Using the latest in marine science gear, researchers track billions of larvae drifting from sea back to bays

Scientists stalk answers: What draws crabs home?

September 13, 2006|By Chris Guy | Chris Guy,Sun reporter

ABOARD THE HUGH R. SHARP -- Here where the Atlantic Ocean meets Delaware Bay, marine researcher Elizabeth W. North and 10 other scientists were in search of secrets kept by billions of tiny crabs bobbing this time of year at the mercy of tides and currents along the continental shelf.

Scientists know that larval crabs, spawned in the relative protection of the Chesapeake or Delaware bays, spend a few perilous but necessary weeks in saltier ocean waters. What puzzles the experts is what causes the minuscule creatures to return to the two estuaries to live out their lives.

"What we know points up everything we still don't understand about what triggers what behavior," said North, the chief scientist for the project, who is based at the University of Maryland Center for Environmental Studies at Horn Point. "Somehow, they seem to sense the need to get back to the bays."

At work in sophisticated, climate-controlled laboratories aboard a state-of-the-art coastal research vessel, the scientific team from Maryland and Delaware spent last week at sea, including a grueling six-day stint at the mouth of Chesapeake Bay and two days about 10 miles offshore from the University of Delaware's marine studies center in Lewes.

The team skirted Tropical Storm Ernesto, then logged 12-hour shifts in a 24-hour operation based largely on the nocturnal habits of immature crabs, which are hitching rides on nighttime high tides that take them home to the two Mid-Atlantic bays.

The scientists were on the University of Delaware's $19.4 million research vessel, the Hugh R. Sharp, studying issues including the physics of ocean currents, and salinity and nutrient content in plumes of fresh water and salty waves, to learn how they affect the billions of drifting blue crab larvae.

The baby crabs have a 95 percent mortality rate even in a good year. Predicting their welfare based on water conditions that can be measured would give a leg-up to conservation efforts, North said.

"It's clear that wind, temperature, salinity, the inflow of fresh water, plus lots and lots of other factors, all affect the crab population," she said.

After years of study of the blue crab, experts have learned a lot about the crustaceans, yet much about their behavior in the ocean remains unclear. Some scientists, for instance, suspect that the crabs catch a whiff of less salty bay waters and leave the ocean.

"The hypothesis is that the crab larvae smell the lower salinity in the bay, then ride a high tide in from the ocean," said Michael Roman, who directs the Horn Point lab near Cambridge. "There is a change in behavior when they get close to the bay that we need to understand."

As part of the eight-day mission, scientists, graduate students and highly trained deckhands and technicians donned safety helmets and scrambled across the ship's flat, open bow to deploy an elaborate nylon net, formally know as an Opening and Closing Net Environmental Sampling System.

The contraption, which includes deep pockets that can trap nearly invisible crabs and other creatures at varying depths, was hoisted off the deck of the Sharp with one of two remote-controlled cranes mounted on the vessel's stern.

Another underwater sample collector was raised and lowered from a platform off the side of the ship by a hydraulic crane operated by one of the ship's crew.

University of Delaware researcher Ana Dittel, an expert in larvae ecology, took a low-tech approach, examining the tiny crabs and other creatures collected by underwater equipment, then carefully labeling and storing them in, of all things, Mason jars.

"You can't really see much to catalog until you get the sample under a microscope anyway," said Dittel. "Analyzing this many samples might take a year."

Another tool researchers hope will pay dividends later is the Scan Fish, an underwater device that looks like an airplane wing. Towed by the Sharp, the device transmits data about the number of larvae at various depths to a bank of computers in the shipboard laboratory.

Like Dittel, the researchers will be spending months back in their labs on land evaluating the samples and other data they've collected in search of answers about the life of a crab.

The Sharp was built on the West Coast and arrived in Delaware in January, replacing a 30-year-old research ship. It has one laboratory permanently set up and wired with a half-dozen computers. The stern can also accommodate two laboratory trailers the size of truck containers that could be customized and delivered to the ship for academic clients.

The ship sleeps 14 researchers and a crew of six to eight. Meals are provided onboard by a full-time cook. Satellite television and radio provide entertainment on long cruises.

"I've been here over 20 years and I've seen a lot of students become Ph.Ds based on their work with us," captain/engineer Jimmy Warrington said. "We all take pride in the science that's being done on this ship."

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