Shark Week, Discovery Channel’s annual homage to the ocean’s most infamous predator, comes to a close this weekend.
But residents of northeastern states like New York—long considered a relatively shark-free zone—might not have to wait until July 2019 to see more, as global warming has been linked with a significant northern shift in the habitats of most marine animals, including most sharks.
“There’s an astounding mass migration of animal life towards the poles,” Malin Pinsky, an associate professor in Rutgers’ Department of Ecology, Evolution, and Natural Resources, told The Daily Beast. In his work with spiny dogfish, a thin, small shark that lives along most of the East Coast, he’s seen their habitat shift “quite substantially.”
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Pinsky isn’t the only scientist to make this observation. In April, researchers in North Carolina published a paper in Nature’s Scientific Resources that documented the northern migration of bull shark nurseries.
By analyzing data from North Carolina’s Division of Marine Fisheries (NCDMF), the researchers found that between 2003 and 2011, when water temperatures in the sound were hovering closer to 22 degrees Celsius, only six juvenile sharks were caught in the area. But as temperatures began to rise, a group of bull sharks migrated from their previous home in Northern Florida and established a nursery in Pamlico, causing a drastic uptick in juvenile shark presence. Between 2011 and 2016 alone, NCDMF found 53.
“You hardly ever saw neonates [infant sharks] in the catch record up until 2011,” Roger Rulifson, a biology professor at East Carolina University and one of the authors of the study, told The Daily Beast. “And in 2011, something changed and we started seeing them every year, and in increasing numbers.” Prior to this change, the northernmost documented nursery was in Indian Lagoon, Florida—approximately 400 miles south of Pamlico.
“It seems more so than any instinctive, biological clock kind of process—they’re responding to how hot or cold the water is, ” said Charles Bangley, a postdoctoral research fellow at the Smithsonian’s Fish and Invertebrate Ecology Lab who also worked on the study. And it’s not just sharks: The study points out that this trend of northward migration “has been observed in hundreds of marine taxa.”
Stephen Kajiura, a professor of Biological Sciences at Florida Atlantic University, observed a similar phenomenon in blacktip sharks. Beginning in 2011, Kajiura and his team flew a small, video camera-equipped plane 500 feet over the ocean’s surface biweekly between Boca Raton and Jupiter, Florida (about 75 kilometers, or 47 miles), and counted the number of blacktips—a species that typically summered in the Carolinas and returned to Southern Florida in the winter—in the resulting videos.
His results, published in the journal PLOS One in 2016, showed that shark presence in Southern Florida was “inversely correlated with water temperature.” That is, the warmer the water was, the fewer sharks returned so far south. When water temperatures rose above 25 degrees Celsius, he discovered, there weren’t any groups of large sharks in the area. If temperatures dropped the following year, however, they returned. The North Carolina researchers found something similar for cold water: when Pamlico Sound temperatures dropped below 22 degrees Celsius, they didn’t see any large groups of bull sharks.
While these two factors were correlated, it doesn’t necessarily mean that increased water temperature caused northward shark migration. Instead, Kajiura wrote, the sharks could also be “following their food,” which would be subject to the same temperature-based pressures.
In the years that followed this study and as Florida’s waters have warmed, Kajiura has also found that most sharks have stopped traveling all the way south. In 2018, as water temperatures averaged about 24 degrees Celsius, he saw less than 3,000—compared to more than 12,000 in 2011, when temperatures averaged closer to 23.
But why are marine animals so sensitive to changes in water temperature, even if it’s just a single degree Celsius? Pinsky explains that it’s because they’re ectotherms, which means that they’re unable to regulate their internal temperature and instead must depend on their external environment to do so. Not all fish nor all sharks are ectotherms, he noted: some, like the Mako or the Great White Shark, can increase their body temperature internally.
But those that can’t are extremely picky about the temperature of their external environment. When ocean water is too warm, shark heart rates and metabolisms spike, increasing their oxygen needs. “There isn’t all that much oxygen in seawater, and the fish can’t get the oxygen they need,” Pinsky said. When the water is too cold, on the other hand, “they don’t move as fast, they’re not as efficient predators, and they can’t get the food that they need.”
The ideal temperature varies species by species. Pinsky’s spiny dogfish prefers water between five and 17 degrees Celsius; Kajiura’s blacktips do best between 21 and 25.
No matter the species, the sharks have one thing in common: When global warming strikes, their habitats can drastically change.
And that means new species of sharks are moving into unusual territory like the Northeast region in greater numbers—especially into areas where humans don’t expect them. On July 20, officials confirmed that a 13-year-old boy was bitten by a shark on the shore of Long Island, marking what is likely New York’s first shark attack in 70 years—and only about the tenth in the state’s history.
But scientists have repeatedly emphasized that shark attacks aren’t the only consequences of the northern migration, and that fears of increasing shark attacks are largely overblown.
In fact, shark attacks are actually one of our most minor problems: As The Daily Beast noted in a previous article about President Trump's antagonistic relationship with the creatures, “You’re more likely to die from heart disease, a car accident, or even from being struck by lightning” than you are from a shark attack. On average, only six people worldwide are killed by shark attacks each year. Just last year in 2017, no shark-related fatalities occurred in the United States.
The northward migration could, however, pose a bigger threat to marine ecosystems. Referencing his ecosystem in southern Florida, Kajiura noted that “This ecosystem has been operating for millions of years, with sharks coming down on a regular basis. If you no longer have this influx of these upper-level predators sweeping in [...] you could have an explosion of these mid-level or lower-level fish, because there’s no big ones coming in to eat them, like there were previously. There could be cascading effects on multiple trophic levels.”
Kajiura said these top-level predators could hypothetically be replaced by other sharks moving north to escape their own warming habitat. But he cautioned that there was no guarantee that these events would happen simultaneously.
He also expressed concern for the northeast’s fisheries. “If you had a fishery for striper on Long Island, and you suddenly have these blacktips showing up eating all your stripers, you could have an economic impact for the fishermen.” Rulifson agreed, adding that some fishermen in North Carolina have had their gillnets ripped apart by the new bull sharks, and that the nets are quite expensive to replace.
But for Bangley, the worst thing about northern migration isn’t the risk of potential ecological or economic damage, or even the risk of a shark bite. “They’re actually fairly cute at the size we’re catching them,” he said.
Instead, it’s the major environmental changes that might be causing it. “The sharks are really kind of an indicator species of the bigger ecological changes that are happening as ocean temperatures increase,” he said. “The scarier aspect of this is just the sign of the larger environmental change that’s going on.”
