Freshwater nurseries for sharks in a changing world
Climate change may be pushing sea surface temperatures closer to the physiological limits of many sharks. But Alyssa is investigating whether bull sharks, with their capacity to move between marine and freshwater ecosystems, might find a thermal refuge in Florida’s natural springs systems. Based on increased sightings of young bull sharks by Crystal River residents, Alyssa is investigating whether bull sharks might have a stable nursery option in King’s Bay and Crystal River – and is working to characterise and protect their potential climate haven.
Alyssa Andres
I grew up in a small beach town in New England, USA, in a Filipino-American family where traditions of a life intimately connected to the ocean are passed down the generations. As a child I caught and handled fish of all shapes and sizes, which fostered my curiosity and a need to understand the life beneath the waves. After graduating from Wellesley College, I worked as an intern with white sharks in South Africa, and to say I was instantly hooked on elasmobranch research is an understatement. Sharks have always fascinated me, but when I saw their grace...
Evaluation of Floridan geothermal springs system as effective bull shark nursery and thermal refuge habitat
This project aims to categorise Florida’s Kings Bay/Crystal River as an official nursery and thermal refuge habitat for bull sharks and to begin evaluating natural spring-fed estuaries as nursery areas for euryhaline elasmobranch species in an era of climate change.
Climate change may be increasing the importance of coastal submarine freshwater spring systems and the environmental stability they offer for euryhaline species. However, this unique habitat and the nursery and thermal refuge services it provides for aquatic species are degrading under human impacts and environmental shifts. By investigating the reliance of a top marine elasmobranch predator on springs for survival at its most vulnerable stage, we provide support for the restoration and enhanced conservation of both springs and sharks.
As the second largest spring system in Florida, King’s Bay/Crystal River acts as a sanctuary for many key marine resources. Thermal and saline stability makes for ideal nursery habitat and a temperature refuge for thermally sensitive estuarine species such as the bull shark. Bull sharks use areas of low salinity to shelter their young and reduce mortality in their most vulnerable life stage. They have also demonstrated thermal sheltering behaviour when the temperature drops below a tolerable level by seeking refuge in the warm-water outflows of power plants. Warm, stable habitats such as these could offer advantages for the survival of euryhaline elasmobranchs (those, like bull sharks, that can adapt to a range of salinities) in an era of climate change. Young bull sharks in such habitats may be protected from temperature extremes and their residence during the cold winter months may be extended, leading to longer protection from predation than in other known nurseries.
Anecdotal evidence from long-term Crystal River residents suggests an increasing presence of young bull sharks, in contrast to the scarce sightings of three decades ago. This suggests Kings Bay/Crystal River may be a newly colonised or growing bull shark nursery, corresponding with observed northern range shifts of Gulf of Mexico species due to tropicalisation and the recent colonisation of more northerly estuaries as bull shark nurseries in North Carolina. However, no other studies to date have investigated movement or nursery habitat in direct relation to springs’ flow or thermal influence for top elasmobranch predators. As the source waters for spring habitats are a shared resource, springs and the services they provide are being degraded by overuse and human-driven changes. The effective conservation and management of springs and the species associated with them necessitates a robust understanding of species’ reliance on springs for survival and of the increasing need for such stable habitats in a changing climate.
- To officially categorise Kings Bay/Crystal River as a bull shark nursery and effective juvenile habitat.
- To identify the extent to which bull sharks seek thermal shelter in Florida’s natural spring systems.
- To evaluate the efficacy of natural spring-fed estuaries as nurseries for euryhaline elasmobranchs.