Tracking a predator in the Delaware Bay

    When you read about where humpback whales go when they migrate, or what Atlantic sturgeon like to eat, it is easy to take for granted how humans know these kinds of things.

    But for those whose job it is to uncover the secrets of the natural world, that knowledge comes only after lots of elbow grease.

    Go to almost any large aquarium and if there is a big, toothy shark on display, odds are it’s a sand tiger shark. Sand tigers are scary looking, with mouths that resemble Freddy Krueger’s hands, but they are relatively docile and easy to manage in captivity.

    Many spend their summers in the Delaware Bay, but scientists do not know much about what they do there or where they go when they leave.

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    “You can think about it as being the lions of the Serengeti, well in Delaware Bay you’ve got sand tiger sharks, so they are essentially ruling the roost,” said Delaware State fisheries professor Dewayne Fox. “(Yet) we know almost nothing about sand tigers in Delaware Bay.”

    The sand tiger shark is at the top of the food chain in the bay. If they were to disappear, their prey could run rampant and alter the entire ecosystem.

    Low reproductive rates, overfishing and accidental catches landed the shark on the federal “species of concern” list 15 years ago, and it remains there today.

    That is why Fox has spent much of his summer on the deck of a boat miles off the coast in Delaware Bay, catching tiger sharks to tag them with acoustic transmitters.

    Shark wrangler

    On a recent afternoon, University of Delaware doctoral student Danielle Haulsee was on the same boat, supervising two undergraduates impaling bait fish on hooks to get the process started.

    “You poke it though the eye, and that’s a good anchoring spot so the fish doesn’t fall off the hook,” Haulsee explained.

    Haulsee is petite, and wore athletic shorts and a long blond braid thrown over one shoulder. She did not look the part of shark wrangler, but on the boat, that’s what she was.

    “It’s a big boy!”, she laughed after snagging a 7.5-foot male that thrashed for a while against the side of the metal boat.

    Haulsee and the team tied both ends of the shark with a thick rope, effectively stringing it out alongside the boat, belly-up, half in and half out of the water.

    She grabbed a scalpel and folded herself over the side of the boat to cut an inch-long incision in the shark’s belly and insert the acoustic transmitter, which looked like a black tube of lipstick.

    Hundreds of acoustic receivers are sunk in the Delaware Bay and along the East Coast. Now, when this shark swims by any of them, those receivers will record and store the unique sound the newly implanted transmitter makes for researchers to download and study.

    Devising a management plan

    Fox and his team have wrestled, tagged and released more than 500 sharks since 2006.

    Estimates vary widely, but Fox said the evidence he trusts most indicates sand tiger shark populations are down as much as 75 percent from 30 years ago. Through his research, he wants to determine if populations are starting to rebound or if they are still in decline. Ultimately, he wants to come up with a management plan for the species.

    “Knowing when they’re here and when they’re not here, we can work with agencies like the Army Corps of Engineers and the state agencies to allow activities like dredging and beach renourishment,” Fox said. “If we know sand tigers are in a given location at a given time of the year, maybe it’s not the best thing to go in there and dredge that area.”

    New tools in tracking sand tigers

    The device they have been implanting since 2006 has its limitations; it only tracks sharks when they are relatively close to the coast where the receivers are. So this year, Fox is collaborating with University of Delaware oceanography professor Matt Oliver to add some new tools to their shark-tracking arsenal.

    They are implanting some sharks with both transmitters and receivers, so researchers will be able to tell not only where sharks go, but who they are with.

    “When a sand tiger leaves, is it following another group of fish, does it hang out with its own species, is it a loner? We don’t know that,” Oliver said.

    Oliver is also launching an unmanned glider that will troll the ocean for two one-month stints this summer to track sharks farther off shore. The bright yellow machine looks like a miniature airplane and will also collect a host of water-quality measurements.

    “Once we can figure out where the sharks like to live, we can then take that idea and basically broadcast it up and down the East Coast, to produce a (predictive) model,” Oliver said.

    Preliminary data collected by Fox confirm a common hypothesis about sand tigers – many migrate to waters near Florida and the Carolinas for the winter. But they also reveal something new, that female sharks might peel off and head east, near the continental shelf, to winter alone in the Gulf Stream.

    Dewayne Fox said he is looking to information gathered by the glider to tell him more.

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