Hammerhead Shark Nursery Discovered In Florida’s Biscayne Bay

Few creatures in the ocean world capture the imagination quite like the great hammerhead shark (Sphyrna mokarran). With its unmistakable head resembling a racing car prototype's spare part, this apex predator stands out even among its fellow sharks. As a member of the Sphyrnidae family, the great hammerhead boasts a head (scientifically called a ‘cephalofoil’) that defies conventional expectations, with eyes flanking an aerodynamic spoiler above the mouth – a design that has puzzled researchers for decades. However, despite being an iconic species, much of the great hammerhead’s life remains veiled in mystery. While they frequent Florida’s waters and are often caught by sports anglers, the location of their breeding grounds has eluded marine scientists... until recently.

Enter Biscayne Bay, a bustling stretch of Florida's southeast coast from Miami to the Florida Keys. In this unexpected urban area, researchers, led by Dr. Catherine Macdonald, director of the University of Miami's Shark Research and Conservation Program, believe they have uncovered a great hammerhead nursery. Macdonald and her team stumbled upon this discovery during routine coastal shark surveys, where the presence of juvenile great hammerheads in Biscayne Bay hinted at a crucial nursery habitat for these enigmatic sharks. “We just caught one,” Macdonald says about the first baby great hammerhead they caught in the area in 2018. “I mean, we weren’t really expecting to, we weren’t out looking for them. We were engaged in our normal coastal shark surveys… we know that this area is a nursery area for some other species we monitor. We’re interested in the reproduction of those other species. And one of them just popped up. And we even had that moment of: ‘Are we sure this is a great hammerhead?’”

The researchers initiated a tagging program, supported by National Geographic, to unravel the mysteries of how great hammerheads utilize this habitat as they grow and age.

In 2021, Macdonald formally identified Biscayne Bay as a probable great hammerhead nursery through a peer-reviewed scientific publication. While the bay serves as a vital nursery for young hammerheads, questions about their migratory patterns persist. Recent research suggests that Florida's great hammerheads may cover vast distances throughout the year, with some individuals consistently visiting Bimini in the Bahamas during winter. However, once believed to be genetically robust, the species show signs of inbreeding. Hannah Medd, head of the American Shark Conservancy, points to overfishing as a significant factor contributing to the decline of the great hammerhead population along the east coast of the United States. “There's a level of inbreeding,” says Medd. “Here are these big animals that make these big movements, and we were making assumptions that they were going to be pretty genetically fit.”

Recreational fishing, a popular activity in Florida, poses a particular threat to the great hammerhead. Even if released, the stress from being caught can be lethal, especially for these powerful swimmers who exhaust themselves trying to escape fishing lines. Medd says the ASC has helped compile the first-ever stock take of Florida’s hammerhead population with recreational fishing numbers included: “It’s the first time we’ve ever looked at that group as a whole… and really count all of the information we have from recreational fishing. It’s synthesising all that information to inform management.” Preliminary findings suggest the number of hammerhead deaths are underestimated. As Florida's recreational fishing hotspots become busier, scientists are advocating for stricter protection measures in Biscayne Bay, where rising sea temperatures and climate change-induced effects could impact not only the sharks but also the overall health of the bay’s ecosystem. This doesn’t mean an outright ban - both scientists believe recreational fishing can be used for good in shark conservation and science.

“We’ve caught juvenile hammerheads that have a recreational fishing hook left in their jaws,” Macdonald says. “And on one hand, that’s a very optimistic sign, right, this animal has encountered fishing gear before and obviously survived. On the other hand, it’s a bit of a marker of the kinds of threats that they face.” Great hammerheads can live up to 44 years, making them among the longest-lived cartilaginous fish on Earth. Yet, their slow reproduction rates mean that their numbers are slow to recover from overfishing and other threats.

The nursery’s existence underscores the challenges faced by young hammerheads, particularly from overfishing and recreational angling, emphasizing the delicate balance required for coexistence between human activities and their conservation. Serving as a critical breeding ground, the bay provides a sheltered environment for the survival and development of juveniles during their vulnerable early stages. This not only unveils key insights into their life cycle, migration patterns, and genetic health but also highlights the urgent need for conservation measures in the face of environmental changes and anthropogenic threats. As we continue to unearth the mysteries that surround these species, each discovery underscores the urgent need to protect this extraordinary species.