Some Tiger Sharks Are Migrating Farther North Due to Climate Change

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A tiger shark is swimming. The predator went farther north in the northwestern Atlantic Ocean. Neil Hammerschlag is a writer.

The waters off the northeastern coast of the United States are some of the warmest in the world. Since the 1980s, this part of the Atlantic has warmed by about 2%.

Some species have moved into new areas and others have disappeared from places they used to call home. The cold water habitats favored by Atlantic cod are predicted to nearly disappear off the coast of New England over the next 60 to 80 years, a shift that will severely complicate attempts to revive the fishery after it collapsed in 1992.

The tiger shark is one of the region's top predator and is being affected by the extreme warming. The sharks, which can grow to 15 feet in length, are venturing farther north in the summer and arriving a month earlier than they did in the 1980s. The changing migratory patterns of the sharks track the changing water temperatures of the species they prefer.

The study shows that this large and toothy species is making rapid and significant changes to its range to cope with and take advantage of climate change.

Neil Hammerschlag, a shark researcher at the University of Miami and lead author of the study, says that apex predators help control and regulate their home ecosystems. We don't know how tiger sharks will affect the environment.

The tigers of the sea are spending more and more time outside of waters with some form of protection from commercial fishing as they move farther north.

The tiger shark population is stable. If the commercial fishing industry kills more sharks that could change. Tiger sharks are more vulnerable to fishing threats because they reproduce and grow slowly.

In the northwestern Atlantic, the tiger shark spends the winter in the tropics near Florida or the Bahamas and only ventures north once things warm up in the summer. These summertime jaunts to points north in search of food usually don't extend past Virginia, but can take the sharks as far as Massachusetts.

The tiger sharks prefer the warm water temperatures of 70 to 80 degrees and are arriving earlier in the year and extending farther north than they did 40 years ago. Hammerschlag and his partners asked how tiger sharks were responding to the changing conditions.

The team captured 47 tiger sharks off southeast Florida, southwest Florida and the northern Bahamas and fitted them with satellite tracking devices to keep an eye on their movements.

The team combined the new tracking data with the times and locations of 8,764 tiger sharks that were tagged by scientists and fishers between 1980 and 2018 as part of the National Oceanic and Atmospheric Administration's (NOAA) Cooperative Shark Tagging Program.

A map of where tiger sharks have been going and when has been created. The team used the tiger shark data and the satellite data to create a model of the environmental factors.

The tiger shark's preferred water temperature range has shifted about 186 miles poleward in the cold season and about 248 miles poleward in the warm season over the last year.

The tiger shark hotspots identified by the 40 years of data from the National Oceanographic and Atmospheric Administration 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 average date of tiger shark capture in the 1980s was in early to mid-August, but in the 2010s it was in early to mid-July.

The data from the satellite tracking shows that the last ten years have been the warmest on record. The tiger sharks were farther north in the hottest years. The researchers estimate that tiger shark migrations extend about 250 miles farther north if the ocean warms by 1 degree Celsius.

The temperature was driving the range shifts among the sharks, not ocean productivity or ocean depth.

A marine ecologist at Rutgers University who has been studying the shifting distribution of ocean species for a decade, says that the oceans are warming and it is scrambling marine ecosystems in ways they are only just starting to understand. Tiger sharks are one species but they interact with many other species in the ocean.

Hammerschlag and Pinksy are not sure what the ecological consequences 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.

According to a shark researcher based at the Monterey Bay Aquarium, juvenile great whites have extended their territory up the California coast as the warmer waters they prefer have stretched farther north. Sea otter have shown an increase in fatal munchings as a result of this range expansion.

The juvenile white sharks are not eating the sea otter, they are eating fur, which is almost worthless. Climate change is putting two species that didn't traditionally interact in conflict.

The tiger sharks may be at risk of conflict with humans due to their movements in response to climate change. The authors of the study found that the sharks were spending less time in the marine protected areas that were mostly used for commercial fishing in the southern part of their range.

The northwestern Atlantic tiger shark population doesn't seem to be 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.

The scientific director of the Canada-based Ocean Tracking Network, who was not involved in the study, says that if we want to create effective marine protected areas, we need to understand where animals are and when. The study suggests that existing MPAs may not be as effective in the future. MPAs may need to be more dynamic under climate change.

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