• Charles Metcalfe

‘The disappearance of Mytilus edulis in the Gulf of Maine’


This case study will attempt to elaborate on a gradual decline in blue mussel (Mytilus edulis) populations in the Gulf of

Maine, and describe some potential solutions to the economic and ecological problems this may cause. Blue mussels are a keystone species in the Gulf of Maine, providing valuable habitat and food for many organisms, as well as playing a valuable role in removing pollutants from the water as they feed.[6], [10] Blue mussels are also an important product of Gulf fisheries, their commercial yield peaking at more than $13 million in 2013.[10] Within the last 40 years, mussel populations along the Gulf coast have decreased by over 60%, and are projected to fall even further as the Gulf continues to change.[12] As the Gulf of Maine is the among the fastest warming bodies of water on Earth at this time,[6] an understanding of the causes and effects of the loss of this keystone species may offer valuable insight into how our marine ecosystems will respond to climate change. The disappearance of the blue mussel is linked to many factors tied into our understanding of climate change in the Gulf, such as the invasion of new species, ocean acidification, rising temperatures, and human activity.[10]


The blue mussel, Mytilus edulis, plays a critical role as a keystone species in Gulf ecosystems. This bivalve helps to remove bacteria and toxins from the water through filter feeding, provides a valuable food and habitat source, and is also a commercially and culturally significant food source for humans.[6] In fact, almost 90% of mussel landings on record come from the Gulf, accounting for a very profitable industry. This also indicates the possibility of total extinction in the wild if populations continue to decline at the current rate.[7] In the last several decades, blue mussels have gone from covering more than 60% of the intertidal surface in the Gulf of Maine to less than 10%.[5]

Ann Thayer searches for mussels on the shores of Bangs Island in

Casco Bay, where they have virtually disappeared over

the past several decades. Photo by Mary Pols

There are many potential factors leading to the decline of the blue mussel in the Gulf of Maine. The most likely candidates include rapidly warming waters, changes in ocean chemistry, overfishing, and the predation and competition of invasive species such as the green crab, Carcinus maenas.[10]

Warming waters are the prime suspect for the massive decline in populations in the Gulf of Maine. In Blue Hill Bay, sea-surface temperature (SST) over the past 20 years has increased by more than 2° C, with notable implications for biodiversity in the region, including a loss of native kelps and blue mussels.[6] The issue is exacerbated by the fact that predominantly south-flowing coastal currents in the Gulf make it difficult for the mussels to make necessary poleward migrations during their pelagic larval stage.[7] An inability to re-distribute and a lack of tolerance for higher temperatures may be playing a significant role in the 60% decline in Gulf blue mussel populations over the last 4 decades.[10]

Carbon dioxide is well known as the greenhouse gas largely responsible for the global warming phenomenon. However, excess carbon dioxide in our atmosphere has another effect; as it dissolves into our oceans, it increases the acidity of the marine environment. Studies indicate that by 2050, the pH of our oceans will be lower than it has been for several million years.[11] Ocean acidification is another potential cause of the recent decline in mussel populations, as an acidic environment can lead to shell degradation. Ocean acidification is often cited as a potential cause for the decline in mussel populations. This is not supported by studies from the University of Maine, showing that the current acidity of the ocean does not notably affect the formation of the byssal threads that mussels use to anchor themselves to their substrates.[3] Additionally, researchers on Bangs island have not noticed the ‘pitting’ which is characteristic of acid-induced shell degradation.[5] However, studies on the West Coast of the United States have found that acidification due to heavy precipitation events can be very harmful to juvenile mussels during the early stages of their shell and byssal thread formation. Commercial hatcheries in the Gulf have noted an 80% decrease in production linked to increased precipitation events.11Another likely factor may be increased predation by the invasive green crab Carcinus maenas, which now outnumbers blue mussels on Bangs island by a 3:2 ratio.[5]

