Every now and then you hear about a shark sighting -- like the hammerhead that washed up on the shores of Fenwick Island earlier this summer. Or Mary Lee, the tagged Great white shark who’s known to wander near the Delmarva coast.
As one fisherman told Delaware Public Media, there’s also been more sharks caught near the shore this summer than usual. And we’re talking adult-size sharks, not puppies. In our latest installment for iSeeChange, Delaware Public Media’s science reporter Eli Chen asked a couple of scientists what might be drawing these sharks closer to shore.
Fishing along the Delaware coast has been great this summer. And usually it isn’t.
“Fishing in the summer in never really that good especially from the land, like the shore, the piers and especially the surf,” said Rich King, a local fisherman.
King also posts fishing reports on a site called delaware-surf-fishing dot com. In a recent post, he says “fishing has been off the hook, pun intended.” There’s been more offshore fish closer to land this year, especially bluefish.
“Usually early morning, late evening, you’re catching the little stuff, what people call ‘trash fish,” said King. “And we’re not seeing as many in the surf and other areas. Some people think that it’s because we had so many bluefish come in that they ate everything.”
But King says there hasn’t just been more bluefish. There’s also been more sharks.
“We’ve caught a lot of sand tigers, sandbars and duskys in the surf. I’m not even trying to catch sharks. If I wanted to go out and catch sharks right now, I could catch… every time I dropped the line I could catch a shark,” said King.
Since the sharks eat the bluefish, he’s wondering….
“Is that would have brought more sharks closer to the shore?” asked King.
Dan Abel, a marine science professor, studies sharks at Coastal Carolina University.
“It may be that the mix of sharks is different. Maybe some are arriving earlier, some are leaving later,” said Abel.
At where he fishes in Winyah Bay, South Carolina, he’s also been catching a sudden flux of sharks.
“We’re catching for about a period of two weeks, a bunch of large bull sharks, fine tooth sharks, black tip sharks, sandbar sharks. That mix is a little bit unusual,” said Abel.
Like Rich King, he’s been seeing food sources come closer to the shore.
“So we had bluefish, redrum, mackerel, as well as the smaller fish,” said Abel.
King is on the right path in thinking that the food source could be drawing the sharks closer. But Abel thinks there’s more factors involved.
“I’m beginning to think that for part of the season, something was going on,” said Abel. “What exactly it is -- I don’t know that science knows yet. I know that water temperatures were higher along our coast in July than they typically would have been.”
Abel says warm ocean temperatures might cue sharks to go where they might find food. But if we really wanted to know how connected shark activity is to ocean temperature -- our best bet is to ask an oceanographer.
“Just like you and me, sharks are complex creatures that cue off of lots of things in the environment,” said Matt Oliver, an oceanographer at University of Delaware. “Answering the question why are these particular sharks found here at this particular time is like asking your teenager why they went to the mall on that particular day.”
For three years, he and his team, involving scientists from Delaware State University and University of Rhode Island, have been tracking sand tiger sharks, a threatened species, using an implanted tag, which is picked up by an underwater drone. The research is partly funded by the Lenfest Ocean Program.
Oliver took me down to the lab to show me one of the drones, which looks like a big long yellow submarine.
“In the middle it’s got this science bay that has receivers to detect individual sharks,” said Oliver. “We get a detection and then it calls us and say “I saw this shark here” and using it sensors and optics packages, and this is what the water is like at the same time. It’s really valuable if we want to piece out what’s driving shark location.”
But he’s not sure if temperature has anything to do with the increase in shark catchings, at least along the Delaware coast. He sent me a few emails saying that water temperatures along Delaware’s beaches have been pretty normal. Oliver says that teasing out what variables dictate where sharks go really depend on scale.
“If you are looking at seasonal timescales and thousands of kilometers, then locating sharks will be a matter of temperature,” said Oliver. “If you’re looking at this beach versus that beach, temperature may not be interesting at all. It may be completely optics.”
By “optics,” Oliver is referring to water clarity. In a paper he and his colleagues published earlier this year, their research showed that sharks spend more time in areas where the water’s a bit dirty.
“There is dissolved organic material in the water,” said Oliver. “The way to think about it is that it’s the compounds that make your tea dark. The further out you go the more clear the water gets. The closer to land, the more stained it is. It’s possible that sharks are cuing off of that.”
But they don’t know why sharks like dirty water. If water clarity’s behind this summer’s increase in shark catchings, it could be that there’s more nutrients running off of the coastal land for some reason. If that’s happening, it could attract phytoplankton blooms, which then brings the smaller fish that eat them, then the larger fish that eat them, and so on. Maybe.
“Could just be normal variation that’s not really explainable,” said Abel.
While Dan Abel and Matt Oliver have ideas for why sharks travel the way they do, neither of them can quite answer Rich King’s question about why he and other fisherman have seen sharks closer to shore. King’s question is really quite practical because sharks are subject to fishing limits and restrictions just like other fish.
“You know, I like to say any braindead idiot can put a fish on a hook and catch a shark. It’s releasing the shark that’s the problem,” said King.
But as the scientists learn more about what cues sharks go where they go, it’s possible that someday fishermen will know when and where they can avoid a hassle.
