Bay Journal

Mixed blue crab survey results show need for safe harvest target levels

  • By Rebecca Hanmer on July 01, 2005
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With our strong focus on restoring the water quality of the Chesapeake, it is easy to forget sometimes that effective fisheries management is an essential component of the Bay’s restoration.

This fundamental element of ecosystem restoration came into focus recently with the release of the 2005 report on blue crabs from the Chesapeake Bay Stock Assessment Committee. The report combines information from four different surveys. While it covers data largely collected in 2004, one survey extended into the early months of 2005.

There are at least three variables in managing blue crab resurgence. Water quality, including dissolved oxygen levels, must be good enough to support the crabs. Appropriate habitat needs to be plentiful, especially lush beds of underwater grasses. And, harvest levels must be managed so that overfishing does not result in an unsustainable population.

The news from this year’s report, like last year’s, is mixed.

On the positive side, the number of juvenile blue crabs entering the Chesapeake Bay population is at an eight-year high, and is above the long-term average, according to the combined surveys index.

The critical spawning stock of mature females, though, continues to be below the long-term average. This depressing trend has been a feature of the Bay’s blue crab stock for 11 of the last 12 years.

Other measures of species health are equally mixed. The overall abundance of blue crabs older than 1 year continues to be below the long-term average. Watermen, meanwhile, had their best harvest in five years in 2004, led by a strong showing in Maryland waters.

So what are we to make of these numbers? Perhaps most obviously, they make it clear that the Bay ecosystem is dizzyingly complex. How can there be a minor population boom in juveniles in the same year that the stock of spawning females is below average? Factors like weather, limitations on survey methods and a host of other factors can leave even the most knowledgeable fisheries scientists humbled.

If one examines the data thoroughly, some clear trends emerge. Perhaps none is as important as the adoption of new regulations beginning in 2001 is beginning to have a positive effect.

Fisheries biologists and watermen alike (not to mention seafood lovers) became alarmed at a big drop in commercial blue crab harvests in the late 1990s. The news only got worse. The combined Virginia and Maryland blue crab landings in 2000 fell to their lowest levels in more than two decades, according to the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service.

The Bay’s water quality, while still significantly degraded, had not declined during the period, so that did not seem to explain the big drop in crab numbers.

Even more puzzling was that underwater Bay grasses, the key habitat for blue crabs, had been making a strong resurgence in the late 1990s and reached nearly 90,000 acres by 2002. So habitat did not seem to account for the major reduction in blue crab numbers.

All of the fisheries-independent surveys were showing a drop in the abundance of crabs. Although watermen were taking in smaller harvests—and working harder than ever to get them—they were catching an unsustainably high number of crabs. The fishing mortality rates from 1998 through 2001 were far greater than the reproductive capacity of the blue crabs to have a sustainable population. The number of blue crabs, the Chesapeake’s signature species, was dropping rapidly.

Many people are surprised to learn that approximately 70 percent of the blue crab population in the Bay is taken every year by commercial and recreational fishermen. The remarkable reproductive success of blue crabs can replace that population annually, but only if all of the conditions are right. Because blue crabs are fished so aggressively, any upset in the system can leave the species vulnerable to a sudden collapse.

In 2001, working with the incomparable science data supplied by the Chesapeake Stock Assessment Committee, the Chesapeake Bay Commission’s Bi-State Blue Crab Advisory Committee recommended that Maryland, Virginia and the Potomac River Fisheries Commission take quick action to stem the decline.

Fisheries managers responded with new restrictions on crabbing, with an aim at doubling the spawning stock of mature females. The key action was to reduce annual harvest pressure by 15 percent. The management plans appear to be working, and the precipitous drop in blue crab abundance has been arrested.

The Stock Assessment Committee has given fisheries managers an upper threshold. If commercial and recreational fishermen take more than this level, they risk a catastrophic crash of the blue crab population. Equally important, the committee provided managers with a safe harvest target level. If the total number of crabs taken in a year is at this level, the species should be able to rebound.

Using the combined survey index, the 2005 Stock Assessment Report shows that average fishing mortality continues to be below the overfishing threshold. One of the methods, though, reveals data that continue to alarm fisheries biologists. The winter dredge survey, which sampled 1,500 stations throughout the Bay from last December to this March, shows unsustainably high fishing mortality for six of the last seven years.

The combined surveys index remains above the recommended target level, and so the recovery of the species continues to be compromised.

At a minimum, a steady hand by managers in maintaining the current set of measures to reduce fishing mortality rates is needed to restore the long-term stability of the blue crab population. In fact, additional restrictions may be needed to bring the harvest down to the target levels.

A Baywide slight drop in spawning stock abundance suggests that more controls are needed. Especially troubling is a continued very low level of spawning stock showing up in the annual Virginia Trawl Survey, which samples 130 stations per month year-round.

There are lessons to be learned from the successful effort to restore the Bay’s striped bass population. Striped bass once formed the basis of one of the most important fisheries—both commercial and recreational—on the Atlantic seaboard. Overharvesting contributed heavily to the striped bass’ decline by disrupting the balance of the spawning stock. Additional influences included loss of habitat, disease and pollution.

A fishing moratorium in the 1980s has been key to the remarkable resurgence of this species. In 1995, the stock was officially declared restored. Continued careful management of the species has resulted in a generally healthy recovery. In other words, an aggressive fisheries management plan, based on sound science, appears to be paramount in restoring this important fish population.

And right now, the science is telling us that we are on the right track with blue crabs, but that we probably need to do more to get our harvests down to the target levels recommended by the Stock Assessment Committee.

Fisheries management is not the only factor in restoring striped bass or blue crabs. But it is clearly a central element of the restoration strategy.

Yes, we must restore water quality and abundant, healthy habitat. Poorly oxygenated waters are a troubling feature of deep Bay waters every summer. And grass beds, even though they saw a 14 percent increase in 2004, are still just 39 percent of our target of 185,000 acres.

So while we redouble our efforts to improve water quality and habitat for the shellfish and finfish of the Chesapeake Bay watershed, we also need a fisheries management plan that appropriately protects vulnerable species. The two go hand-in-hand.

Perhaps there’s a lesson here for native oysters, too.

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About Rebecca Hanmer
Dick Williams, LEED AP® BD+C, consults on high-efficiency construction and sustainability and writes and blogs to address his curiosity about humankind’s impact on the biosphere. Distributed by Bay Journal News Service.
Read more articles by Rebecca Hanmer

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