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The Biology of Abundance

Counting Crabs

The Summer of '96: Maryland Blue Crab Regulations

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Blue Crabs in the Chesapeake

Blue Crabs:
The Biology of Abundance

By Merrill Leffler

How threatened is the blue crab fishery in the Chesapeake Bay? Could crabs plummet like the Bay's once bountiful oyster fishery? Or the striped bass fishery before its rescue?

In 1995 reports based on an annual Baywide monitoring of blue crabs spurred anxiety over a possible decline. Some newspapers reported a decline of adult crabs as high as 62 percent, compared with the previous five years, a number that was quickly corrected and revised downward to 34 percent. Scientists and resource managers alike scrambled to understand not only the short-term but the long-term trend of Chesapeake crab stocks.

Last year annual Baywide monitoring of blue crabs showed a 34-percent drop in abundance compared with the average of five previous years.

The cause for public unease appears to rise not only from scientific reports, but from an underlying intuition that we may be too fond of the blue crab. With consumer demand growing for hard and soft shell crabs, and with seafood processors competing for the picked crab market, many Marylanders and Virginians worry that a vast array of pots, trotlines, dredges and dipnets are taking too many blue crabs out of the Bay.

For Bay researchers and resource managers, the question became this: Was the 34-percent drop the first sign of a crisis, or was it merely a low point on the chart of booms and busts that characterize most fishery harvests - more the result of natural processes, such as storms, winds, rains, and temperature shifts, all of which affect the number of young crabs that inhabit the Bay each year? Or did it indicate some other problem as well - such as the loss of underwater grass beds, a key habitat for crabs?

"We had been identifying a number of developing problems," says W. Pete Jensen of Maryland's Department of Natural Resources. An increase in fishing effort, for example. That was one reason, says Jensen, for limiting entry into the commercial fishery in 1994 and for putting limits on fishing hours and numbers of crab pots.

In January 1995, at the annual East Coast Fisherman's Expo in Ocean City, Anne Lange of the NOAA Chesapeake Bay Office presented a graph suggesting a decline. As this and other data emerged, Jensen grew especially concerned about "a declining trend in mature female crabs." These were signs, says Jensen, that meant Maryland had to act: "We have a very valuable fishery, and we are just not going to wait for a crisis to occur."

Maryland did act, by imposing in the fall of 1995 a temporary set of emergency crabbing restrictions. Additional, though less stringent, regulations have been proposed for 1996 (see Sidebar, "The Summer of 1996").

The actions by fisheries managers in Maryland and Virginia, says Bill Goldsborough of the Chesapeake Bay Foundation, represent a historical shift from crisis management to conservation. "However," he says, "it means that if we're going to act to avoid a crisis, we're acting with less definitive information. If we want more perfect information, we'll have to wait for a collapse. So it's a matter of being sensitive to warning signs - even if they're not definitive."

Although Chesapeake watermen harvest nearly 100 million pounds of blue crabs a year, the number of crabs available for catching fluctuates. According to CBSAC scientiest, crab stocks peaked during the 1980's, then declined in the 1990's. While the 1995 winte dredge survey showed a 34-percent decline of adult crabs, preliminary results from the 1996 survey appear to show a rebound.

Taking Stock of Blue Crabs

Based on the 1995 survey, the subcommittee started from the position that there was a stock collapse. But after an analysis of long-term monitoring and other data, they "simply were not picking up the problem."

To get a better handle on the status of blue crab stocks, the Chesapeake Bay Stock Assessment Committee (CBSAC) Technical Subcommittee brought together available data from blue crab research and monitoring efforts in order to do an analytical stock assessment. According to fisheries scientist Louis Rugolo of the Maryland DNR, who chairs that subcommittee, stock assessment is a mathematical tool that can give managers a quantitative idea of whether the fishery is being harvested above or below its highest sustainable rate. As Rugolo points out, the mathematics are only as good as the quality of data that are available, including basic assumptions about crab biology.

