Researchers release juvenile blue crabs

Hatchery-reared animals may help restore the Chesapeake’s ailing crab fishery

(June 28, 2004) Researchers at the Virginia Institute of Marine Science released 11,540 hatchery-reared blue crabs into the York River last week in a collaborative effort to enhance Chesapeake Bay’s historically low blue crab stocks.

The population of female blue crabs in Chesapeake Bay has declined more than 80% during the last ten years.

Maryland scientists released a similar number of juvenile crabs into the Rhode River, which empties into Chesapeake Bay near Annapolis. Together, the 23,000 crabs represent the largest single experimental release of juvenile blue crabs ever attempted to test the feasibility of stock enhancement for the species.

The release program is a collaborative effort between VIMS, the University of Maryland’s Center of Marine Biotechnology (COMB) in Baltimore, and the Smithsonian Environmental Research Center (SERC) in Edgewater, Maryland. It complements efforts to reduce the harvest of adult crabs through the establishment of blue crab sanctuaries.

The VIMS release effort was headed by Drs. Rom Lipcius and Rochelle Seitz and Mr. Jacques van Montfrans. The effort was aided by graduate students Russ Burke, Dave Hewitt, Deb Lambert, Chris Long, and Bryce Brylawski; staff members Mike Seebo and Kristie Erickson; and REU college interns Francisco Soto Santiago and Nicole Rohr and VIMS Governor's school students Ian Keene-Babcock and Jenny Geldermann. The team will now focus on monitoring the crabs’ survival and their interactions with other organisms that make up the bottom-dwelling community into which the crabs were released.

The released crabs were reared in a hatchery at COMB by Drs. Yoni Zohar and Odi Smora and colleagues, who have spent the last several years overcoming previous obstacles to raising blue crabs in captivity, including cannibalism.

Lipcius and Seitz collected the crabs from SERC, where resident and VIMS scientists had completed the laborious task of tagging each of the 23,000 tiny crabs. The tags, which consist of a tiny wire inserted into leg muscle, allow researchers to track the crabs despite repeated molting of the external skeleton. Researchers can easily identify a tagged and recaptured crab by passing it across a metal detector. Each of the crabs was also tagged with a dye visible through the transparent exoskeleton of the swimming legs.

After transporting the crabs to VIMS in iced coolers, the researchers released the animals into shallow mud coves on the Catlett Islands, a pristine salt marsh habitat owned and managed by VIMS’ Chesapeake Bay National Estuarine Research Reserve.

Last week’s release builds on experiments by Lipcius, Seitz and van Montfrans during the last two summers in which they released young crabs from York River and Tangier Island grass beds into the same Catlett Island coves. High survival rates among these transplanted crabs show that the coves contain far fewer crabs than they are capable of supporting.

“Last year’s experiments suggest that the ecosystem is below its carrying capacity, and that the blue crab is limited by recruitment, not resources,” says Lipcius. In other words, low crab numbers aren’t due to a lack of food or excessive predation, but to a shortage of young crabs.

“That’s why enhancement using hatchery-reared crabs seems like a viable method for restoring the Chesapeake’s blue crab population,” says Seitz .

The VIMS research team will spend the rest of the summer and fall monitoring whether the hatchery-reared crabs fare as well as last year’s transplanted crabs, and how they might affect other organisms in the coves, including the thin-shelled clams (e.g., Macoma balthica, Macoma mitchelli, and Mya arenaria) that make up a substantial fraction of the blue crab diet.