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)