Crab Population Survey Finds Record Low Juveniles, More Females
Back in 2007, the number of blue crabs in the Chesapeake Bay plummeted to an all-time low, in part because of chronic over-harvesting. Watermen every year caught almost two thirds of all the crabs alive in the Bay.
The crisis got so bad that the federal government declared the Chesapeake Bay an economic disaster, allowing watermen to collect disaster relief payments.
To address the problem, the administration of Maryland Governor Martin O’Malley worked with Governor Tim Kaine of Virginia to protect female blue crabs, which are critical to crab management because each female can each produce up to three million eggs a year.
Virginia outlawed the destructive practice of watermen dredging up hibernating females – full of fertilized eggs -- from the southern portion of the Bay during the winter. Maryland banned the recreational harvest of female blue crabs.
In years that followed, the number of blue crabs in the Bay more than tripled, from 251 million in 2007 to 765 million in 2012, leading O’Malley to declare victory. However, since then, the numbers of crabs have been tumbling, falling to an estimated 282 million in the most recent survey – nearly as few as during the Bay’s blue crab disaster of 2007.
So what happened?
I asked Michael Luisi, director of fisheries assessment at the Department of Natural Resources. He said that blue crab populations in the Bay vary widely from year to year, because they are a short-lived species whose larvae can be easily killed by unpredictable wind and temperature conditions.
It does not appear that any increase in harvesting – or any loosening up of the regulations – caused the drop in recent years, Luisi said.
“It really comes down to a little bit of mother nature,” Luisi said. “When the females release their eggs, and the larvae go out into the ocean. And then tides and winds and currents are supposed to draw them back into the Chesapeake Bay. Sometimes that doesn’t happen as well as it does in other years. And so this was one of those years when those juvenile crabs that came back to the bay – we saw fewer of them than we saw in the past.”
While the number of juvenile crabs in the Bay hit a record low last winter, the number of spawning-age female crabs actually increased and appeared to be reasonably healthy. For this reason, Luisi said, the state is not proposing additional restrictions on crabbing.
Tuck Hines is the Director of the Smithsonian Environmental Research Center and a blue crab expert. He said the recent decline in young crabs in the Bay is worrisome – and could be caused by a die-off of eelgrass and other aquatic plants in the southern part of the bay.
“Rockfish and lots of other things feed off of blue crabs, especially in the lower bay,” Hines said. “And seagrasses have been a great refuge habitat for juvenile blue crabs. With the loss of seagrasses, and last year was another decline in the abundance of seagrasses, probably due to rainfall and nutrient inputs, and also due to the hot summer weathers, which tend to inhibit the seagrasses. That is reducing that refuge habitat that allows those juveniles to hide out and grow large and become part of the fishery.”
The only way to stop this loss of aquatic vegetation in the Bay is by attacking much bigger environmental issues than crab catch limits: Notably, global warming, which is cooking heat-sensitive Bay grasses and increasing rainfall and runoff pollution; and the over-fertilization of farmland, which is feeding algal blooms that are darkening the Bay’s waters.
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