Distributions of Sharks across a Continental Shelf in the Northern Gulf of Mexico

Abstract

Declines in shark populations have sparked researchers and fishery managers to investigate more prudent approaches to the conservation of these fish. As managers strive to improve data collection for stock assessment, fisheries‐independent surveys have expanded to include data‐deficient areas such as coastal regions. To that end, a catch series from a nearshore survey off Alabama was combined with data from a concurrent offshore survey with identical methodology to examine the depth use of sharks across the continental shelf (2–366 m). The combined data set contained 22 species of sharks collected from 1995 to 2008: 21 species in the offshore data set (1995–2008) and 12 species in the nearshore data set (2006–2008). Depth was a significant factor determining species' distributions, primarily for Atlantic sharpnose Rhizoprionodon terraenovae, blacknose Carcharhinus acronotus, and blacktip C. limbatus sharks. Blacknose sharks had the highest catch per unit effort (CPUE) in the middepth stratum (10–30 m), blacktip sharks had consistently higher CPUE in the shallow depth stratum (<10 m), and Atlantic sharpnose sharks showed high abundance throughout both the shallow and middepth strata. Length frequency and sex ratio analyses suggest that Atlantic sharpnose and blacknose sharks are using waters greater than 30 m deep for parturition, whereas adult blacktip sharks are probably using shallow waters for parturition. Our abundance patterns illustrate a continuum of depth use across the inner continental shelf. Surveys that do not encompass the entirety of this ecosystem fail to accurately characterize the distributions of these important predators.

Feeding ecology of three coastal shark species in the northwest Gulf of Mexico

“…Several studies have shown an overlap of range for these 3 species in the GOM (Drymon et al 2010 and along the coast of Texas (Froeschke et al 2010). However, until now there have been few observations regarding their compared feeding strategies in the northwest GOM.…”

“…Sharks within the Order Carcharhiniformes are among the most common species in coastal waters of the GOM (Burgess et al 2005, Drymon et al 2010. Among the most abundant of these are the Atlantic sharpnose Rhizoprionodon terraenovae, bonnethead Sphyrna tiburo, and blacktip Carcharhinus limbatus sharks .…”

“…Atlantic sharpnose sharks also had the lowest mean δ 13 C values, indicating they fed the furthest offshore of the 3 species. They have been shown to have a relatively small home range, having a 95% estimated home range of < 9 km (Carlson et al 2008) as juveniles, little range separation via ontogeny , and are thought to pup offshore (Drymon et al 2010). These trends correlate to the low mean δ 13 C values suggestive of offshore feeding where Atlantic sharpnose shark abundance is highest.…”

Multiscale analysis of factors that affect the distribution of sharks throughout the northern Gulf of Mexico

“…Small-scale sampling occurred at depths up to ~20 m, and large-scale sampling occurred primarily at depths >20 m. Discrete depth preferences for Blacknose Shark have previously been documented. Analyzing the same 2 bottom-longline data sets used in our analyses, Drymon et al (2010) showed a discrete depth preference of 10-30 m for Blacknose Shark. Our data support these fi ndings yet provide no additional insight into why Blacknose Shark occupy these depths.…”

“…Previous studies have examined the distributions of coastal sharks in the northern Gulf of Mexico (Drymon et al, 2010), and we sought to further the approach in these studies by relating spatial trends in shark species assemblages to abiotic and biotic data, including the degree to which these patterns were driven by the availability of potential prey items. Ultimately, we wanted to determine whether patterns in the structure of shark communities and the factors that drive them are independent of scale.…”