Bull shark presence as a function of hydrographic data.

<p>Box and whisker plots (median, interquartile range) of temperature, salinity and dissolved oxygen are shown during periods of telemetered bull shark absence (A) and presence (P) in the lower (red) and upper (yellow) portions of Mobile Bay. Mean values are shown with diamonds, and circles indicate outliers.</p

Stomach contents of red snapper and vermilion snapper from cohabited (sympatric) and single-species (allopatric) sites.

<p>Only categories with %IRI > 1 for either red snapper or vermilion snapper are shown. %W = mean percent by weight without unidentified stomach contents, ±SE indicates the standard error. %IRI = index of relative importance, expressed as a %.</p

Bull shark dynamic habitat use in the upper bay.

<p>Dynamic habitat use of the upper Mobile Bay by bull sharks acoustically detected in 2009 (left column) and 2010 (right column): Temperature vs. Salinity (A, B), Temperature vs. Dissolved Oxygen (C, D), and Salinity vs. Dissolved Oxygen (E, F) are shown. Areas in yellow indicate the available dynamic habitat (data from the NEP mooring stations), and open circles indicate the presence of telemetered bull sharks.</p

Bull shark dynamic habitat use in the lower bay.

<p>Dynamic habitat use of the lower Mobile Bay by bull sharks acoustically detected in 2009 (left column) and 2010 (right column): Temperature vs. Salinity (A, B), Temperature vs. Dissolved Oxygen (C, D), and Salinity vs. Dissolved Oxygen (E, F) are shown. Areas in red indicate the available dynamic habitat (data from the NEP mooring stations), and open circles indicate the presence of telemetered bull sharks.</p

Red and vermilion snapper experimental prey consumption.

<p>(A) Shrimp consumed per fish in each of four treatment groups. Data shown represent consumption by: red snapper alone (RS), red snapper in the mixed treatment (RS mixed), vermilion snapper (VS) in the mixed treatment (VS mixed), and VS alone. (B) Shrimp consumed per attempt across the same four treatments. The boxes represent the interquartile ranges (IQR); the horizontal black line is the median; the red diamond indicates the mean. The whiskers indicate the most extreme value within 1.5 × IQR. All black dots are outliers (further than 1.5 × IQR from the median).</p

Pianka's niche overlap indices.

<p>Values are given for overlap indices for stomach contents collected from all sites, cohabited sites (sympatric), and single-species sites (allopatric).</p

Results of boat-based range testing the WHS 2000 acoustic receivers.

<p>Mean proportion of transmissions detected are shown as a function of distance from the receiver, in meters. Error bars are SE.</p

Summary of predictive model results.

<p>Summary of GLM results for predicting the probability of detecting at least one bull shark as a function of region (upper bay and lower bay) and year (2009 and 2010). Significant values (α<0.005) are indicated in italics.</p

Map of area sampled.

<p>Forty reef locations, twenty artificial (white circles) and twenty natural (red circles) were sampled in the northcentral Gulf of Mexico off the coast of Alabama and Florida. White numbers on the map indicate bathymetry (in meters). Image generated using Esri ArcGIS v 10.3</p

Spatial and Dietary Overlap Creates Potential for Competition between Red Snapper (<i>Lutjanus campechanus</i>) and Vermilion snapper (<i>Rhomboplites aurorubens</i>)

<div><p>Understanding the complex nature of direct and indirect species interactions is a critical precursor to successful resource management. In the northern Gulf of Mexico fisheries ecosystem, red snapper (<i>Lutjanus campechanus</i>) and vermilion snapper (<i>Rhomboplites aurorubens</i>) are two commercially harvested species within a larger reef fish complex. These two species share similar habitats and diets; however, little is known about how these species partition habitat and dietary resources. In this study we examined the extent of spatial and dietary overlap between red snapper and vermilion snapper, and experimentally compared their feeding behavior. Field data from multiple gear types demonstrates that red snapper and vermilion snapper frequently cohabited reefs in the northern Gulf of Mexico, and Pianka’s niche overlap indices suggest significantly overlapping diets. Experimental manipulations show that red snapper are the dominant forager of the two species, as red snapper foraging alone ate more shrimp per fish than vermilion snapper in both the single species (p = 0.003) and mixed species (p = 0.02) treatments. In addition, red snapper ate significantly more shrimp per fish in the mixed species treatment than in the single species treatment (p = 0.04). Vermilion snapper shrimp consumption per fish did not differ significantly between mixed and single species treatments. Cumulatively, our results suggest that spatial and dietary overlap could lead to competition between red and vermilion snapper in the study area; however, conclusively determining the existence of such competition would require further research.</p></div