Sightings of nursing bull sharks in coastal estuaries like North Carolina's Pamlico Sound are on the rise, leading experts to warn of a "potentially strong" impact on humans.
The shift is credited to ocean warming, a phenomenon that is encouraging bull sharks to "colonize" the sound as part of their nursing habitat, say biologists involved in a multiyear study.
The findings were published Monday in Nature.com and revealed that bull shark nursing activity in the Pamlico Sound went from minimal prior to 2010 to an annual occurrence starting in 2011.
That's troubling news for humans as bull sharks can grow to more than 10 and a half feet long and are known to attack large prey, including humans. The Pamlico Sound is one of the biggest attractions on the Outer Banks, drawing tens of thousands of visitors annually.
"These results suggest that increasing water temperature and salinity have allowed bull sharks to expand their nursery habitat," says the report. "This shift will have unknown, but potentially strong, impacts on both the local ecosystem and interactions with humans."
The study was co-authored by experts from the North Carolina Division of Marine Fisheries, Simon Fraser University and East Carolina University. The researchers involved in the report called the evidence "dramatic."
Only six juvenile bull sharks were captured in Pamlico Sound from 2003 to 2011. From 2011 to 2016, the number jumped to 53, says the report.
The findings are just the latest in a series of reports to show the impact ocean warming is having on sharks.
Experts with Florida Atlantic University published data in late winter showing blacktip sharks were less inclined to migrate to Florida for the winter, because waters off the East Coast are warmer than decades ago. That is leading to more blacktips being found year-round off the Carolinas. They can reach lengths of 8 feet.
Bull sharks can tolerate brackish and fresh waters, which means they can be found in any body of water connected to the ocean, experts say.
"To our knowledge, this study is the first to attempt to associate environmental factors with the apparent colonization of a nursery habitat by a marine apex predator," says the report. "The use of Pamlico Sound as a primary nursery may increase further if water temperatures remain warm and females born in the system reach maturity and return to give birth."
The study goes on to note there is evidence that bull shark populations are recovering from previous declines, which may cause a range expansion into new areas where they once roamed. However, the Pamlico Sound was not one of those areas, the experts say.