Warmer sea surface temperatures have led to a bull shark population increase, scientists say

Not all animal species are experiencing a detrimental effect due to warming global temperatures.

The number of bull sharks -- one of the most aggressive species of shark in the world -- increased significantly in a 20-year period, and researchers say warmer seawater is to blame.

Increasing sea surface temperatures over the past 20 years have coincided with a five-fold increase in the abundance of juvenile bull sharks in Mobile Bay, an estuary in Alabama, according to a study published Thursday in Scientific Reports.

Bull sharks are among the shark species most likely to have a negative interaction with humans, along with the great white shark and the tiger shark.

Researchers measured changes in the distribution and abundance of bull sharks in Mobile Bay using data from 440 bull sharks captured and released during surveys conducted between 2003 and 2020 by the Alabama Department of Conservation and Natural Resources, which has been conducting a gill net survey of the Alabama coastline, Lindsay Mullins, a Ph.D. student at Mississippi State University's wildlife, fisheries and aquaculture department and lead author of the study, told ABC News.

Environmental factors associated with these changes during the same period were collected using remote sensing data, according to the paper.

The number of individual sharks captured per hour of surveying increased five-fold between 2003 and 2020, the data showed. In addition, all of the bull sharks surveyed during the study period were juveniles.

The uptick in population coincided with an increase in the mean sea surface temperature in Mobile Bay from 22.3 degrees Celsius -- 72.1 degrees Fahrenheit -- in 2001 to 23 degrees Celsius -- 73.4 degrees Fahrenheit -- in 2020, according to the paper.

Sea surface temperatures above 22.5 degrees Celsius were associated with an increased likelihood of bull shark presence, the researchers found in data produced by computer modeling.

The study also revealed that the average probability of capturing a bull shark during surveys increased throughout Mobile Bay between 2003 and 2020, despite increases in coastal urbanization since 2000. The likelihood of tagging a bull shark was highest near the city of Daphne and along the western shoreline of the bay, according to the researchers.

The species is typically found in warm, shallow coastal waters in both fresh and saltwater environments, which is why they are thriving in the warmer waters while other animal species are beginning to decline, Mullins said.

The findings highlight the resilience of the species in response to climate change and coastal urbanization, a "surprising" result, Mullins said.

"It is, sort of, contrary to that narrative that we normally think, which is that for many species, warming water is a detriment," she said.

While sharks near U.S. waters have benefitted from aggressive coastal conservation efforts, overall, the global outlook for sharks is still relatively grim due to overfishing or being caught as bycatch, Mullins said.

However, it's unclear how the bull shark population could respond to even more increases in sea surface temperature. It is also unclear how a continued uptick in populations will affect the local ecosystem.

While bull sharks are known for their ability to help balance and maintain the health of coastal ecosystems by regulating prey populations, the scientists speculate that increased numbers could affect fishing opportunities in the region and could potentially lead to increases in interactions with humans, according to the paper.

People who recreate along the Mobile Bay Coast don't have much to worry about, Mullins said, adding she wouldn't hesitate to continue to take her family.

"This increase in abundance is not going to lead to an increase in shark bites," she said.