Such an assessment had not been done for the blue crab, says Bess Gillelan, chief of the National Oceanic and Atmospheric Administration's Chesapeake Bay Office in Annapolis and chair of CBSAC, because long-term Baywide monitoring information wasn't available. "We realized the limitations of that data on blue crab in the '80s," she says, "and that was a major reason for beginning the annual winter dredge survey."

The Baywide dredge survey is a comprehensive monitoring effort covering crab stocks in Maryland and Virginia (see "Blue Crabs in Winter," Marine Notes, Vol. 12, no. 1).

Rugolo's technical subcommittee gathered statistics from commercial harvests and from state monitoring efforts, in addition to laboratory and field studies reported in the scientific literature (see Sidebar, "Counting Crabs"). The committee included representatives from Maryland and Virginia and the NOAA Chesapeake Bay Office, as well as from outside the Bay region: Vic Crecco of the Connecticut Department of Environmental Protection and two scientists from the National Marine Fisheries Service - Mark Terceiro from the Northeast Fisheries Science Center at Woods Hole and Douglas Vaughn of the Southeast Fisheries Science Center at Beaufort, North Carolina.

"When our subcommittee met last July in Beaufort," says Rugolo, "we were starting from the position that there was a stock collapse. After the announcement of the blue crab's 34-percent decline, we believed that we were going to find a stock problem."

As the subcommittee members went through the data and finished a preliminary analysis, they were "stunned and flabbergasted," says Rugolo. "We simply were not picking up the problem."

How Long Does a Blue Crab Live?

"The first order of business in doing an assessment is literally to define the characteristics of the species," says Rugolo. For the blue crab, like other species, this means identifying its life history, how soon it matures, how fast it grows, how long it lives - these are examples of biological reference points that can then help scientists calculate levels of catch or exploitation that will leave enough crabs to sustain stocks at a desired level.

The blue crab's natural longevity is an important factor here and one still disputed by scientists. So far, no reliable method exists for calculating the age of blue crabs, a process made difficult by molting, where the crab leaves its shell - and evidence of its age - behind.

The question of aging is crucial. According to John McConaugha of Old Dominion University, if the crab reaches maturity quickly, only living to age three, as many scientists have believed, then one could argue that fishermen may as well harvest them, since market-sized crabs will have already reproduced and will die anyway. If, on the other hand, crabs can live to age six or seven or eight, then fisheries managers may want to establish regulations based on longer lives. For instance, females are thought to mate only once, though they can spawn more than once - a second or third spawning could depend on how much sperm she is still carrying from the original mating.

If larger (and, therefore, older) male crabs carry and deliver more sperm, one management consideration may focus on protecting large males. Jacques van Montfrans of the Virginia Institute of Marine Science argues that protecting females is also important since larger females produce more eggs. An important management issue, then, would be to protect the largest spawners, not just the egg-carrying females, known as sponge crabs. "Fishing on them [the large males and females]," he says, "could potentially have a great impact on the crab population."

Clearly, the age used in stock assessment models can significantly influence a model's projections. While long-time Bay scientists like Eugene Cronin and Willard Van Engel have taught for years that crabs live three, at most four, years, there have been recent indications that they may live five or six or even eight years. Though one tagging study from the 1940s showed that blue crabs lived to at least five years, that study took place farther south, in Albemarle Sound, North Carolina.

Are such results applicable to the Chesapeake Bay as well? According to some researchers, new information is causing scientists to rethink several basic assumptions about blue crab biology - not only the natural life span of a crab, but multiple spawnings by females, and possible molting after reaching sexual maturity, what had traditionally been considered the "terminal" molt.

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Jimmies or sooks -- which are more important for maintaining blue crab stocks? Some believe that protecting egg-bearing females is more important, but others argue that size may be as important as sex, and that we should protect older, larger crabs. This is complicated by the fact that scientists do not yet have an accurate means for determining the natural life span of the blue crab.

In 1989 and 1990 McConaugha was funded by CBSAC to conduct a tagging study to try to determine the longevity of female crabs. His group tagged 17,000 female crabs and got about a thousand back, most in the first and second years. When he received calls of tag returns five years later, in 1994 and 1995, he remained skeptical because he never saw the crabs himself. This spring, however, a female crab aged seven or eight years was returned from a source considered reputable.

