Bay Journal

Blue crabs remain at low levels despite decline in fishing pressure

The assessment suggests considering separate harvest thresholds for male and female crabs

  • By Karl Blankenship on December 01, 2005
In addition to overharvesting, some have suggested that poor water quality, loss of habitat, increased predation by striped bass and other fish, or other factors are also affecting crab populations. 

 (Chesapeake Bay Program)

Fishing pressure on blue crabs has declined in recent years, but the abundance of the Bay’s most commercially valuable species remains at one of the lowest levels on record, according to the most comprehensive look at the Bay’s blue crabs in nearly a decade.

The first Chesapeake Bay blue crab stock assessment completed since 1997 concluded that the crustaceans were being overfished from 1998 through 2002, which contributed to the current low population levels. “There is yet to be convincing signs of recovery from this period of low abundance,” the assessment said.

The assessment, which drew on of dozens of studies that had been completed in recent years, made numerous management recommendations, including suggesting that management be more dynamic to adjust to changing population levels, and that male and female populations be managed differently in the future.

The assessment was funded by the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office and completed by a team of scientists from the University of Maryland, the Maryland Department of Natural Resources and the Virginia Institute of Marine Science.

It received high marks from an independent peer review conducted by the Center of Independent Experts, which is affiliated with the University of Miami. An international review team assembled by the center said the assessment was “a major advance over the [previous] assessments.”

Using the new information, the assessment established a new, easier to measure, overfishing “threshold”—the maximum catch that should be allowed—as well as a new management “target” which is intended to provide a wider margin of safety.

Under the new threshold, no more than 53 percent of the harvest size crabs should be taken each year. After accounting for natural mortality, that should protect 10 percent of the spawning stock, the current management goal. (The actual crab population is estimated each winter by a Baywide dredge survey.)

The new target is a harvest limit of 46 percent of the adult population, which would protect 20 percent of the spawning potential, which is the current management target, but has never been reached since it was established in 2000.

Using the new threshold, the assessment concluded that crabs were not overfished in 2003, the final year examined by the assessment team.

But more recent data shows that the crabs were overfished again in 2004, said Tom Miller, a fisheries scientist with the University of Maryland Center for Environmental Science, who led the stock assessment team. “Our best prediction for 2005 is that it is back down below the overfished definition again,” he added.

Miller said he believed the 2004 figure was a “bit anomalous” and that the general pattern is a slow decline in fishing pressure.

That suggests that management efforts taken in 2001 and 2002 to reduce fishing pressure may be having an effect. But so far, they have not resulted in the desired goal of increasing the number of female spawners.

Surveys by the Virginia Institute of Marine Science have shown an 80 percent decline in the abundance of spawning females in spawning grounds during the spawning season since 1992. Other indicators do not show the same trend, but none show evidence of a recovery, and the assessment said the Virginia trends deserved “close monitoring.”

In addition, the assessment said that although overall fishing pressure had declined, it had fallen less for females than it had for males. Because of the difference in fishing pressure on males and females, it said a separate assessment of the male and female populations should be considered in the future, which could lead to a different fishing threshold for each sex.

“If we don’t manage them differently, we make the assumption that a crab taken from any part of the Bay is equivalent, and that likely is not the case,” Miller said. “There are times when you wouldn’t want to harvest particular crabs.”

The assessment included an extensive effort to reconstruct long-term stock abundances along with reconstructed harvest levels that took into account the effect of past changes in the way landings were reported. That effort showed that fishing pressure on crabs remained nearly constant regardless of the size of the stock. As a result, fishing pressure—relative to the size of the stock—is highest when the population is lowest.

“That is exactly the opposite of what you want, because what that will do is drive a stock further down,” said Rom Lipcius, a blue crab researcher at VIMS who was a member of the assessment team.

As a result, the assessment suggested that management needed to be more dynamic to adjust for changing population levels. Right now, management agencies primarily regulate effort—the amount of time or gear people can use crabbing. But the same amount of effort with low numbers of crabs can mean a larger fraction of the stock is taken when populations are low.

Derek Orner, a fisheries biologist with NOAA’s Chesapeake Bay Office, said the next step is for the states to adopt “decision rules” about what they will do if the harvests exceed the targets and thresholds in the assessment.

Although an overfishing threshold and precautionary target for harvests was established in 2000, surpassing those levels never triggered any management action. “If we go above the threshold, that’s bad, but there really is no action or decision that goes along with that,” Orner said.

He said state managers agreed to begin developing those actions at the October Fisheries Steering Committee meeting, where the stock assessment was officially approved. The committee seeks to coordinate fishery management actions among Bay jurisdictions.

“It can go the other way too,” Orner added. “If you happen to be 10 percent under our target, maybe the decision would be to loosen the regulations a little bit to allow a few more to be taken.”

Another concern that came out of the reconstructed historical population estimates is that the stock has remained low for so long without an obvious sign of recovery.

“There have been periods of high and low abundance.” Miller said. “What marks this period as different is that it has been low for a prolonged period of time, longer than we have seen in the past. In the past, we have had blips, but they have appeared to recover within a four– or five-year period. We have not seen that. We have seen a decade of relatively low abundances.”

Miller said that current harvest levels would be expected to result in an increase in abundance over time — unless something else is “fundamentally different” in the Bay.

“If for some reason the capacity of the Chesapeake Bay to support crabs has changed, we may not get back up there,” he said.

Some have suggested that poor water quality, loss of habitat, increased predation by striped bass and other fish, or other factors are also affecting crab populations.

“I think we have to start looking at other things,” Lipcius said. “We have to start looking at environmental issues. Are we continuing to degrade the nursery habitats so that the production is going down?”

Information from the assessment will also be used in the development of a new Chesapeake Bay blue crab fishery management plan, which is expected next year.

The new fishery management plan under development is expected to take a broader look at ecosystem factors affecting blue crabs than plans in the past.

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About Karl Blankenship

Karl Blankenship is editor of the Bay Journal and Executive Director of Chesapeake Media Service. He has served as editor of the Bay Journal since its inception in 1991. .(JavaScript must be enabled to view this email address).

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