A shark nursery may be a stone's throw from Miami's bright lights

Great hammerheads are odd-looking and mysterious creatures, but scientists may have found the first ever nursery for these enormous sharks – in one of the busiest fishing spots in the world.

Few shark species can grab your attention like the great hammerhead.

As with all members of the Sphyrnidae family, the great hammerhead sports a head that looks less like something a living creature would have and more like an spare part from a racing car prototype. Two eyes sit on either side of an aerodynamic spoiler (technically a "cephalofoil") above the mouth, like lifting surfaces on an aircraft.

After decades of research, we still don't quite know the evolutionary forces that led to the hammerhead's oddly shaped cranium. It's most likely because it helps the species better pick up the minute electric signals generated by their favourite prey – stingrays – hidden under sand. The underside of their head is studded with electroreceptors – pores that detect electromagnetic fields produced by living creatures, although tests have shown they are no more adept at using this sense than other sharks with normal shaped heads. The hammerhead shape can also be used as also be a weapon to help immobilise the sharks' prey. The position of their eyes either side of the cephalofoil gives them better depth perception, and the separation of the nostrils also allows them to "smell" more water as they swim.

Great hammerheads (Sphyrna mokarran) also have another attention-getting quality other than their outlandish head; their sheer size. The largest great hammerhead ever caught was 20ft (6.1m) long, about as long as the biggest confirmed great white. The great hammerhead is one of the largest predatory sharks of all, and an imposing animal to see swimming towards you.

For a shark so large and so physically distinctive, however, much of their lifespan is a mystery. They haunt shallow waters and open oceans across the world, from Australia to the west coast of Africa. The location of their nursing grounds ­– the place where they breed – and give birth to their young – has largely evaded marine science.

But in Florida, marine biologists think they have finally found one.

Great hammerheads are no strangers to Florida's water. They're often targeted by the state's sports anglers and can often be caught close to shore at night when they enter shallow waters to hunt for prey. But a steady population of adult sharks does not necessarily mean they are breeding locally.

Catherine Macdonald, director of the University of Miami's Shark Research and Conservation Program, believes researchers have finally found a great hammerhead nursery. It's not in some remote corner of the state's remaining protected mangrove forests, but in one of the most developed stretches of the state's southeast coast: Biscayne Bay, which stretches south from Miami to the Florida Keys.

Like many other large shark species, great hammerheads give birth to live young rather than laying egg cases. The tiny juveniles spend their first year or so feeding on small fish in the sheltered shallow waters of the bay, and blend in with other young hammerhead species, making them much more difficult to identify.

"We just caught one," Macdonald says, referring to the first baby great hammerhead 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?'

"We were like, 'that's weird'. But we catch weird stuff from time to time. There's plenty of species that we only see maybe a couple times a year. but then we kept catching them [great hammerheads] in these smaller size classes. And at a certain point, we said, 'This is more than a fluke'."

With the support of National Geographic, Macdonald and her team started a tagging programme in an attempt to find out how great hammerheads' habits change and expand their use of this habitat as they grow and age.

Macdonald emphasises that, for many shark species, there isn't enough evidence to determine exactly where nursery areas are. "Even for very well studied species, like the great white, there are still questions about pupping locations and juvenile habitat use, although we're starting to get answers to some of those. And of course, mating, which can be even more difficult to track down."

It's much easier to understand shark habitats for species that give birth in shallow coastal areas, continues McDonald. "Because our understanding of shallow coastal habitats that we spend a lot of time in and near is a lot better."

In 2021, Macdonald published a paper identifying Biscayne Bay as a probable great hammerhead nursery. Juveniles are seen there year after year, and juveniles are more likely to be found there than elsewhere, although continuing research will answer questions about whether young sharks remain in the Bay for extended periods.

Tagging small sharks like juvenile great hammerheads can often be a challenge, Macdonald says, because it requires a totally different approach than tagging big ocean-going sharks.

"One of the main ways that we gather data on large pelagic sharks is with satellite tags. But those tags are awfully large for small sharks to carry," she says. "We study our juveniles mainly using acoustic tags, which are internally implanted and have a 10-year lifespan. But they only work if an animal passes near receivers. They work much better for shallow coastal environments that have good receiver coverage than they do for sort of pelagic and offshore environments."