The question of age is crucial. Stock predictions depend on the life span fisheries modelers assume for blue crabs, and that can vary from 4 to 6, or even 8 years.

In the face of changing information Rugolo's committee assumed a longevity of, at first, six years - a biologically conservative age limit - and then in response to scientific reaction, of four years - a more traditional assumption. In their analyses an assumption of six years led to the conclusion that crab stocks were "close to being [fully] exploited." When an assumed longevity of four years was used, the model pointed to "no evidence of overfishing."

Then again, if one assumes that blue crabs can live to seven or eight years, the model may imply that current harvesting could be at the edge - or over the edge - of what blue crabs need to successfully reproduce.

The CBSAC report caused a good deal of controversy: some in the public media focused on the lack of a crisis and downplayed the call for strong conservation measures. Some scientists criticized the assumptions about blue crab age estimates and questioned the stock assessment analysis itself. According to Don Boesch, President of the University System of Maryland Center for Environmental Science, "The stock assessment should have had more inclusion of blue crab biologists from the start, more factoring in of the uncertainty involved in stock assessment models, more review and quality control, and more direct communication with managers."

"It is difficult to get consensus," says van Montfrans. "There are things we don't know and things we don't agree on - people tend to think scientists ought to have a unified view."

The stock assessment subcommittee continues to refine its analysis, which will be reviewed by scientists before its release this fall.

For fisheries managers, stock assessment analysis is only one measure of the state of a fishery and only one tool in gauging its health. They have to consider other issues in setting regulations to ensure a sustainable fishery. Bess Gillelan points to the importance of bi-state consistency between Maryland and Virginia, to the need for long-term stability for the industry in the face of fluctuating stocks, and to the overall economics of the fishery - all significant considerations in addition to the question of age and abundance.

Future Management Issues

While a better understanding of blue crab biology and population dynamics is critical to manage the fishery more effectively, there are unquestionably social, cultural and economic considerations as well. Doug Lipton, a marine economist with the Maryland Sea Grant Extension Program, points out that whatever decisions managers make will have large economic impacts. And, adds Lipton, we need to consider the potential economic impacts of that decisionmaking from the outset.

Lipton and economists James Kirkley at the Virginia Institute of Marine Science and Leonard Shabman of the Virginia Polytechnic Institute are beginning to pose the kinds of questions that could influence the future of blue crab management in the Chesapeake Bay. For example, asks Lipton, are there less costly ways of controlling the fishery? Are we achieving our biological goals of sustainability in an efficient manner? For example, declining trends in the effort it takes to catch crabs (the catch per unit effort) could be due to too much fishing gear - if so, every additional pot or trotline results in everyone harvesting fewer crabs.

We need to ask hard questions, Lipton says, questions that in the Chesapeake Bay are likely to be controversial. For instance, some fisheries in other coastal areas have instituted Individual Transferable Quotas (ITQs), a limited entry system in which fishermen are given quotas on how much they can harvest. This system of management eliminates the race to harvest crabs that can lead to what Lipton calls "excessive use of gear" and a waste of resources. With ITQs, he says, the crabber is free to harvest his or her quota at the lowest possible cost.

For example, says Lipton, every year some 40 million pounds of crabs are being harvested from the Maryland portion Chesapeake Bay, on average. "How many pots do we need to catch that?" he asks, adding that "every pot over that is a waste."

In addition to all these questions are questions about the impact of regulations on Bayside fishing communities, and on those who depend on the blue crab for a livelihood - not to mention the half a million recreational crabbers who spend their free time chasing crabs.

Already a Fisheries Management Plan is being finalized for the Chesapeake blue crab, though important gaps in biological information still remain. In particular, researchers are considering new analytic tools - such as the measurement of lipofuscins, compounds that accumulate over time - to accurately age blue crabs. Meanwhile, resource managers will continue to adjust regulations according to the best information that fisheries science can provide. In the end, there is no doubt that decisions about the blue crab fishery will provide a fascinating history of how scientists, citizens and decision makers interact to manage a much sought-after public resource.

(Jack Greer contributed to this article.)

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