Important Blue Crab Habitat Again Suffers Die-Off in Southern Chesapeake Bay
from Bay Daily by Chesapeake Bay Foundation

Eelgrass A form of aquatic vegetation called eelgrass that is a critical habitat for blue crabs appears to have suffered another die-off in the southern Chesapeake Bay this past summer -- the second setback for these grasses in five years, according to a report in the Bay Journal.

The Bay Journal article, based on research by Robert Orth, an aquatic vegetation expert at the Virginia Institute of Marine Science, says that scorching summer heat might have caused the die-off this year, as it did in 2005.

However, Mike Naylor, an assistant director at the Maryland Department of Natural Resources who has also studied Bay grasses for years, offers a cautionary note, suggesting that the weather conditions this past summer should not be exaggerated.

Naylor said that while temperatures hit unusual highs early in the summer, the rest of the summer wasn't that hot. "Summer water temperatures were near normal in 2010. June was a hot month, setting a number of records for high temperatures, but after than it cooled down," Naylor wrote in a comment on Bay Daily this morning, after the initial publication yesterday of an article suggesting extreme summer temperatures had killed the eelgrass.

The Bay Journal quotes Orth, the Virginia grasses guru, as saying he finds it worrisome that two temperature-related diebacks of eelgrass have taken place within six years, when none had been observed since the annual monitoring was initiated in 1984. A more extensive die-off of eelgrass denuded parts of the southern Bay in 2005. "If this continues to happen, the likelihood of eelgrass remaining as a persistent member of our community could be really thwarted," Orth said.

What caused the recent die-offs of eelgrass?

The Virginia Institute of Marine Science blog on aquatic vegetation suggests high temperatures are to blame.

"It does appear that eelgrass here in the lower bay has suffered another setback this summer due to the really hot summertime temperatures," the blog says. "While the springtime aerial survey showed abundant SAV and has documented the continued recovery of eelgrass from the 2005 dieback, many of these beds now have almost no eelgrass present except for a few small shoots."

A report by the Chesapeake Bay Foundation detailed the important role that eelgrass -- which is unusually sensitive to heat -- plays in the blue crab lifecycle.

"The disappearance of eelgrass is especially harmful to crabs because a crucial part of their lifecycle unfolds in the southern Bay, among these underwater plants with long, slender leaves.

Blue crabs spend the first months of their life as larvae, floating free in the Atlantic Ocean. Then they are swept by currents into the southern Bay. At this point,they look like tiny lobsters and they are called megalopae. But essentially they are a form of plankton (or fish food).

The megalopae settle out of the water column in the southern Bay into the eelgrass where they find shelter from predators like red drum and croakers. Without the hiding places, the megalopae would be gobbled up. Scientists have counted 30 times more< young crabs in grass beds than on the barren bottom.

Eating smaller crustaceans that thrive in the grass, the megalopae molt and grow into juvenile crabs. Over a period of two or three weeks, the crabs molt about a half dozen times in the protective jungle of underwater plants. When they are big enough to move out—about a half inch—the crabs start migrating northward up the Bay, where many spend much of their lives."

By Tom Pelton

Chesapeake Bay Foundation

(Note this article was written on 11/11/2010, and then updated with Naylor's comments on 11/12/2010)

To learn more about conditions in the Bay, visit the "Eyes on the Bay" website.
Important Blue Crab Habitat Again Suffers Die-Off in Southern Chesapeake Bay-1.jpg
(Photo of eelgrass at top from VIMS)