It's now believed that great hammerheads may cover large migratory distances over the year, but exactly where Florida's population head to remains a mystery. The same sharks seem to visit Bimini in the Bahamas during winter year after year, according to research from shark scientist Dave Portnoy.

The larger the great hammerheads grow, the more they become a target – both for commercial shark fishing operations, and recreational anglers, says Hannah Medd, the head of the American Shark Conservancy (ASC). Based in West Palm Beach, north of Miami, Medd wanted to find out more about Florida's population of great hammerheads and how genetically healthy they are, which might be a possible clue as to how far they're travelling.

One recent study into the genetics of great hammerheads, conducted by Nova Southeastern University, has provided some surprising insights. "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."

Medd's programme tagged 23 great hammerheads over a five year period, mostly thanks to the help of local sport anglers. The programme revealed that even in an area with a known resident population, apex predators like the great hammerhead are a rare sight – and the population seems to have declined thanks to the effects of overfishing.

"I think their numbers are naturally low, and they're big animals," she says. "They have been impacted by overfishing for sure, along the east coast of the United States."

Florida has a lot of recreational anglers. More than a million people in the state currently have a licence for salt water fishing. "Florida is such a hotspot for sport fishing," says Medd. "And guys usually target the large sharks, that's the more fun fight."

Medd is keen to examine the impact that recreational fishing is having on the great hammerhead population in Biscayne Bay. Part of the reason for this is because great hammerheads are a species which are more affected by catching than some other species. Even if the fish are put back after being caught, the effects from being caught can be lethal.

Great hammerheads are such powerful swimmers that they essentially exhaust themselves to death trying to escape a fishing line, explains Macdonald. "They exceed their capacity to breath, to process the toxic by-products generated by exercise… to the point that they can't survive."

Nurse sharks, by contrast, cope relatively well with being caught. "When we hook a nurse shark, it goes in, it sits on the bottom, and it waits till you haul it up. And so it's not tired at all when you bring it up, right, it's just kind of grouchy." Commercial longline fishing boats may have their lines in the water for up to 12 hours at a time, she says. "If one is hooked for that length of time, survival rates are usually very low."

The hammerheads killed by recreational fishing are harder to count, especially if a shark is alive when unhooked but dies later from shock. "Those mortality numbers aren't always captured," says Medd. "So they're not considered in the management, which may be leading to over-harvesting."

Medd says the ASC has helped compile the first-ever stock take of Florida's hammerhead population. "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." While the study won't be published before the end of the year, preliminary findings suggest the number of hammerhead deaths are underestimated, says Medd.

Medd and Macdonald, however, both believe the recreational fishing industry can be a help rather than a hindrance as long as it's carefully managed.

"Even if you don't care about it for other reasons, there tends to be a sense that you want this stuff to be around for your kids and your grandkids if it's meaningful to you," says Macdonald.

Biscayne Bay is already the site of a national park, but scientists are calling for the park to have stricter protection measures. It's already thought that climate change-induced rising sea temperatures will have an adverse effect on the corals in and outside the bay. They could also affect the health of young, developing sharks.

"We've had just an unprecedentedly hot summer here in Miami, says Macdonald. "Inside the bay, we had water temperatures at or approaching 100F (38C). That's human blood temperature."

As cold-blooded animals, the temperature of the water around great hammerheads helps the sharks regulate their internal temperature, says Macdonald. "In general, we know that we see a more pronounced stress response to something like capture in warmer water. I also worry that some of these human-related threats will become more intense as water warms because of the animal's metabolic response to that warmer water." Even if the threat of recreational fishing is the same, she continues, a shark might be less likely to survive an encounter with a fisherman.

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"Our evidence says that, yes, these animals are using habitats outside of Biscayne National Park," says Macdonald. "Those habitats are important to them. Biscayne National Park has slightly more stringent fishing regulations than the rest of Florida waters, but not vastly so. I think there's a lot of room to make adjustments."

Left untouched, great hammerhead sharks can live up to 44 years, making them among the longest-lived cartilaginous fish on Earth. They give birth to relatively few young after a year-long gestation, meaning their numbers are slow to recover. Yet they also have surprisingly large brains crammed into their oddly shaped skulls – they are fairly large compared to their body mass and have high levels of foliation, a measure of brain complexity.

It is clear there is still much we have left to learn about these strangely shaped and mysterious creatures. And, if they rely on the warming, busy waters off Florida 's coast to nurse their young, they have a tough future ahead.

"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."

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