September 5, 2008

Blue Crab Mystery

Visitors to Baltimore’s Inner Harbor were in for a surprise earlier this week. Hundreds of blue crabs lined the shallow water between the Maryland Science Center and the Rusty Scupper restaurant, about a 150-yard distance.

Though an iconic image throughout the Harbor, blue crabs are usually more likely to be seen on a dinner plate at Phillips Restaurant than in the murky waters of the Patapsco River.

The sheer number of crabs – I estimated over 350 – led me to suspect something was up. Could it be a jubilee – an event where crabs gather near the surface and on shore to escape oxygen-deprived water?

Mike Naylor, a biologist at Maryland Department of Natural Resources (MDNR), doesn’t think so. After looking at the pictures he noted that had it been a jubilee, he would have expected to see crabs breaking the surface. Instead, these crabs seemed perfectly content to stay in the water. Naylor also noted that some of the crabs were exhibiting normal mating behavior, which may be unlikely if they were stressed.

crabs swimming

Click to play video of blue crabs in Baltimore’s Inner Harbor. Photo and video by Kyle Smits

When I went back to the spot the following evening, the number of crabs had dropped significantly – probably less than 50 remained.

So what caused this unusual crab gathering? No one seems to know for sure. Dissolved oxygen readings taken about 36 hours after the event were within the normal range. But levels can change quickly. I’ll be sure to post again if I get an answer as to what brought the crabs together in such large numbers.

Whatever the reason, it was a great view of nature in my urban neighborhood. And it seemed to remind those walking by that the ecosystem of the Inner Harbor is more than just its seafood restaurants and T-shirt shops.

Information on blue crabs:

http://www.mdsg.umd.edu/issues/chesapeake/blue_crabs/

Chesapeake Quarterly, Counting Crabs in Winter, Volume 5 Number 4

3 Responses to “Blue Crab Mystery”

  1. John says:

    maybe there was a food supply.

  2. Ivy McClellan says:

    Were there any boating events near the harbor that day? The water seems awfully clear and shallow. Was it overcast? Was there a weather fluctuation? Lots of things it could have been. Wish I was still home to have seen that.

  3. Sandy Rodgers says:

    It was a clear sunny day in September with no big weather fluctuations and no events that Jessica remembers. We never figured out the cause. Maybe someone at the Institute of Marine and Environmental Technology, located in the Inner Harbor, might have an idea. Here’s their web address: http://www.umces.edu/imet/about-imet.

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