Blue Crabs

Back from the Brink or Still in Trouble?


The fact that we can't control Mother Nature makes it even more imperative that we focus on the elements we can control, especially reducing pollution to the Bay and maintaining sufficient numbers of spawning-age crabs.

Also known as "beautiful swimmer," the blue crab is one of the more resilient of Chesapeake species, but its fate depends on many factors. With the drastic decline of the Bay's oysters in the 1980s, watermen began extending their crabbing efforts much later into the fall, the time they would normally have shifted to oystering. A decade or so later, the crab population had been cut in half to around 300 million. Not only was the blue crab itself depleted, so too were the oyster reefs and underwater grasses that provide it with food, shelter, and oxygen.

The Complexity of Restoration

The Maryland Department of Natural Resources and Virginia Institute of Marine Science monitor the Bay's crab population with a comprehensive survey conducted in the winter when crabs are semi-dormant. Survey data shows that the population remained at a low level until 2008, when decision makers finally agreed to put in place new, science-based guidelines for the fishery, requiring a reduction of the catch of female crabs to a sustainable level. Within two years the population had doubled and watermen and fisheries scientists were hopeful the population was coming back from the brink. But 2012-2013 survey numbers dropped back to 300 million—a drastic loss even considering that the crab population has always been prone to fluctuations.

The 2016 survey found the total numbers were up to 553 million due to increases primarily in both adult male and female blue crabs. Most significantly, the number of adult female blue crabs rose from 101 million to 194 million, meaning the population, although still below healthy target levels, is getting closer 215 million target.

These most recent numbers while encouraging should still serve as a reminder that continual vigilance is necessary if we are to bring back not only a sustainable crab fishery, but a healthy, sustainable Bay.

Despite increases, it is clear the Bay's blue crab population would be strengthened by improved water quality and crab habitat. Reducing the levels of nutrients reaching the Bay from farms and lawns and better managing stormwater runoff before it gets into rivers and streams, will help mitigate weather extremes, improve water quality, and contribute to the recovery of both blue crabs and Bay grasses.

Virginia, Maryland, Pennsylvania, the other Bay states, and the District of Columbia all committed to reducing pollution from all sources as their part of their Clean Water Blueprint.

Creating a Sustainable Fishery

"The crab population is truncated, meaning we catch crabs so quickly, there are very few crabs bigger than legal size," Goldsborough said. "As a result, the fishery is very dependent on each year class that comes in.

"And that creates instability, which is not a good thing for the crabs or the crabbers. We want more older crabs in the crab population so that we have ongoing high reproductive potential—which helps stabilize the population, and larger crabs on average in the catch—which are the most valuable in the market."

Essential for building stability in the crab population is for Maryland and Virginia to continue applying the science-based guidelines for managing the fishery adopted in 2008, which help avoid the overfishing of previous years. Fisheries managers in Maryland and Virginia have been keeping total annual harvests below 25.5 percent of all females.

"The blue crab was really affected most by overexploitation by fishing," said Dr. Thomas Miller, Director of the Chesapeake Biological Lab. "A decade ago, they were experiencing 70 percent removal rates. And you can't take 70 percent of the trees and still have a forest. And if you take 70 percent of the crabs, you no longer have a healthy crab stock."

The Fight to Shrink "Dead Zones"

Another strategy to improve the long-term stability and health of the Chesapeake's crab populations is to cut significantly the nitrogen and phosphorus pollution into the Bay. Nitrogen and phosphorus stimulate explosive growth of algae. Algal blooms darken the water, blocking light and killing underwater grasses that crabs need for shelter. Algal blooms fed by polluted runoff quickly die and decay, sucking up oxygen and creating "dead zones." The dead zones force crabs to find oxygen in shallow waters where they are more easily caught.

"Dead zones" also kill the food that crabs eat, destroying or preventing the growth of 75,000 metric tons of clams and worms a year in the estuary, according to a scientific study in the journal Science. That is enough food to support half the commercial crab harvest.

In an effort to shrink the Bay's "dead zones," the U.S. Environmental Protection Agency issued pollution targets for the Chesapeake Bay in December 2010. These targets have come under attack by industry lobbying groups, however, and CBF is fighting to defend them in court and in Congress. 

At the center of this fight are blue crabs and the more than 6,000 watermen and other workers who depend on the crabs for their livelihoods.

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