Drymon et al.: Factors that affect the distribution of sharks throughout the northern Gulf of Mexico 
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Figure 1 
Spatial extent of the area used in our small-scale analysis of shark distribution in the northern Gulf of Mexico during 
2006-09. (A) Eight blocks (1-8, west to east), which spanned depths from 1 to ~20 m, where the shark bottom-longline 
survey was conducted during all months by the Dauphin Island Sea Laboratory. (B) Sample locations for the small- 
scale bottom-longline data during 2006-09 (filled circles) and trawl data during 2007-09 from the Southeast Area 
Monitoring and Assessment Program database (http://seamap.gsmfc.org) (open circles). 
concerning spatial patterns and distributions of shark 
communities in coastal marine systems is still needed 
before resource managers can successfully incorporate 
sharks into sustainable ecosystem management plans 
(Heithaus et ah, 2007). 
Long-term, fishery-independent monitoring pro- 
grams are one of the most common ways to assess 
spatial patterns for marine vertebrates. The NOAA 
Southeast Fisheries Science Center (SEFSC) Mississip- 
pi Laboratories have been conducting annual bottom- 
longline surveys to assess patterns of shark distribu- 
tions across the entire northern Gulf of Mexico since 
1995, and the data from these surveys are incorporated 
into stock assessments that ultimately shape fishery 
management plans for these animals. Given the im- 
portance of merging biological scales with the scales 
of fisheries management, we sought to examine spatial 
patterns in assemblages of shark species on the scale 
of the northern Gulf of Mexico and to investigate to 
what extent those patterns in shark communities were 
present regionally, along the coasts of Mississippi and 
Alabama. 
The goal of this investigation was to characterize 
the spatial distribution of shark communities in coastal 
waters of the northern Gulf of Mexico. 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 po- 
tential prey items. Ultimately, we wanted to determine 
whether patterns in the structure of shark communi- 
ties and the factors that drive them are independent of 
scale. We predict this multifaceted approach will allow 
for a more precise understanding of the determinants 
of the spatial distributions of these predators in na- 
ture and for a definition of appropriate management 
measures. 
Materials and methods 
Small-scale study site 
A bottom-longline survey was initiated in May 2006 by 
the National Marine Fisheries Service (NMFS) and the 
Dauphin Island Sea Lab (DISL). During this survey 
sharks were sampled from waters at depths of 1-20 m 
along the Alabama and Mississippi coastlines (Fig.l). 
Sampling occurred during all months (January-Decem- 
ber) on NMFS research vessels (all 20-30 m in length), 
such as the RV HST, RV Gandy, and RV Caretta. A 
stratified random block design was used and 8 blocks 
were established along the combined coast of Missis- 
sippi and Alabama. Each block was ~10 km east-west 
and extended from the shoreline to approximately the 
20-m isobath. Blocks 1-4 were located west of 88°00 W 
(western blocks), and blocks 5-8 were located east of 
88°00 W (eastern blocks) (Fig. 1A). Sampling was allo- 
cated evenly and replicated within each block. For this 
study, we analyzed data collected in 2006-09 as part 
of this survey. 
Small-scale sampling methods 
Between 12 and 16 stations were randomly selected 
and sampled each month using a stratified random 
