Factors shaping the co-occurrence of two juvenile shark species along the Texas Gulf Coast
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2017, Springer-Verlag Berlin Heidelberg. As top predators, sharks play important ecological roles in coastal marine ecosystems. Yet, environmental changes in many coastal regions are likely altering the composition of ecological communities, the interspecific interactions sharks have within coastal waters, and subsequently the ecological roles sharks play within these ecosystems. As such, understanding interactions among sharks, and how extrinsic factors shape these interactions, is important for predicting the consequences of future human impacts and environmental changes. Elucidating the contexts under which species co-occur and the implications of co-occurrence is an important step in developing such a predictive framework. Blacktip shark (Carcharhinus limbatus) and bull shark (Carcharhinus leucas) concentrations were quantified using long-term coastal gill net survey data across five bay systems in Texas. Relationships between co-occurrence, and environmental factors and shark sizes were examined within and across species. Co-occurrence of blacktip sharks and bull sharks varied spatially and temporally, with a significant increase in interspecific co-occurrence from the 1970s to 2010s, and a significant decrease in bull shark concentrations through time. Changes in environmental conditions, specifically increasing salinities, may have been responsible for increased blacktip and bull shark co-occurrence, and potential interspecific competition, which in turn may have led to decreased bull shark concentrations to reduce intraspecific competition. However, more refined questions are needed before predictive frameworks can be developed concerning the contexts under which co-occurrence is prevalent. Quantifying resource use among coastal sharks will help elucidate the drivers and implications of co-occurrence, and the potential for competitive interactions within and across species.
