A four-year Virginia study found that so-called ghost fishing carried on by lost and abandoned crab traps takes a very real — and lethal — toll on the Bay's blue crabs and other aquatic dwellers.

Researchers found that the roughly 32,000 crab pots pulled from Virginia waters during four winters of collection efforts held more than 25,000 crabs. Three-fifths of them were females, the gender fishery managers have targeted for increased protection.

In addition, the lost pots captured more than 30 other aquatic species, including large numbers of oyster toadfish, black sea bass, Atlantic croaker, spot and flounder. Among the species were a number of rare diamondback terrapins.

Scientists have expressed concern for years about the potential impact of lost crab pots in the Bay. The new findings are the most comprehensive picture yet of their impact in the Chesapeake, but the scientists cautioned that, if anything, their estimated numbers are low.

"It's a very, very conservative estimate," said Donna Bilkovic, a research assistant professor with the Virginia Institute of Marine Science's Center for Coastal Resources Management.

That's because the data was collected during the winter when most species are less active, and therefore less likely to be caught.

"It is a little misleading to extrapolate what we saw in the derelict traps removed in the winter," Bilkovic said. "We think it is actually a much larger impact, and we would expect to see different species."

In fact, she estimated that annually, at least 1.25 million blue crabs are likely to be caught by crab pots that have been abandoned or lost in Virginia's portion of the Bay. Those crabs would have a value of more than $400,000, the researchers said.

But the researchers also said the impact of ghost pots could be limited if watermen were required to install biodegradable panels that would create an exit for species trapped by lost pots.

Their results stem from a four-year program that ran from late 2008 through early 2012 in which the Virginia Marine Resources Commission employed watermen to use sonar to locate abandoned pots and remove them. VIMS scientists helped design the program and collected the data on the species found in the traps. The program was financed using a portion of the $15 million in federal money awarded for conservation projects after the National Oceanic and Atmospheric Administration's Fisheries Service declared the blue crab fishery a failure in 2008.

Crab pots are wire mesh cubes measuring roughly 2 feet on all sides. The pots are baited to lure crabs through a funnel, which also keeps them from escaping. Pots are typically deployed with lines attached to buoys so watermen can easily locate them.

But many pots are lost each year. Buoy lines can be severed by the props of commercial or recreational boats, or ripped off by storms. Storms also move pots around so they can't be found. In other cases, pots might be vandalized or simply abandoned by watermen — nearly 40 percent of all pots found in the Virginia program still had buoy lines and had simply been left in the water.

Whatever the reason, the VIMS scientists estimated that about 20 percent of the crab pots fished each year are lost. The commission licenses as many as 385,000 commercial pots that can be fished each year, although the actual number fished is typically smaller.

In addition, an unknown number of pots are used by recreational crabbers. "We don't have a strong understanding of just what the extent of the recreational fishery is," Bilkovic said. That is a concern, though, as recreational fishing tends to take place in shallower water where diamondback terrapins are more likely to come in contact with the pots.

Some lost pots are damaged or fall apart quickly. In high salinity waters, they may break down in two years, but in lower salinities they may persist for four years or more. During that time, they continue to "fish."

The Virginia collection program took place after crab season ended in mid-December each year, but about 40 percent of the pots were actively fishing — that is, they contained crabs or other species.

Even empty traps can attract some fish and crabs, and whenever something is caught it effectively re-baits the trap, luring even more animals to their doom.

Field tests by VIMS scientists have shown that empty crab pots capture an average of 50 crabs a year, while re-baited pots can capture an average of 100.

Bilkovic estimated that 1.25 million crabs would be caught in the missing pots each year, assuming that just 250,000 pots are fished a year and 10 percent of those are lost but still capable of catching crabs, and that each pot catches 50 crabs.

The Baywide number of adult crabs has been between 175 million and 180 million during the last two years. That means ghost pots may be catching 1 percent or more of the adult population in Virginia.

(Maryland has also conducted a smaller derelict crab pot collection effort. Those figures are still being analyzed, state officials say.)

Understanding the number of crabs caught through ghost fishing could be important for management. "That is a number that, if we can quantify it in some way, will be important to the stock assessment," said Joe Grist, the VMRC's deputy chief of fisheries.

Normally, when scientists do stock assessments, they assume that any crabs not taken in the fishery are lost to natural mortality. If a portion of those crabs are actually lost through an activity that can be managed, it gives agencies another way to influence the overall population, Grist said.

There could be a relatively simple way to help mitigate the problem, according to VIMS researchers. Watermen are required to install cull rings on their pots that are essentially escape hatches that allow small crabs to escape while retaining legal size crabs.

In research supported by the National Fish and Wildlife Foundation, VIMS scientists have experimented with replacing cull rings with larger, biodegradable cull panels. The panels, made of a plant-based material, polyhydroxyalkanoate, are consumed by bacteria if left in the water.

The lost pots would still be a lethal trap for crabs and other species for several months until the panels degrade, but would then be relatively harmless for most wildlife, which would be able to move freely through the opening.

"That is the idea, to not make too much of a burden on the actual commercial watermen," said Kirk Havens, assistant director of the VIMS Center for Coastal Resources Management. "We are still saving two or three years or more of bycatch."

When the pots are pulled out of the water — as normally happens every few days during the fishing season — UV rays from the sun kill much of the bacteria, prolonging the life of the panel so it would typically last an entire fishing season.

Other materials might degrade more rapidly, but would have to be replaced several times each year, Havens said. The panels cost a little less than a dollar each, and to be most effective, each pot would need two. If the panels had to be replaced multiple times a year, it would become time-consuming and costly for watermen with large numbers of pots, he said.

Scientists tested the panel with several watermen in different parts of the Bay who fished them alongside unmodified pots. "There was no difference in the catch rate at all," Bilkovic said.