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A Perfect Storm for Blue Crabs

BabybluecrabHere’s a paradox about the Chesapeake Bay.

A report by the University of Maryland revealed the Bay had the murkiest waters on record last year and declined in overall health to a grade of D plus. Heavy spring rains and Tropical Storm Lee in 2011 flushed tons of mud and pollutants into the estuary.

A second report, however, paints an entirely different picture. At press conference last week, Governor Martin O’Malley released the results of a scientific survey showing that blue crab populations have tripled over the last five years. Despite the polluted waters, the number of juvenile blue crabs last year set a record.

This was good news for the Bay -- but a challenge to logic.  How could the crabs do better when the water got worse?

Dr. Thomas Miller, Director of the Chesapeake Biological Lab, explained that water quality was not a factor in the crab resurgence of the last five years. The comeback of the once-troubled species was caused, he said, mostly by restrictions on catching female crabs imposed by Virginia and Maryland in 2008. The protection of mothers allowed more reproduction.

"The blue crab was really affected most by overexploitation, by fishing,” Dr. Miller said. “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.”

Why wasn't water quality a bigger factor?  For decades, we’ve been hearing how important it is to stop pollution in the Bay.  And yet, the Chesapeake’s best-known tenants –- the blue crabs –- apparently care little about it?  Why didn’t last year’s deluge of silt suffocate the crabs?

Part of the answer, Dr. Miller said, is that crabs are highly mobile and tend to stay in shallow waters.  Unlike, say, oysters and other bottom-dwellers –- crabs can scuttle or swim away from the most-polluted, low-oxygen zones on the bottom. 

“Blue crabs are an extremely resilient species,” said Lynn Fegley, Deputy Fisheries Director for the Maryland Department of Natural Resources. "I’ve seen crabs survive in a bilge for quite a while. And I’m certainly not comparing the Chesapeake Bay to a bilge. But they are tolerant.”

The real question, Fegley said, is how many more crabs would there be today if the water were cleaner?  There is a strong but indirect connection between crabs and pollution, in that pollution kills the clams and worms that crabs eat, and the underwater grasses that young crabs hide in to avoid predators.  Bad water quality limits the amount of crab food and shelter in the Bay, which puts a cap on the overall population.

That makes logical sense.  But here’s something counter-intuitive about last year’s storms.  Although they flushed a lot of pollution into the Bay, they could have also helped the Bay -- in a way-- by sweeping more crab larvae into the estuary from the Atlantic Ocean.

Crabs spawn at the mouth of the Bay, and their larvae are dependent on ocean currents to drive them back into the Chesapeake, where they find shelter and grow.

Tom O’Connell, Director of Fisheries for the Maryland Department of Natural Resources, said that some scientists have concluded that, during heavy rains, fresh water pours out of the Bay on the surface. But at the same time, deeper down, heavier salt water -– full of crab larvae -– rushes into the Chesapeake.

“That could be one of the reasons we saw a greater abundance of juveniles, because we had a greater influx of those juveniles from the ocean,” O’Connell said.

In other words, last year may have been a perfect storm for blue crabs: A combination of regulations that protected fertilized female crabs so they could spawn, and weather conditions that helped the larvae.

By Tom Pelton

Chesapeake Bay Foundation 

(Photo from the Chesapeake Bay Program)




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One of the surveys stated that the female carab population was down which doesnt make sense when attributing the population increase to conservation efforts.
What about the record mild winter and spring, could that be the reason for a population spike due to minimal over-wintering mortality loses? We all know that wild animals have a constant struggle to survive the winter and crabs are no different.
People cant take credit for the weather so they only focus on "their" conservation efforts.
Lets wait until next year to see what the population looks like before we give credit to conservation.

is a map available which shows the low oxygen areas of the Bay?

I agree we can't manipulate the weather and nature is going to surprise us with some resiliency.
An estuary like the Chesapeake Bay is an awesome treasure. Conservation efforts in Tampa Bay Estuary have brought unbelievable success in cleaning up that once-extremely-polluted bay.
The Chesapeake's a bigger bathtub, but if the Chesapeake could see that kind of progress it would be awesome, and re-pay, in countless ways. Meticulous study and conservation efforts (in tandem with luck, nature and a few prayers) are so crucial for management and restoration.
Kudos and congrats to our conservationists, scientists, volunteers, businesses and citizens for their on-going work-in-progress. Be great to see more oyster mariculture happening in the Chesapeake!

Interesting points, Mark. One theory is that the number of female crabs seemed to be "down" this past winter simply because the weather was so warm, the females did not retreat to their normal deep water bottoms to hibernate (where the scientists look for them). Perhaps they remained in shallow waters, where the survey did not count them.

Kathy, many of the low oxygen zones are commonly in the deepest parts of the bay during the summer. Although sometimes dead zones pop up elsewhere, and sudden drops in oxygen levels can kill fish even in shallow embayments and tributaries.

I'll see if I can find a map.

If just restrictions made such a difference, image the change improved water quality could make! Having restrictions while also improving water quality could really help the population levels and keep the crabbing industry alive and well.

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