Research sheds light on sharks off Delmarva coast

The sand tiger shark, one of the top predators in Delaware Bay and nearshore Atlantic Ocean, isn't entirely driven by its quest for food. New research suggests one survival strategy may cause juvenile sand tigers to bypass the Delaware coast and head north to sheltered waters in New England, to avoid being eaten by larger relatives.

The adult sharks that migrate in and out of our waters also seem to have some habitat preferences – among them, being closer to shore especially as they move south in the late summer and fall.

Researchers at the University of Delaware and Delaware State University have been tagging sharks with tracking devices for several years. Some 300 sand tiger sharks have transmitters that allow scientists to track their locations.

Danielle Haulsee, a graduate student in the College of Earth, Ocean and Environment at the University of Delaware Lewes Campus, said researchers would like to develop a model to forecast where the sharks will be, "almost like a weather map."

The reason: these sharks may be top predators in the marine ecosystem, but the population is in decline, the species is slow to mature and has low reproductive rates, and the animals often are caught when commercial fishermen target other species.

"Every adult female is really important," Haulsee said.

There are many other fish species that also have unique transmitter pings from Atlantic Sturgeon to striped bass. So when scientists are tracking a species, they can pick up hundreds of other pings.

Dewayne Fox, assistant professor of fisheries at Delaware State, started implanting the sand tigers with lipstick tube-sized transmitters in 2007.

"The model we thought was just a north-south migration," Fox said.

Then he started to notice he was only capturing adult sharks in Delaware Bay. That raised a question: Where do the young sharks spend the summer?

Working with researchers at the University of Rhode Island, they discovered that young sand tiger sharks migrate past our coast and spend the summer in Plymouth Bay off Massachusetts. Fox said these smaller animals are likely trying to avoid becoming prey for larger, adult sand tiger sharks.

"We think that risk of cohabitation may force that extended migration," he said.

The adults always seem to leave Delaware Bay about Sept. 21. That coincides with the fall equinox.

"My guess is that daylight is playing a role," he said.

They've also found that females don't return to Delaware Bay every summer. Fox said that because mating is difficult, they may spend their off-years further south, away from large congregations of male sharks.

"That's what we thought," he said.

But lately they have discovered that females may not just stay in nearshore, southern waters – they also may migrate to deeper waters well off shore, he said. These new discoveries come from cooperative tagging and monitoring efforts.

Fox and other researchers have set up dozens of devices "listening for anything with a tag " along the coast and in Delaware Bay, Fox said. Delaware State manages the Atlantic Cooperative Telemetry Network to manage all those pings in a massive data set.

"Our focus historically has been sandbar and sand tiger sharks and Atlantic sturgeon, he said. But other species, tagged by researchers up and down the coast are also picked up when they pass through the array of listening devices. The devices are also called hydrophones.

Fox also set up two gates of hydrophones in the Atlantic Ocean to go from nearshore to the shipping channel. The idea is that a shark passes through one gate at the north end of the study area and then passes through the south gate. Researchers can then estimate how long it takes the fish to get from one place to another, the direction of travel and the length of time it spends in the area, Fox said.

And that is where Haulsee picks up the story.

In 2012, she and her advisers decided to add a new technology to the mix. They programmed an underwater robot named Otis to run a zigzag pattern off the coast. The robot collected water samples and listened for the pinging of tagged sharks. The goal was to have the robot swimming where the sharks were swimming to get a better idea of where they were moving.

Haulsee said there were some naysayers – folks who said it would be difficult to track sharks in the open ocean because the area is so large – but once the robot started to run its programmed course, it picked up the pings of hundreds of different sand tiger sharks.

Over 19 days of shark tracking, they never heard the same shark twice, she said.

What they think might be happening is that water temperature doesn't matter so much. Instead, the sharks may prefer fresher water sites with lower salt content – areas where freshwater was coming out of the coastal bays from Delaware south to Chincoteague. In addition, the sharks migrating south favored sites closer to shore.

Thanks to the added water sampling capabilities of the robot, Haulsee discovered that the sharks prefer habitats with high levels of color-dissolved organic material.

"Color-dissolved organic matter has a certain yellowish color and scientists don't know exactly what it's comprised of, except to say it's the dissolved plant and animal material left in the water," she said. "We found that this organic material was not coming from the estuary or from the surface water; rather, it was coming from the bottom and being re-suspended in the water column we suspect by wave action, weather or tides."

The thought: Sand tiger sharks may be drawn to these nutrient-rich areas because they are important in the larger food web. In other words, smaller fish may be drawn in, which attracts the sharks or the sharks could simply be drawn in because of the rich organic matter. No one is sure. Either would support the idea that the Delaware Bay and coastal waters are critical resting and feeding areas for sand tiger sharks during the summer and these nutrient-rich nearshore areas as the fish migrate south each fall.

"It's hard to say whether the sand tiger sharks use this area of the Delaware coastal ocean as a landmark," she said. "Or whether they are there because they are looking for food. ... Other researchers have used robotics and acoustic telemetry separately; we were the first to bring those two technologies together to study habitat selection."

Reach Molly Murray at (302) 463-3334 or mmurray@delawareonline.com. Follow her on Twitter @MollyMurraytnj.

Sand tiger shark

•4 to 10 feet in length

•Upper side is light brown or light greenish-gray with the underside a grayish white

•Teeth are large needle-like and protruding from mouth

•Delaware Bay is habitat

Source: Delaware Department of Natural Resources and Environmental Control