The waters off the northeastern coast of the United States are some of the fastest warming in the world. Human-caused climate change has warmed this part of the Atlantic about 2.7 degrees Fahrenheit since the 1980s.
These rapid, profound alterations have rewired marine ecosystems, causing some species to move into new areas and others to disappear from places they once called home. The cold water habitats favored by Atlantic cod, for example, are predicted to nearly disappear off the coast of New England over the next 60 to 80 yearsâ€”a shift that will severely complicate attempts to revive the fishery after it collapsed in 1992.
New research published today in Global Change Biology shows that this extreme warming is also changing the migrations of one of the regionâ€™s apex predators: the tiger shark. These sharks, which can exceed 15 feet in length and dine on anything from sea turtles to lobsters to car parts, are venturing roughly 270 miles farther north in the summertime and arriving about a month earlier than they did in the 1980s. The sharksâ€™ changing migratory patterns almost perfectly track the shifting water temperatures the species most prefers.
The study, which combines nearly ten years of satellite tracking data with 40 years of catch records, shows that this large and toothy species is making swift and significant changes to its range to cope with and take advantage of climate change.
â€œApex predators help control and regulate their home ecosystems,â€ says Neil Hammerschlag, a shark researcher at the University of Miami and lead author of the study. â€œBut we donâ€™t know exactly how tiger sharks will impact the ecosystems theyâ€™re moving into.â€
The findings also have implications for the speciesâ€™ conservation because as these tigers of the sea move farther north, theyâ€™re spending more and more time outside of waters with some form of protection from commercial fishing.
â€œThe wild tiger shark population is relatively stable,â€ says Hammerschlag. â€œBut if these sharks start getting killed by the commercial fishing industry in greater numbers that could change. Tiger sharks reproduce and grow slowly, which makes them more vulnerable to threats like fishing.â€
In the northwestern Atlantic, the tiger sharkâ€™s typical seasonal patterns are not unlike those of certain wealthy retirees: they spend the winter months in the tropics near Florida or the Bahamas and only venture farther north once things warm up in the summer. These summertime jaunts to points north in search of food usually donâ€™t extend past Virginia, but can take the sharks as far as Massachusetts.
As climate change has cranked up the heat in the ocean, the balmy water temperaturesâ€”between 70 and 80 degreesâ€”that tiger sharks prefer are arriving earlier in the year and extending farther north than they did 40 years ago. That shift led Hammerschlag and his collaborators to ask the question: how were tiger sharks responding to these changing conditions?
To figure out the answer, the team captured 47 tiger sharks off southeast Florida, southwest Florida and the northern Bahamas and outfitted them with satellite tracking devices to monitor the sharksâ€™ movements from 2010 to 2019.
To add historical context, the team combined these new, detailed tracking data with the times and locations of 8,764 tiger sharks that were caught and tagged by scientists and fishers between 1980 and 2018 as part of the National Oceanic and Atmospheric Administrationâ€™s (NOAA) Cooperative Shark Tagging Program.
These combined datasets created a multi-decade map of where tiger sharks in this region have been going and when. The team also layered NOAA satellite measurements of sea surface temperatures, phytoplankton presence and ocean depth onto the corresponding times and locations from the tiger shark data so the researchers could assess the relative influence of these environmental factors.
The study found that the northern edges of the tiger sharkâ€™s preferred water temperature rangeâ€”which measurements taken from thermometer-equipped shark tags suggest is between 78 and 82 degreesâ€”has shifted about 186 miles poleward in the cold season and about 248 miles poleward in the warm season over the last 40 years.
In parallel, the seasonal tiger shark hotspots identified by NOAAâ€™s 40 years of tagging data saw their northern edges shift about 186 miles north in the cold season and 270 miles north in the warm season compared to the 1980s.
The catch data also showed the tiger sharks were moving earlier in the year: the average date of a tiger shark capture in the 1980s was in early to mid-August, but in the 2010s that date rolled all the way back to early to mid-July.
The satellite tracking data spanning nine out of ten years in the warmest decade on record for ocean temperatures (the 2010s) echoed these findings. In the hottest years, the tiger sharks roamed farther north earlier in the year. Based on these data, the researchers estimate that for every 1 degree Celsius (1.8 degrees Fahrenheit) of anomalous ocean warming, tiger shark migrations extend roughly 250 miles farther north and begin their summertime road trips about 14 days earlier.
Additional statistical analysis suggested that it was indeed temperature that was primarily driving these range shifts among the sharks, rather than ocean productivity or ocean depth.
â€œThe oceans are warming and itâ€™s scrambling marine ecosystems in ways weâ€™re only just starting to understand,â€ says study co-author Malin Pinsky, a marine ecologist at Rutgers University whoâ€™s been studying the shifting distribution of ocean species for a decade. â€œTiger sharks are just one species but because theyâ€™re top predators they also interact with many other species in the ocean.â€
Hammerschlag and Pinksy arenâ€™t sure what the ecological fallout of a large predatory shark pressing farther north as the seas heat up will be, but off the coast of California, an even more famous apex predator is offering an example of the unpredictable new interactions that can occur.
Salvador Jorgensen, a shark researcher based at the Monterey Bay Aquariumâ€™s Research Institute who wasnâ€™t involved in the study, says juvenile great whites have extended their territory up the California coast as the warmer waters they prefer have stretched farther north. This range expansion has led them to overlap with sea otters, which have shown an accompanying uptick in fatal munchings.
â€œItâ€™s not that these juvenile white sharks are eating the sea otters,â€ says Jorgensen, â€œthey insulate themselves with fur not blubber so as food theyâ€™re almost worthless.â€ But the rising incidence of these fatal exploratory bites are an example of â€œclimate change putting two species that didnâ€™t traditionally interact in conflict,â€ he says.
Unfortunately for the tiger sharks, their movements in response to climate change may bring them into increased conflict with an exceptionally dangerous species: humans. When the study authors overlaid the sharksâ€™ new movements with marine protected areas (MPAs) they found that the farther north the animals ventured the less time they were spending in the MPAs that had mostly protected them from commercial fishing in the southern part of their range.
Right now, the northwestern Atlantic tiger shark population doesnâ€™t appear to the showing any ill effects, but the findings of this study suggest we need to keep a close eye on these sharks in the years to come, says Mikki McComb-Kobza, a conservation biologist at the University of Colorado and the executive director of the Ocean First Institute who wasnâ€™t involved in the research.
â€œIf we are concerned about marine biodiversity and want to create effective MPAs we have to understand where animals are and when,â€ says Sara Iverson, scientific director at the Canada-based Ocean Tracking Network and who was not involved in the study. â€œThis study suggests that for this species existing MPAs may be less effective going forward. Under climate change, MPAs may need to be more dynamic depending on what we are trying to protect.â€