The decline of the blue mussel is important not only to marine biologists and environmentalists, but to commercial hatcheries and mussel harvesters. The problems indicated by the decline may also affect other shelled species such as scallops and soft shelled clams. As 87% of Maine’s commercial fish catch consists of shelled species, the recent decline in mussel populations may be an early sign of a much larger problem to come.[11] One independent fisherman in southern Maine has seen his mussel catch decline from over 5000 pounds to less than 50 pounds in the past three decades.[5] Mussel farming accounts for the vast majority of commercial mussel yields today, but these mussel farms are dependent on wild mussels to colonize the ropes on which the commercially raised mussels grow.The decline in wild mussel populations, therefore, is putting constraints on a industry which in 2014 brought more than $10 million in at the dock nationwide, Maine and Massachusetts being the largest commercial producers of wild mussels.[10] Although dragging and overfishing in the 1980s is sometimes cited as a potential cause of the recent downturn, it still does not account for the vast numbers of mussels that have virtually disappeared in the rocky intertidal zones of the Gulf. The decline in mussels over the past several decades is unprecedented, and likely linked to warming waters, changing ocean chemistry, and increased predation. This decline will have far-reaching consequences for commercial fisheries and marine ecosystems in the Gulf of Maine, as well as serving as an early warning sign of the negative effects of climate change worldwide.

A green crab, Carcinus Maenas, on Bangs island. Photo: Mary Polls


Though this population decline is clearly a troubling problem, few solutions have been developed. Currently, much research on the subject is focused on attempting to find the sources and impacts of this issue. Nonetheless, there are several ongoing projects to manage this decrease. First, scientists are experimenting with raising mussels in the lab from larval stage to harvest, to potentially ease the economic stress of declining fisheries.[11] Other studies are researching the potential benefits of growing mussels in areas in close proximity to kelp forests, investigating the potential ‘halo effect’ of some native kelps.[4] Finally, there are notable studies based on identifying the adaptive abilities of the blue mussel, understanding the extent of their ability to move their range northward, and to cope with warming seas and invasive species. One such study uses shell chemistry and elemental fingerprinting to pinpoint the origin locations of juvenile mussels, a method which correctly identified the origins of 80% of collected juveniles from seven populations along ~500 km of Gulf coastline.This method may become important to understanding where to concentrate efforts and research on mussel disappearance, and clarify the ability of blue mussels to deal with a changing environment through redistribution.[7] Each of these studies may be particularly influential in mitigating blue mussel population decline in the Gulf of Maine, as well as the ecological and economic issues that come with it.

A mussel farmer holding fresh mussels in the Gulf of Maine.

Photo: John Ewing

At the Darling Marine Center in Walpole, Maine, an experiment with hatchery-cultivated mussels are underway, hoping to help manage the decline of wild blue mussels in the Gulf as they face increased predation and ocean acidification. This experiment is one of many to determine the most productive way to farm mussels in the ocean. The study is incomplete, but is aimed at finding the most effective type of collector lines for mussel farming. One way declining fisheries might manage decreasing populations is to switch to new, more productive lines.[11] Although this may allow those in the industry time to adapt and find new sources of income, it is likely a short term solution in the Gulf of Maine, as the predicted sources of the mussel decline would be very difficult to reverse. These include acidifying waters, and increased predation by invasive species.[11] More research and data are required to develop similar solutions in the Gulf, if the blue mussel problem is to be managed.

Bill Mook, owner of Mook Sea Farms in Walpole, adjusting water flow in his

hatchery. Mook now has to treat the sea water entering his facility due to

the increased acidity which occurs after large storms. Photo: Portland Press Herald

At Bigelow Labs in East Boothbay, Maine, researchers are investigating the effects of the changing Gulf on blue mussels, as well as investigating kelp farming as a potential solution to the mussel population decline of the last several decades. One experiment is looking directly at the phytoremediation ‘halo’ effect of kelp on mussel farming, with the hypothesis that the kelp will improve growing conditions for the mussels by sequestering carbon and lowering the acidity of the surrounding marine environment. The preliminary data suggest that kelp farms do indeed improve growing conditions for blue mussels.[6]

This experiment offers a potential solution to the problem of the mussel population decline with the added benefit of improving water quality and creating a second crop, kelp, for farmers to harvest from fisheries in the Gulf. Hopefully more research will be published on this matter, and kelp farming can be implemented in conjunction with mussel farms in the Gulf of Maine.[11]

Nichole Price, senior research scientist at the Bigelow Laboratory for Ocean Sciences in East Boothbay. Price is researching the potential phytoremediation effects of kelp. Photo by Gregory Rec


