A Green Crab is not Distinctly Green.  The Top of Their Shell Can Range Between Dark Brown and Green With Flecks of Yellow.  The Bottom of Their Shell Can be White, Orange or Red.  (Photo Provided by Friends of Casco  Bay.)

Data from the Friends of Casco Bay Yarmouth Continuous Monitoring Station shows that Casco Bay is about 4 degree F warmer than the past seven winters, on average according to a report issued Thursday, March 2nd, from the Friends of Casco Bay.  This observation comes as scientists continue to report year-round temperatures in Casco Bay and the Gulf of Maine are rising faster than most other water bodies on the planet.

“While 4 degrees may not seem like much, this seeingly small change in temperature can kick off a cascade of changes throughout Casco Bay’s ecosystem,” says Staff Scientist Mike Doan.  Green crabs provide a compelling example.

Originally from Europe green crabs are estimated to have arrived  in Maine around 1900 after hitching a ride on ships crossing the Atlantic Ocean.  Their preferences for feeding on shellfish like softshell clams became a clear problem in the 1950s, when an increase in water temperatures returned in the 1950s, when an increase in water temperatures coincided with a boom in Maine green crab populations.  Softshell clam harvests plunged in those years.  The fishery rebounded after coldeer water temperatures returned in the 1960s and the green crab population declined.

The connection between warmer water temperatures and more green crabs has been seen repeatedly in Maine.  Scientists think this is partly because green crabs struggle to survive cold winters, but fare much better when water temperatures warm.

As we think about this winter’s warm water temperatures, green crabs and their potential impacts, it is important to remember a common saying that any scientist will know:  correlation does not imply causation.  In a complex ecosystem like Casco Bay, there are many factors that influence the size of green crab populations.  Temperature is only one of them.

“It would be misleading to say that we will certainly see more green crabs in the Bay this summer,” says Mike.  “But with what we know from science and Maine’s history, its fair to say there is a good chance we will see more green crabs in the coming months.”