Multi‐tissue stable isotope analysis and acoustic telemetry reveal seasonal variability in the trophic interactions of juvenile bull sharks in a coastal estuary

Abstract

Summary1. Understanding how natural and anthropogenic drivers affect extant food webs is critical to predicting the impacts of climate change and habitat alterations on ecosystem dynamics. 2. In the Florida Everglades, seasonal reductions in freshwater flow and precipitation lead to annual migrations of aquatic taxa from marsh habitats to deep-water refugia in estuaries. The timing and intensity of freshwater reductions, however, will be modified by ongoing ecosystem restoration and predicted climate change. 3. Understanding the importance of seasonally pulsed resources to predators is critical to predicting the impacts of management and climate change on their populations. As with many large predators, however, it is difficult to determine to what extent predators like bull sharks (Carcharhinus leucas) in the coastal Everglades make use of prey pulses currently. 4. We used passive acoustic telemetry to determine whether shark movements responded to the pulse of marsh prey. To investigate the possibility that sharks fed on marsh prey, we modelled the predicted dynamics of stable isotope values in bull shark blood and plasma under different assumptions of temporal variability in shark diets and physiological dynamics of tissue turnover and isotopic discrimination. 5. Bull sharks increased their use of upstream channels during the late dry season, and although our previous work shows long-term specialization in the diets of sharks, stable isotope values suggested that some individuals adjusted their diets to take advantage of prey entering the system from the marsh, and as such this may be an important resource for the nursery. 6. Restoration efforts are predicted to increase hydroperiods and marsh water levels, likely shifting the timing, duration and intensity of prey pulses, which could have negative consequences for the bull shark population and/or induce shifts in behaviour. 7. Understanding the factors influencing the propensity to specialize or adopt more flexible trophic interactions will be an important step in fully understanding the ecological role of predators and how ecological roles may vary with environmental and anthropogenic changes.

Conservation challenges of sharks with continental scale migrations

“…The current study was not intended to identify the role that the GBR plays in the ecology of C. leucas, but given the well-known role of rivers and estuaries as nursery areas for this species, and the size of the animals utilizing the GBR, it is likely that it may be an important foraging ground for adults. Previous research has indicated that juvenile C. leucas move to exploit pulsed resources (Matich and Heithaus, 2014) which may be a factor in the movement patterns here, although direct links to feeding on fish spawning aggregations cannot be made at this stage. As such, the potentially large number of individuals that occur on the GBR likely have a key role as large predators in coral reef environments.…”

Sympatric elasmobranchs and fecal samples provide insight into the trophic ecology of the smalltooth sawfish

“…Continued evaluation of fecal material via alternative mitochondrial gene analyses, or via isotopic analysis of structural components with higher rates of protein turnover (e.g. plasma) could provide further insight into dietary differences on short temporal scales (Hussey et al 2012a, Matich & Heithaus 2014. Even with this consideration, our combined results do specify that the smalltooth sawfish feeds on teleost and elasmobranch fishes throughout its life.…”

All nurseries are not created equal: large-scale habitat use patterns in two smalltooth sawfish nurseries

“…The contrasts in habitat use were attributed to differences in geomorphology and habitat availability between estuaries. Similarly, seasonal prey availability influenced movements of juvenile bull sharks Carcharinhus leucas in Florida's Shark River estuarine nursery (Matich & Heithaus 2014), but abiotic variables such as salinity have been more influential on the species in the Ca loosahatchee River (Heupel & Simpfen dorfer 2008). Understanding how species use available habitats is critical for understanding range-wide ecosystem function and for effective management of economically important and protected species.…”

“…Little data from the Peace River contributed to these studies. However, based on lessons learned from other species (Stevens et al 2007, Matich & Heithaus 2014, it is important to study nurseries with a variety of geomorphologies and habitat types to understand how they are being used so conservation can be maximized. Four nursery hotspots were identified in the Caloosahatchee River with varying importance over different years depending on freshwater flow conditions (Poulakis et al , 2013.…”