Smithsonian scientist Eric Johnson picked through the flopping contents of a 50-foot seine net: a half-dozen baby crabs, each one spawned in a hatchery, raised to the size of a nickel and released earlier that day into the Rhode River.

And a dozen fish, some of which would love to make a meal out of Johnson's experiment.

Marine scientists are claiming a measure of success in a four-year study to determine whether the Chesapeake Bay's fading blue crab population can be replenished with brethren grown in a hatchery.

The crab population in the bay has declined 80 percent in the past 15 years, said Anson "Tuck" Hines, director of the Smithsonian Environmental Research Center in Edgewater. Curbs on commercial fishing haven't reversed the losses. So scientists have released 150,000 lab-grown crabs into the bay since 2002, each one tagged for tracking.

By periodically sweeping the bay with nets, researchers are slowly learning how well the crabs compete in the wild: where they travel, how quickly they grow, mature and mate, and how many survive the marine mean streets to reach adulthood.

"Our goal is not to add crabs to the system so the crabbers can take them out," Johnson said. "Our goal is to produce mature females," which can reproduce a half-dozen times in their lifetime. "It's the gift that keeps on giving."

Early results are encouraging. Survival rates for the tank-raised crabs are "exactly the same" as for those spawned in the wild, Hines said, based on results to date. Between 6 percent and 30 percent of the released crab babies reach adulthood, depending on where and when they are released. Many fall prey to hungry fish and to each other; crabs are notorious cannibals.

"So far, this works, on a small scale," Hines said yesterday, after a successful baby-crab release. "And it's been very encouraging in that regard."

The 6,800 crabs released yesterday were spawned about two months ago at the University of Maryland's Center of Marine Biotechnology at Baltimore's Inner Harbor, the first hatchery for blue crabs.

The Smithsonian researchers have released 25 batches of crabs, a total of about 100,000 babies, into the Rhode River and the South River, both in Anne Arundel County. Each crab is injected with a tiny length of wire and a droplet of colored dye, both forms of identification. Dyes are color-coded and injected in different places to allow crabs released from different batches to be told apart.

Researchers at the Virginia Institute of Marine Science have released 50,000 crabs into the York River.

Lab-grown crabs start life with at least two natural disadvantages. After being raised in tanks, they don't know how to burrow into the muck to escape predators; and they tend to have shorter spines than crabs reared in the bay, making them a somewhat easier fit inside the jaws of a fish. But scientists have learned that their crabs overcome these deficiencies quickly once released into the wild, learning how to burrow within two days and even, quite literally, growing a spine when exposed to the wild.

Hines and his colleagues have barely made a dent in raising the population of reproductive female crabs, which, even after the decline, is estimated at 10 million. But now that they are getting the hang of crab-raising, the time may be near to go into mass production.

"We need to scale it up incrementally, building confidence as we go," he said.

Research technician Midge Kramer, left, scientist Eric Johnson and interns Emily Gamelin and Kathryn Chop load coolers containing thousands of baby crabs onto boats to be tossed into the Chesapeake.