Certain kinds of wildlife are notorious for thriving in urban settings. Think rats, rock pigeons and even the occasional coyote. Now, Florida scientists have added another creature to the list: sharks.
While many large predators show little appetite for city living, an intriguing project tracking the movements of sharks as fearsome as hammerheads revealed the fish are unexpectedly tolerant of life up close to the 6 million humans of greater Miami.
“We were surprised to find that the sharks we tracked spent so much time near the lights and sounds of the busy city, often close to shore, no matter the time of day,” said Neil Hammerschlag, director of the University of Miami’s Shark Research and Conservation Program.
Ecologists group animals into two main categories when it comes to their tolerance for human development. Some, like raccoons or rats, have figured out how to capitalize on the trash we make and the nooks and crannies we build. They are “urban exploiters.” Then there are the animals like mountain lions, lynxes and wolves that generally give human infrastructure a wide berth, often abandoning habitat where roads or buildings encroach. These are the “urban avoiders.”
As that list suggests, on land, big, toothy predators generally keep their distance from the din of the city. But less is known about their aquatic counterparts. So, a group of researchers set out to see if the sharks of Miami’s Biscayne Bay might shed some light on the matter.
The bay, a 730-square kilometer lagoon extending from the Atlantic Ocean, offers a kind of natural laboratory for seeing whether sharks prefer city living or, like many other large predators, seek quieter waters. The bay’s northern end is home to the crowded shores of downtown Miami, thick with marinas, noisy boat traffic and light pollution. At the southern end of the bay, meanwhile, lie the mangrove forests and quiet reefs of Biscayne National Park.
To monitor shark behaviour, the researchers caught three different species of sharks there—the relatively sedate nurse sharks, bull sharks, and the biggest of the bunch, great hammerheads, which can grow as long as a minivan. They attached small radio transmitters to the sharks, either by surgically implanting them or, in the case of the hammerheads, piercing the skin and leaving it outside the shark’s body.
Once the fish were released, the scientists tracked their movements with a network of 40 underwater receivers that picked up signals from nearby sharks, telling one individual from another, as well as measuring how long they stayed close to a particular spot. Between 2015 and 2019, the receivers heard 27 nurse sharks, 13 bull sharks and 14 great hammerheads.
These scientists expected the sharks would show a preference for the less urban areas. To the degree that they did swim near the city, they thought the fish might spend more time there at night or on weekdays compared to places further south, because there would be less boat traffic and other human activity on the water.
But when they crunched the data, that’s not what they found. Instead, all three species showed no significant preference for either type of location. Nor did the sharks show any particular preference for nights or weekdays in the city when compared to other parts of the bay, according to findings published June 16 in the journal Marine Ecology Progress Series.
In short, when it came to cities, they acted more like raccoons than wolves. The scientists aren’t sure why. It’s possible that aquatic environments are different, because humans erect less hard infrastructure directly in the shark’s habitat, such as traffic-filled roads. Sharks might also be lured close to cityscapes by the promise of an easy lunch from angles discarding fish carcasses at marinas or the Miami Seaquarium, much like grizzly bears once feasted at Yellowstone National Park’s dumps. In fact, sharks lingered near the Seaquarium far longer than anywhere else.
Much as the grizzlies ran into trouble, Florida researchers warn this seeming tolerance of urban living could be bad for both sharks and people. Sharks in Biscayne Bay have high levels of mercury and algae-based poisons, and evidence of poorer diets. On the flip side, anything attracting sharks to human activity could increase the risk of people being bitten (a relatively rare occurrence).
“By spending so much time close to shore, sharks are at risk of exposure to toxic pollutants as well as fishing,” said Hammerschlag, “which could impact their health and survival.”
Hammerschlag, et. al. “Urban sharks: residency patterns of marine top predators in relation to a coastal metropolis.” Marine Ecology Progress Series. June 16, 2022.
Image: A researcher leaning over the water releases an acoustically tagged nurse shark off the coast of Miami. (Photo courtesy Robbie Roemer)