Sharks Seem To Like City Life. But What Does This Mean For Us?

Our world is rapidly changing.

As the planet continues to become increasingly urbanized, it is expected that by 2050, 70% of its population will live in cities. Today, the World Urban Forum (WUF) is convening in Katowice, Poland until the 30^th of the month, focusing on the theme, ‘Transforming Our Cities for a Better Urban Future.’ On their agenda, individuals will discuss how cities are disproportionately affected by global crises like climate change, the COVID-19 pandemic, armed conflict, and natural disasters.

Perhaps, they will also talk about the urbanization of coasts, which brings with it coastal development… and damage to coastal ecosystems through the destruction of coastal marine habitat and through run-off of sediments and pollution. As we continue to populate our coasts, it is crucial for scientists to figure out how animals will adjust their behavior and habitat use to cope with coastal development.

Based on studies of land predators, scientists at the University of Miami Rosenstiel School of Marine and Atmospheric Science predicted that sharks residing off the greater Miami coast would avoid the densely populated coastline, only appearing when people were scarce (such as weekdays and late hours). But according to a new study by those scientists, they found the opposite.

To determine shark residency patterns across Biscayne Bay, the researchers established an array of underwater receivers to track the movements of three species of sharks off Florida. Between 2015 and 2019, a total of 24 bull sharks (Carcharhinus leucas), 27 nurse sharks (Ginglymostoma cirratum), and 36 great hammerheads (Sphyrna mokarran) were tagged either within or just offshore of Biscayne Bay. The team then set out to evaluate shark residency patterns in relation to proximity to urbanization; to do this, they took the distance from urban area (DUA within the boundary of the city of Miami or Key Biscayne) to a receiver station.

Animals are generally labelled as ‘urban adapters,’ or ‘urban avoiders.’ Urban adapters include the wildlife you normally see scurrying around cities, lurking amongst the shadows like rats, raccoons, pigeons, and opossums. The avoiders are those we rarely see prowling amongst us humans, like wolves, cougars, and bears. When looking at the tracking data, scientists found that “space use patterns of tracked sharks were consistent with that of ‘urban adapters.’” Not only that, but that the “modeling also revealed that an unmeasured spatial variable was driving considerable shark residency in areas exposed to high urbanization.”

Sharks aren’t the only ones who seem to like the city life. For example, previous research found that dolphins in west central Florida have adapted to the construction of a bridge by establishing feeding locations outside of the construction zone, and by shifting their activities to a time of day when construction was minimal. Due to this, scientists have banned together to come up with a framework and guiding principles for monitoring the effect of coastal development on marine mammals.

But why are these sharks hanging around so closely? And could this pose a risk to us? Three reasons why the predators could be more attracted to populated coastlines have been proposed by the research article, pointing to nutrient runoff and sewage discharge creating bottom-up food webs that attract sharks; marine food waste (e.g. the remains of fish being dumped here); and leftover fish from the Miami Seaquarium being discarded into the water. One of the study’s authors, Dr. Neil Hammerschlag told the University of Miami that these sharks could be at risk of “exposure to toxic pollutants as well as fishing, which could impact their health and survival” if they continued to stay too long near the bustling cities.

As for whether the sharks off our coastal urban landscapes poses an increased threat… well, the risk of being bitten by a shark remains extremely low. While Florida has topped global charts in the number of shark bites for decades, with this trend continuing in 2021 with 28 unprovoked bites, the total number of unprovoked shark bites worldwide is extremely low (73 for 2021), given the number of people participating in aquatic recreation each year. The 2021 worldwide total was in line with the most recent five-year (2016-2020) average of 72 incidents annually, according to the Florida Museum of Natural History. The International Shark Attack File (ISAF) offers resources for reducing your risk of a shark bite and instructions for what to do if you encounter a shark, for those who are interested in learning more.