near a major biogeographic divide

Nearshore waters are utilized by elasmobranchs in various ways, including foraging, reproduction, and migration. Multiple elasmobranch species have been previously documented in the nearshore waters of North Carolina, USA, which has a biogeographic break at Cape Hatteras on the Atlantic coast. However, comprehensive understanding of the elasmobranch community in this region is still lacking. Monthly year-round trawling conducted along two ocean transects (near Cape Lookout and Masonboro Inlet in 5 to 18 m depth) in Onslow Bay, North Carolina provided the opportunity to examine the dynamics and seasonal patterns of this community using a multivariate approach, including permutational multivariate analysis of variance and nonparametric BIO-ENV analysis. From November 2004 to April 2008, 21,149 elasmobranchs comprised of 20 species were caught, dominated by spiny dogfish (Squalus acanthias) and clearnose skate (Rostroraja eglanteria). All species exhibited seasonal variation in abundance, but several key species contributed the most to seasonal differences in species composition within each transect. Spiny dogfish was most abundant in the winter at both locations, comprised mainly of mature females. Although clearnose skate was caught in all seasons, the species was most abundant during the spring and fall. Atlantic sharpnose (Rhizoprionodon terraenovae) was one of the most abundant species in the summer, and two distinct size cohorts were documented. Temperature appeared to be the main abiotic factor driving the community assemblage. The extensive year-round sampling provided the ability to better understand the dramatic seasonal variation in species composition and highlights the relative abundance of several understudied elasmobranch species that may be of significant ecological importance. Our results underscore the importance of inner continental shelf waters as important elasmobranch habitat and provide baseline data to examine for future shifts in timing and community structure at the northern portion of the biogeographic break at Cape Hatteras.

Methods
For a complete description of this dataset, please refer to our publication in Plos One. Briefly, elasmobranchs were surveyed with a bottom trawl on a monthly basis between November 2004 and April 2008. Sampling was conducted during all months of the year, although certain months are missing during specific years due to funding limitations. Sampling was conducted along two transects that extended from 0.4 to 8 km from shore, one transect was just west of Cape Lookout, the second was north of Cape Fear off Masonboro Inlet. All elasmobranchs (sharks, skates and rays) were counted by species, and up to 30 individuals of each species was measured for length and sexed. This dataset consists of the raw catch data of elasmobranchs, including numbers by species, and lengths and sex of a subsample (no subsample if n < 30 for a give haul and species).