Assessment to weigh impact of fishing on blue crab stock
Latest survey finds 15 percent decline
Scientists will gather next month to wade through stacks of blue crab numbers - numbers of males, numbers of females, numbers of juveniles, numbers of crabs harvested by various techniques.
For two days, they will crunch numbers in computers, and feed them into models that replicate blue crab life cycles.
It will be the first-ever "stock assessment" done on the Bay's most valuable fishery - and one that is increasingly the focus of concern. The purpose of the assessment is to better understand the condition of the blue crab population and to deter mine the degree to which it is affected by fishing pressure.
"Some of the survey data indicate that there is a connection between fishing, harvest and abundance, and the models will help us determine that more precisely," said Bess Gillelan, director of the National Oceanic and Atmospheric Administration's Chesapeake Bay Office.
NOAA's National Marine Fisheries Service established a special Chesapeake Bay Stock Assessment Committee in 1985 to evaluate the health of fish species. Over the past seven years it has also funded the development of a long-term winter dredge su rvey that is the most comprehensive measure of blue crab abundance in the Bay.
Fishing pressure has increased on blue crabs in recent years as stocks of other Bay fish and shellfish have declined. Both Virginia and Maryland have adopted regulations to limit harvests, but several recent surveys indicate a continued decline a nd have caused some scientists to question whether those actions go far enough.
"There are warning signs that the population is under stress," said Bill Goldsborough, a fisheries scientist with the Chesapeake Bay Foundation. "We ought to heed the signs."
Maryland has already formed a special "blue crab management steering committee," consisting of scientists, state Department of Natural Resources fisheries managers and industry representatives. The committee will review blue crab issues and hold hearings to "keep an active discussion going on," said Pete Jensen, DNR Fisheries Director.
At this point, he said, it is still an "open question" as to whether new regulations will be proposed. "We're going to watch it very closely," he said. "If we see anything that seems to warrant additional regulations, we will."
A representative of the Virginia Marine Resources Commission suggested at a recent Chesapeake Bay Commission meeting that the VMRC may also need to reexamine the issue.
The latest Baywide winter dredge survey showed a 15 percent decline in the blue crab population from last year. Over the past six years, the population has fallen 34 percent, according to the survey.
The 15 percent drop was slightly better than had been indicated by preliminary survey figures released earlier this year, but was still enough to concern many blue crab scientists who reviewed the results at a special meeting convened by the Stoc k Assessment Committee in April. "There was still lots of cause for concern," Gillelan said.
The outcome of the upcoming stock assessment will not only help determine the health of the stock, but what harvest levels will help maintain a healthy population. It may also provide clues about which segment of the population should be targeted for protection to preserve the overall stock.
Much of the thinking among the scientists who attended the April meeting was that any further restrictions should be targeted to protect female crabs so they can reproduce. A large portion of the crab harvest is aimed at female crabs that migrate down the Bay during the fall and winter.
"There's really not any protection for mature females," Goldsborough said. He suggested that a deep-water sanctuary be created running north to south along the middle of the Bay, allowing a clear path for females migrating to the mouth of the Chesapeake.
In the summer, females that have migrated to the lower Bay extrude their eggs; a single female may produce anywhere from 750,000 to 8 million eggs. The larval crabs leave the Bay to swim in coastal waters for up to two months before returning to the Chesapeake.
While protecting females was a top concern of scientists, there were also questions about the health of the male crab population. The average size and weight of males, as well as their numbers relative to the number of females, have declined in recent years.
Although studies have shown that most - between 90 percent and 97 percent - of females are impregnated by the time the fall migration begins, the amount of sperm provided is widely varied. The fewer males per female means that males are mating mu ltiple times, reducing the amount of sperm transferred to each female. More study is needed to determine if reductions in the amount of sperm available to females is affecting their ability to to produce large numbers of viable eggs, the scientists said.
While fishing pressure was the leading concern cited by scientists, other factors may be affecting populations as well. Among some of those mentioned:
- Climate: Water temperature and other climatic factors are important to the blue crab, though there is no clear evidence that recent declines were related to any fluctuations in climate.
- Predation and cannibalism: Studies suggest cannibalism of small blue crabs may be high in some years, but there is no evidence of an increasing trend for this kind of mortality. Predation by striped bass is also a concern, though scientists do not believe this has a significant impact on overall population levels.
- Habitat: Decreases in habitats important to blue crabs could contribute to a decline, though the amount of Bay grasses in the Chesapeake - one of the most important habitats for blue crabs - has generally increased in recent years.
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