Blue Crabs

Back from the Brink or Still in Trouble?

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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.

New science-based guidelines were established in 2008, 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 in 2013, survey numbers dropped back to 300 million. In 2011 a recommended target of 215 million spawning age female crabs and a threshold of 70 million were adopted by Maryland, Virginia, and the Potomac River Fisheries Commission. The 2017 winter dredge survey found the abundance of these females surpassed the recommended target, but both the numbers of juvenile crabs and males declined. This most recent survey placed the total number of crabs at 455 million.

The Bay's blue crab population has always been prone to fluctuation, but it is clear it 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.

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.

"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."

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.

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, resulting in the cooperative efforts of federal and state agencies in what is called the Chesapeake Clean Water Blueprint.

At the center of this effort 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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