While the Bay’s blue crab population no longer appears to be overfished, scientists are increasingly perplexed as to why the population of the Chesapeake’s most valuable economic species has not shown signs of returning to former levels of abundance.

The estimated blue crab abundance has rebounded from the record low of 90 million adults in 1999—which spurred new catch restrictions—but the overall population has now remained at lower-than-average abundances for a decade, according to recent stock reports.

Blue crab numbers have dropped to low levels in the past, but always bounced back. Such a protracted period of low abundance is unprecedented in the Bay and worrisome to scientists.

Scientists and managers say Baywide fishing restrictions implemented in 2001 and 2002, which were aimed at reducing fishing pressure by 15 percent, appear to have stabilized the population, as harvest rates declined.

While stable, it’s unclear whether numbers will increase over time, or whether the population is being kept in check by other factors.

“When is it going to respond?” asked Derek Orner, a fisheries biologist with the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office. “How much patience do you have?”

That’s a question scientists and fishery managers are increasingly asking.

“I do think the extended period of low abundance is a cause of concern,” said Tom Miller, a fisheries scientist with the University of Maryland Center for Environmental Science. “And for me, it is just as concerning that we can’t explain it.”

In the 1990s, huge numbers of adult crabs filled the Bay, reaching nearly 500 million in 1991, when they were so numerous that commercial buyers curbed their purchases as the market became glutted.

But numbers—and crab catches—fell sharply in the late 1990s, raising concern about the health of the stock. A special panel, the Bi-State Blue Crab Advisory Committee, was created by the legislative Chesapeake Bay Commission to bring together fishery managers, scientists, stakeholders and others to grapple with the problem.

To guide future management, they set the first-ever threshold and target for the blue crab fishery.

The threshold is considered the maximum fishing pressure that could be sustained by the crab population; cross the threshold, and the risk of a stock collapse greatly increases, especially if the low abundance of crabs were mixed with other stresses, such as climatic conditions unfavorable for larvae survival. The threshold, intended to maintain 10 percent of the spawning potential of an unfished population, is reached when 53 percent or more of the adult crabs are harvested.

The target is a desired harvest level, which allows for a margin of safety. It aims to keep harvests below 46 percent of the adult population. That level, over time, is expected to maintain 20 percent of the spawning potential of an unfished population—which would double the spawning stock when compared to a populstion fished at the threshold. That would eventually lead to a larger, more stable population, scientists say.

A recently completed report by the Chesapeake Bay Stock Assessment Committee, which includes scientists and fishery managers from around the Bay, had good and bad news about progress in meeting those goals, but concluded that the overall condition of the stock “warrants concern for the ninth consecutive year.”

The good news was that in 2005—for the first time since 1997—the percentage of the population that was harvested fell below the target level. Only about 37 percent of the estimated 170 million crabs were caught, according to the report.

But crab abundance is affected by more than just harvest, and the reduced catch rate of 2005 did not translate into increased abundance last year. Recently compiled figures put the 2006 adult crab population at 130 million—the lowest number since 2002.

Other reports are mixed as well. There has been no clear improving trend in reproduction, according to the CBSAC report. Harvests have been relatively steady, but below average.

Meanwhile, the number of spawning females has remained steady, with no obvious Baywide trend. But a survey by the Virginia Institute of Marine Science of spawning females in the spawning grounds near the mouth of the Bay has been showing a slow but steady decline—a concern because the ultimate goal is to double the spawning stock.

“In my view, we are going to have to take some sort of action if we don’t see any kind of recovery at the end of this year,” said Rom Lipcius, a VIMS researcher who has worked extensively with crabs.

Female blue crabs are capable of producing huge numbers of eggs—up to 8 million—that they release in the summer and fall as they migrate to the mouth of the Bay. Blue crab larvae are swept into the ocean, with postlarvae returning to the Bay four to six weeks later. Weather patterns along the coast determine, in part, how many crabs are pushed into the Chesapeake and “recruited” into the Bay’s crab population.

Because females can produce so many eggs, the population is capable of rebounding rapidly, even from low levels if good environmental conditions are present. Yet blue crab abundance has been below the long-term average of about 225 million every year since 1996.

“The reason why we are not seeing the recovery is really not clear,” said Miller, who led the most recent stock assessment of Bay blue crabs, which was completed two years ago. “They have been at low levels in the past and recovered to a very healthy level in a fairly short period of time. Yet this time, they have been at a relatively low level of abundance for a fairly extended period of time.”

Miller said it was premature to call for further restrictions on catches because other factors may be at play.

He and other scientists say there could be ecosystem changes affecting the blue crab population in ways that scientists did not anticipate when they set the threshold and target. For instance, it’s possible that predation on blue crabs has increased over time, perhaps as a result of the robust striped bass population.

Other possibilities could include changes in local ocean currents, which influence the return of crabs into the Bay. Perhaps fecundity—a measure of the number of eggs produced by a female—has changed.

“There are a lot of potential causes out there, and I think if we continue to see this pattern, we should begin to ask those questions,” Miller said.

One problem that could loom on the horizon is the potential impact of the massive die-off of underwater grasses in the lower Chesapeake Bay in the summer of 2005 because of warm temperatures. The die-off of eelgrass deprived small crabs of critical habitat, potentially making them more vulnerable to predation and cannibalism by larger crabs when they entered the Bay in late 2005 and early 2006.

Further, eelgrass beds remained very sparse last year, meaning small crabs may still face a critical habitat shortage.

If that’s a key factor in crab survival, Lipcius said, it could show up in the form of fewer legal size crabs this year and next.

“We’ll know just how valuable eelgrass is to the blue crab,” Lipcius said. “That’s going to be interesting. We’ve always assumed it’s important to the production of blue crabs, but now nature has actually given us a test of that.”

One activity that scientists and managers say could help is examining regulations to see if they are as effective as thought.

The Virginia Marine Resource Commission, for instance, is considering changing rules on the collection of females with egg-filled “sponges” that are nearly ready to spawn. Current rules require watermen to toss back certain sponge crabs—to protect their eggs—but recent research shows that most of those eggs die anyway as the result of handling.

As a result, the VMRC is contemplating expanding its existing spawning sanctuary to cover an area heavily used by sponge crabs along the Atlantic Coast, south of the Bay’s mouth. “It’s not a heavily fished area, but it is a good sponge area, and we wouldn’t want to see it become a heavily fished area,” said Rob O’Reilly, a natural resource manager with the VMRC.

And many believe that the Bi-State Blue Crab Committee, which was disbanded after funding was cut by the states, should be reassembled to better guide crab research and management.

“I think it was disbanded because of the controversial nature of its work,” said Bill Goldsborough, senior scientist with the Chesapeake Bay Foundation, who noted that after the threshold and target was established, many of the committee’s other recommendations to improve Baywide management were left undone.

“They had a lot of potential for improving the lot of both the crab and the crabber, yet we are staying at this low level, and neither one of them is optimized right now,” he said.