Blue Mussels: Hanging by a thread in Casco Bay

Posted on Nov 13, 2015

Ann Thayer searches for mussel beds along the shores of Casco Bay.

Ann Thayer searches for mussel beds along the shores of Casco Bay.

When Ann Thayer goes out in her Boston Whaler, it’s not just to enjoy time on beautiful Casco Bay. This Friends of Casco Bay Board Member is scouting out mussel beds, and more often than not, she is not finding them.

Blue mussels (Mytilus edulis) are the common mussels found along the Maine coast. Ann explains, “In addition to providing a rich habitat for other sea life, dense mussel beds can provide protection to the shorefront against the effects of storm surges. Mussels are filter feeders, and they can siphon up to 25 gallons of water a day as they feed on microscopic algae and nutrients in the water column. In short, they are important contributors to the Bay!”

For the past few years, anecdotal accounts suggest that mussel beds once piled high with layers of living mussels are now all but gone. Ann, an environmental scientist by training, offered to lead a volunteer effort to survey the Bay to see if reports of disappearing mussel beds are true. Over the past two summers, she and a handful of other observers have looked at more than 25 areas between Portland and Harpswell, surveying by foot, kayak, and small boat.

Baby mussel spat, the floating plankton phase of mussels, appear to be plentiful in the water column. Commercial aquaculture growers are getting plenty of natural seed set on their ropegrown mussels. Juvenile mussels are being found on mooring ropes, wharf pilings, and floating docks. “It’s the intertidal and subtidal horizontal mussel beds that are missing,” Ann reports.

Dr. Brian Beal, Professor of Marine Ecology at University of Maine at Machias, and others suspect that bottom-dwelling crustaceans, such as lobsters, rock crabs, green crabs, and Asian shore crabs, may feed on baby mussels trying to establish a foothold on the ocean floor. But Brian says field testing is needed to prove or disprove this hypothesis. He suggests cordoning off some bottom areas from predators with cages or nets to see if juvenile mussels survive there.

Ann has found small juvenile mussels on flats in Brunswick. “What is disturbing is that when you do find pockets of mussels, they are just individuals of one age. You don’t find whole beds of mussels, with new mussels growing on older ones, like we used to see.”

Everyone seems to have a different theory as to why the once ubiquitous blue mussels have disappeared: green crabs and other predators, dragger nets destroying mussel beds, warming sea temperatures, and ocean acidification. Cathy Ramsdell, Friends of Casco Bay Executive Director/Casco Baykeeper Pro Tem, cautions, “There are a lot of theories, but there isn’t much research being done on the change in distribution patterns of blue mussels and the possible causes. All we have is speculation. Our current objective is to try to get a handle on presence or absence of beds along the coastline of Casco Bay.”

Ann Thayer says, “This project with Friends of Casco Bay is totally driven by citizen scientists. It’s another reason why our volunteers are so important. We are looking for people with small boats who can survey the eastern part of the Bay from Brunswick to the New Meadows River.” If you are interested in joining the search, contact Friends of Casco Bay at keeper [at] cascobay [dot] org.