2007
Volume 5, Number 4
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Tracking Crabs on the Move

By Jack Greer

It's hard enough for scientists to count blue crabs in winter, when the crabs are sitting still. It's even tougher to track them the rest of the year, when they're on the move. But that's what researchers must do if they are to understand the basic elements of blue crab biology — behavior, migration patterns, and growth. This is a daunting challenge, since the Bay is 200 miles long, with countless creeks and coves where crabs can hide.

Anson "Tuck" Hines and his colleagues at the Smithsonian Environmental Research Center (SERC) have searched for crabs on the move, using every trick in the book. They have tagged young crabs by injecting them with tiny microwires and elastomer paints. They have marked older crabs with large metal labels and strapped electronic transmitters on their backs.

   
tagged juvenile crab by Alicia Young
tagged adult crab by Alicia Young
   
hair thin microwire on a thumbnail by Alicia Young

Hair-thin injected microwires (above) that a metal detector can sense and injected red elastomer (above left) are two methods used to mark small crabs for recapture. Thousands of tiny crabs come from a hatchery at the University of Maryland Center of Marine Biotechnology, where researchers spawn and raise them in closed systems. Scientists at the Smithsonian also mark large crabs with external tags (below left) — and offer cash rewards for their return. Photos by Alicia Young.

For those of us who wonder what blue crabs do during the day and where they go, Hines and his colleagues have come up with a surprising number of answers.

Working with colleague Tom Wolcott of North Carolina State University, the research group has followed female crabs with data-logging backpacks all the way down the Bay. Coupled with tag returns from watermen, the study showed that inseminated females tended to move along the Bay's deep channel, mostly the Eastern edge — and mostly they walked. From mid-Bay this slow crawl can take more than a month.

Hines's group has also tracked small crabs coming the other way, heading north from Virginia. For three years SERC Researcher Eric Johnson has used trawls and seines to spot juvenile crabs as they migrate up the Bay. He's watched them arrive in the Potomac, beginning in late August and continuing through September and October. He's followed them north to the Magothy River, above Annapolis.

Hines and Johnson want to know: Are these young crabs walking? Swimming? Do they favor the Eastern or Western shores?

Johnson found some answers by trawling for small crabs as they passed between Cove Point and Taylor Island, where the Bay narrows. He caught a number of these intrepid juveniles in the middle of the Bay, he says, traveling at the surface, at night, on a flood tide. They appeared to ride the tides like a conveyer belt, Johnson says, to get up the Bay. Though preliminary, these findings fit well with other research that shows larval crabs moving out of the Bay and then back in, as they ride Bay and ocean currents.

But tracking crabs as they move through the whole Bay is tough work. What if scientists could shrink the nation's largest estuary down to the size of one small tidal tributary and then track the movement, behavior, and distribution of crabs?

Hines and his colleagues have in a sense done just that. Their miniature Bay is the Rhode River, south of Annapolis, where SERC resides. Hines has set out to determine how many crabs are in this one river, how they distribute themselves, and when they come and go.

During the past two years they have tagged a remarkable 5,137 crabs. Even more remarkably, over 2,400 have come back to them — a very high rate for such surveys. Of those returned, 31 percent came from recreational crabbers. Given the information they get from both recreational and commercial crabbers, Hines and his group can document how many crabs were caught using trotlines or crab pots, or by week-enders dangling a chicken neck.

All this tracking has led to a detailed picture of crabs in this one tributary. Hines estimates that there are about 60,000 to 120,000 adult crabs in the Rhode River, over an area of about 585 hectares. Work in the river has also shown that blue crabs tend to "partition" themselves by sex and by size.

Their crab census has also calculated an average fishing rate in the river — about 58 percent over two years. That's in line with removal rates often seen in the whole Bay, says Hines. It's also well above the 46 percent fishing target recommended by the Bi-State Blue Crab Advisory Committee in 2001. At times, he says, fishing pressure in the river can be "intense."

For about five years Hines and his team have worked with homegrown crabs produced by a crab hatchery led by Yonathan Zohar and Oded Zmora at the University of Maryland Center of Marine Biotechnology (COMB). They've also collaborated closely with Rom Lipcius and Rochelle Seitz at the Virginia Institute of Marine Science (VIMS) to compare blue crab ecology and the fate of released hatchery crabs in the upper and lower portions of the Bay. These hatchery crabs provide a unique opportunity to study growth rates and survival because they are all of a known age.

The team tags small juvenile crabs with a microwire that stays with the crab even through molting. They detect these microwires by waving a wand-like metal detector over the crabs, and they can even locate them after a small crab has landed in the belly of a fish. They also tag different hatchery classes (cohorts) — up to tens of thousands of crabs — by injecting their legs with tiny drops of different-colored elastomer paints.

They can distinguish between crabs that stay near or journey far, go deep or stay shallow, burrow in bare sites or seek out woody debris.

Their hope, Hines says, is to be able to replicate this information about movement and behavior for other tidal tributaries around the Bay, to develop a more detailed picture of how crabs behave in the Chesapeake.

Our understanding of crab ecology has come a long way, according to Hines. So has our application of science to policy. He gives credit to researchers like Tom Miller at the University of Maryland Center for Environmental Science who have taken on the challenge of Baywide blue crab stock assessment. And to the winter dredge survey conducted by the Maryland Department of Natural Resources and VIMS, which despite its limitations, is the best measuring tool we have, he says. He points to major funding from Congress and federal and state agencies that has brought great advances in data collection and research during the past two decades.

"Go back and read William Warner's book, Beautiful Swimmers," he says, referring to the 1976 classic. "We've learned a lot since then."

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