Though several potential solutions to this problem have been proposed, more research is necessary to determine their effectiveness. Because the decline in blue mussel populations is potentially linked to warming waters and changing ocean chemistry, it will be important to understand the capacity the blue mussel has to adapt, and the impacts that extirpation in the wild would have on our society and the Gulf ecosystem.[11] Based on the research in this case study, the most promising solution is the use of kelp forests and kelp farming in close proximity to mussel farming, offering a new crop to aquaculturists and maintaining mussel populations through the expected phytoremediation effect of the kelp. This solution is related to an ongoing study, and so it is difficult to make concrete conclusions about its effectiveness.[4] In any case, it is critical that research continue on the decline in the blue mussel population, so that reasoned decisions can be made in the global race to mitigate the effects of climate change.

Brian Beal, marine ecologist at the University of Maine Machias,

holding a handful of juvenile mussels being raised inside

mesh bags in an experiment at the Downeast Institute.

Photo by Gregory Rec


Auker, Linda A., et al. “Exploring Biotic Impacts from Carcinus Maenas Predation and

Didemnum Vexillum Epibiosis on Mytilus Edulis in the Gulf of Maine.” Northeastern

Naturalist Pub. Eagle Hill Institute, 2014, doi:10.1656/045.021.0314.

“Blue Mussel, United States/Northwest Atlantic.” Seafood Watch, The Safina Center and

Monterey Bay Aquarium, 9 Jan. 2017,


Dickey G, Preziosi BM, Clark CT, Bowden TJ (2018) The impact of ocean acidification on the

byssal threads of the blue mussel (Mytilus edulis). PLoS ONE 13(10): e0205908.


Pols, Mary. “Bigelow Lab's 'Time Machine' Aims to Find out How Shellfish Will Fare in the

Future.” Press Herald, Portland Press Herald, 27 July 2018,


Pols, Mary. “Where Have Maine's Mussels Gone?” Press Herald, Portland Press Herald, 28

Aug. 2015, www.pressherald.com/2015/08/30/where-have-maines-mussels-gone/?rel=related.




Society of America, 7 Nov. 2018, doi:10.1130/abs/2018AM-323992.

Sorte CJB, Etter RJ, Spackman R, Boyle EE, Hannigan RE (2013) Elemental Fingerprinting of

Mussel Shells to Predict Population Sources and

Redistribution Potential in the Gulf of Maine. PLoS ONE 8(11): e80868.


Steeves, Laura & Filgueira, Ramón & Guyondet, Thomas & Chassé, Joël & Comeau, Luc.

(2018). Past, Present, and Future: Performance of Two Bivalve Species Under Changing Environmental Conditions. Frontiers in Marine Science. 5. 184.


Thompson, McKenzie, "Implications of Body Size and Habitat Distribution of Carcinus

Maenas for Predation on Mytilus Edulis in the Gulf of Maine" (2017). Honors College. 257. https://digitalcommons.library.umaine.edu/honors/257

WHITTLE, PATRICK. “Mussel Population Loses Strength in Gulf of Maine.” Press Herald,

Portland Press Herald, 30 Aug. 2016,


Woodard, Colin. “Shellfish Can't Keep up with Shifting Ocean Chemistry.” Press Herald,

Portland Press Herald, 4 Feb. 2016, www.pressherald.com/2015/10/29/ocean-acidification-threatens-future-aquaculture-shellfish-industries/.

ZIPPAY, M. L. and HELMUTH, B. (2012), Effects of temperature change on mussel, Mytilus.

Integrative Zoology, 7: 312-327. doi:10.1111/j.1749-4877.2012.00310.x


Charles Metcalfe is a graduate of Kennebunk High School and currently a freshman at Tufts University. Charles has a deep appreciation for the natural world and particularly the Gulf of Maine coastline, and spends much of his time outdoors: sailing, hiking, rock climbing, and teaching at youth camps. He also has diverse academic interests; from creative writing, philosophy, and literature, to engineering and biology. Tentatively, he will be majoring in biochemistry and environmental studies, with the hope of eventually helping to solve pressing environmental problems in innovative new ways.

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