Ruby-throated Hummingbirds

Blue Mussels in Decline

by Sue Pike

October 15 2016  The York Weekly/York County Coast Star/Portsmouth Herald/Fosters Daily/Exeter News

My neighbor’s brother lives along the coast in Ipswich Massachusetts.  All of his life he has gathered oysters, clams and mussels from the mudflats and rocky shoreline near his house.  He had noticed their population declining over the past couple of years but as of this summer all of his mussels were gone. 

I like to mussel, it’s easy and I love those chewy orange nuggets, especially steamed with garlic and

photo by Sue Pike  Blue mussels at low tide

butter and wine.  Like my neighbor’s brother, I too have been having to search farther and farther afield for these tasty morsels, but I didn’t really think about it being a widespread problem until my neighbor called and asked about what was happening at her brothers.   I had missed a report that came out back in July; researchers (Sorte et al., Global Change Biology) compared modern population sizes of blue mussels (Mytilus edulis) in the Gulf of Maine with historic populations and found a greater than 60% decline since the 1970s.  Intertidal communities that used to be composed of large beds of blue mussels and a rich assortment of other species are now largely composed of algae and barnacles.

This finding is huge, not only because it spells the end to my carefree mussel-collecting days, but more importantly because of what it means to the ecology of the coast, and by extension the health of all ocean and terrestrial communities that rely upon the coast.  Blue mussels are foundation species.  Foundation species are organisms that have an important role in structuring a community.  Corals and beaver are foundation species for obvious reasons.  Mussels are foundation species because their presence influences the diversity of intertidal habitats-when mussels are abundant there are higher numbers of other intertidal species.  If you think about it, having masses of mussels attached to intertidal rocks creates a reef—a complex environment that provides shelter for some species and novel attachment sites for others. 

In addition to their role as foundation species mussels are filter feeders-they clean the ocean around them-filtering out bacteria, heavy metals and toxins (I have a friend who refuses to eat all filter feeders for this reason).

Why are mussel populations in decline?  It appears to be a combination of factors; warming ocean temperatures (the Gulf of Maine is warming faster than almost any other place on Earth!), increased harvesting by humans and predation by invasive species. For example, since I moved back to Maine in the mid-1990s the invasive Asian shore crab (Hemigrapsus sanguineus) has also moved in.  These tiny crabs have a broad diet and are wreaking havoc in the intertidal-they feed upon seaweeds, salt marsh grass, larval and juvenile fish and many native invertebrates such as barnacles, snails and bivalves (like mussels).  One study in Long Island Sound found that Asian shore crabs accounted for as much as 25% of the blue mussel mortality at the site studied.

Human harvesting has also increased.  Maine and Massachusetts account for the majority of wild blue mussel harvesting on the east coast.  These novel and highly efficient predators (humans and crabs) coupled with warming ocean temperatures might be enough to extirpate blue crabs from the Gulf of Maine.

Most studies of biodiversity decline indicate that we are in the midst of the sixth mass extinction, an extinction event comparable to the one that saw the end of the dinosaurs. This is the first mass extinction to be caused by a single species (humans).  I find this hard to comprehend, I feel like if we’re in a mass extinction animals and plants should be dropping dead all around us on a daily basis. I am usually able to convince myself that it really isn’t that bad….and then I personally experience something like the decline of the mussels and realize that it is.