Environmental influences on the occurrence of coastal sharks in estuarine waters

Abstract

Long-term fisheries independent gill net surveys conducted in Texas estuaries from 1975 to 2006 were used to develop spatially explicit estuarine habitat use models for 3 coastal shark species: bull shark Carcharhinus leucas, blacktip shark C. limbatus, and bonnethead shark Sphyrna tiburo. Relationships between environmental predictors and shark distribution were investigated using boosted regression trees (BRT). Bull shark was the most abundant species (n = 5800), followed by blacktip (n = 2094), and bonnethead sharks (n = 1793). Environmental conditions influenced distribution patterns of all species and relationships were nonlinear, multivariate, and interactive. Results showed very good model performance and suggested shark distribution is most closely linked to salinity, temperature, and proximity to tidal inlets. By interpolating the BRT models, maps of the probability of capture were produced using ordinary kriging. Results showed that the central region along the Texas coast contains the most important estuarine shark habitat. This area was characterized by warm temperatures, moderate salinities, and abundant tidal inlets. Bull sharks also extended into low salinity estuaries, while blacktip and bonnethead sharks were restricted to areas near tidal passes with moderate salinities. Juvenile sharks were frequently captured, suggesting the Texas coast may constitute important nursery areas for all 3 species. The development of these spatially explicit models allows for prioritization and conservation of areas in a region that has great potential for human disturbance and climate change impacts. These results provide new insight into the habitat requirements of coastal sharks in the northwestern Gulf of Mexico and practical information for managing this resource. KEY WORDS: Shark · Boosted regression trees · Essential fish habitat Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 407: [279][280][281][282][283][284][285][286][287][288][289][290][291][292] 2010 developing spatially explicit habitat maps for management purposes as animal abundance or productivity is directly linked to the amount of suitable areas available (Stoner 2003, Valavanis et al. 2008. Despite this recognition, delineation of essential habitat has been slow for many species in part because necessary data are often unavailable or analytical techniques have been unable to reliably identify critical habitat from available data. Moreover, predicting distributions of large, rare animals based on habitat characteristics can be difficult (Rooper & Martin 2009). Sampling requires adequate spatial and temporal coverage and must account for a large number of 'zero observations' in the assessment of species such as sharks.A suite of environmental variables has been hypothesized to influence elasmobranch distributions including temperature (Morrissey & Gruber 1993, Matern et al. 2000, Ortega et al. 2009), oxygen concentration (Parsons & Carlson 1998, Heithaus et al. 2009), salinity (Heupel ...

Abundance and Distribution of Sharks in Northeast Florida Waters and Identification of Potential Nursery Habitat

“…Similarly, Castro (1993) and Ulrich et al (2007) documented the presence of sharks in South Carolina estuaries after water temperatures reached ∼19-20 • C. Increasing shark abundance at higher temperatures is also expected. In the coastal waters of Texas, Froeschke et al (2010) showed that shark catch rates increased as temperatures increased between 20 • C and 30 • C, a trend also seen in the present study. Also, coastal waters tend to be warmest during summer months when parturition for species like Atlantic Sharpnose and Blacktip sharks occurs (Castro 2011:509-513), resulting in increased shark catches, particularly of age-0 individuals (Parsons and Hoffmayer 2007).…”

Two‐Stage Boosted Regression Tree Model to Characterize Southern Flounder Distribution in Texas Estuaries at Varying Population Sizes

“…Many estuarine species (including Southern Flounder) spawn offshore and juveniles recruit into estuaries via tidal inlets (Nañez-James et al 2009). As a result, the abundance of such species is often greatest near inlets during the fall spawning season (Whaley et al 2007;Froeschke et al 2010Froeschke et al , 2013b. Our results also indicate greater abundance in the areas furthest from the inlets.…”

“…Soak time was used as an additional explanatory variable to control for effort-related effects on catch (Maunder and Punt 2004). The distance from each sample location to the nearest tidal inlet was calculated using the cost-distance function in ArcGIS (Whaley et al 2007;Froeschke et al 2010). Two cost-distance surfaces were created, one without Packery Channel inlet (which did not open until June 2005; Figure 1) and one that included Packery Channel inlet (which applied to all sampling events after June 2005).…”

“…This is a model-averaging (ensemble) method that allows for both explanation and prediction (Elith et al 2008). Ecological applications of BRTs are recent but increasing rapidly (Friedman 2001;Leathwick et al 2006;Elith et al 2008;Froeschke et al 2010;. This type of model allows for continuous or categorical predictors, accommodates missing values, and is unaffected by data transformations or extreme observations.…”

Anthropogenic modifications to a barrier island influence Bonnethead (Sphyrna tiburo) movements in the northern Gulf of Mexico

“…Species that exhibit such site fidelity provide an opportunity to examine how environmental changes affect movement patterns [14]. While factors that influence Bonnethead movements have been well studied in the eastern GOM [6,16], comparatively less is known about the movements of this species in the northern GOM [but see 17]. Identifying the region-specific factors influencing the distribution of this species is essential for understanding potential impacts induced from anthropogenic alterations to coastal ecosystems.…”