A new study from the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science finds that juvenile great hammerhead sharks (Sphyrna mokarran), a critically endangered species, rely heavily on the resources of Florida's Biscayne Bay as a nursery habitat during their earliest and most vulnerable years.
Nestled within the heart of the Miami metropolitan area, Biscayne Bay is a shallow, subtropical estuary known for its clear waters and ecological richness. However, in recent decades, the bay has suffered from declining water quality, reduced freshwater inflow, and physical degradation—largely driven by urban development and rapid population growth—posing growing risks to the species that depend on it.
"We analyzed the feeding and habitat use patterns of 62 great hammerheads sampled between 2018 and 2025," said John Hlavin, the lead author of the study and a doctoral student in the Department of Environmental Science and Policy at the Rosenstiel School. "Our team employed a research technique called multi-tissue stable isotope analysis to track both short and long-term resource use, allowing us to gain new insights into the species' dietary habits across different life stages."
The findings indicate that young sharks rely on Biscayne Bay's shallow inshore habitats and prey species year-round for the first two years of their life. After age two, subadult hammerheads shift to foraging on coastal reefs, but return to Biscayne Bay seasonally, from late spring through early summer. Many adult hammerheads continue to feed on bay resources, indicating that nearshore habitats remain important throughout the great hammerhead's life cycle.
"Juvenile great hammerheads show a constrained diet and habitat use, potentially feeding heavily on small inshore stingrays early in life," said Catherine Macdonald , a research associate professor in the Department of Environmental Science and Policy, and the director of the Shark Research and Conservation Program at the Rosenstiel School. "This reliance on a narrow range of prey and habitats makes juvenile great hammerheads particularly vulnerable to human activity and environmental change."
Miami is a popular recreational fishing destination; however, great hammerheads are exceptionally sensitive to capture stress. The study's findings point to a need for responsible fishing practices, especially from March through July, when both juveniles and subadults inhabit the bay.
"Reducing interactions with these sharks—particularly avoiding catch-and-release fishing in key nursery areas—can dramatically improve survival for this endangered species," said Hlavin. "If a shark is accidentally caught, the best action is a quick, in-water release, without photographs that delay the release process."
Methodology
Researchers used stable isotope ratios of carbon and nitrogen in muscle and blood plasma samples to evaluate great hammerhead diet and habitat use across age classes and seasons. This non-lethal method provides insights into both recent and long-term feeding behaviors, revealing how their reliance on Biscayne Bay changes as these sharks mature.
A call for increased conservation measures
This study offers critical data to inform conservation strategies and marine policy in Florida and beyond. Protecting essential nursery habitats like Biscayne Bay are vital to giving great hammerheads a fighting chance in the face of environmental degradation, habitat loss and overfishing.
This research was supported by the National Geographic Society, Nature Trust of the Americas, Florida Sea Grant-Guy Harvey Fellowship, and the Annual Mary Roche Fellowship at the University of Miami Rosenstiel School.
The study titled, "Nursery resource use dynamics in great hammerheads (Sphyrna mokarran) across ontogeny," was published in the journal Ecology and Evolution on June 15, 2025. The authors are John F. Hlavin*1,2, Catherine C. Macdonald1,2
1.Shark Research and Conservation Program, University of Miami Rosenstiel School for Marine, Atmospheric, and Earth Science, Miami, Florida. 2. Field School Scientific Training, Coconut Grove, Florida.
About the University of Miami and Rosenstiel School of Marine, Atmospheric and Earth Science
The University of Miami is a private research university and academic health system with a distinct geographic capacity to connect institutions, individuals, and ideas across the hemisphere and around the world. The University's vibrant academic community comprises 12 schools and colleges serving more than 19,000 undergraduate and graduate students in more than 180 majors and programs. Located within one of the most dynamic and multicultural cities in the world, the University is building new bridges across geographic, cultural, and intellectual borders, bringing a passion for scholarly excellence, a spirit of innovation, and a commitment to tackling the challenges facing our world. The University of Miami is a member of the prestigious Association of American Universities (AAU).
Founded in 1943, the Rosenstiel School of Marine, Atmospheric, and Earth Science is one of the world's premier research institutions in the continental United States. The School's basic and applied research programs seek to improve understanding and prediction of Earth's geological, oceanic, and atmospheric systems by focusing on four key pillars:
*Saving lives through better forecasting of extreme weather and seismic events.
*Feeding the world by developing sustainable wild fisheries and aquaculture programs.
*Unlocking ocean secrets through research on climate, weather, energy and medicine.
*Preserving marine species, including endangered sharks and other fish, as well as protecting and restoring threatened coral reefs. www.earth.miami.edu .
