Illustration by Leo Clark

The Great Mussel Die-off

Risk of mussel extinction in the Pacific Northwest threatens ecological stability

Western freshwater mussel populations are dying. According to the Xerces Society for Invertebrate Conservation, a nonprofit organization dedicated to the conservation of invertebrate species, freshwater mussels are under threat from the construction of dams, pollution, the effects of climate change and other changes to their aquatic habitat. In an article titled “Extinction Risk of Western North American Freshwater Mussels,” co-authors Emilie Blevens and Sarina Jepsen stated that freshwater mussel species diversity “declined 35% across western watersheds by area, and among the most historically diverse watersheds, nearly half now support fewer species/clades.”


In a blog post titled “The Merit of Mussels,” Blevens wrote that “for biologists and the many people who recognize the inherent value of biodiversity and rare species, the conservation of freshwater mussels and preservation of their habitat is easy to understand. These are animals that share our rivers and enrich our natural and cultural connections.”


Blevens argued that conservation and habitat restoration efforts should prioritize reducing water pollution, investing in sanitation technologies and encouraging a reduction in industrial, agricultural and residential water use.


The threat of mussel bed die-offs has increased in freshwater species like the western ridged mussel (Gonidea angulata), which has heightened conservation concerns. In another post, “Working From the (River) Bottom Up to Conserve the Western Ridged Mussel,” Blevens wrote, “the stakes are high, and yet, there is still an opportunity to provide meaningful protection to the western ridged mussel.” Listing an animal under the Endangered Species Act has saved over 200 species of plants and animals from extinction, she stated—an outcome which will hopefully be shared by western ridged mussels.


A Xerces Society publication, “Freshwater Mussels of the Pacific Northwest,” stated that mollusks are extremely “sensitive to changes in their environment” and act as a “canary in the coalmine” regarding the health of aquatic ecosystems. Because mollusks are slow-moving and highly responsive to alterations in their environment, they are a useful monitor of habitat conditions. Mussels can only live in “permanent bodies of water,” including lakes, ponds and rivers. They typically require a stable environment, as too much variability in factors such as water flow or dewatering can make it difficult for juvenile mollusks to establish themselves.


Mussels play an important role in maintaining aquatic ecosystems. They filter materials in the water like algae and sediment, providing food for bottom-dwelling organisms and controlling nutrient levels. Mussels act similarly to garden earthworms, promoting species diversity and availability of organic matter. They provide benefits to humans as well—freshwater mussels filter pharmaceuticals and bacterial populations like E. coli out of water bodies.


Freshwater mussels are also a key food source for many predators, including river otters, raccoons, gulls and some fish. While healthy mussel populations can survive a standard amount of predation, fragmented or declining populations might be wiped out.


“Freshwater mussels are one of the most endangered groups of animals on Earth,” according to Xerces Society. Out of almost 300 North American freshwater mussel species, 35 have gone extinct in the past century. The United States Endangered Species Act lists 25% of freshwater mussel species as endangered or threatened, and individual U.S. states list 75% of species as threatened, endangered or of special concern.


The decline in freshwater mussel populations, the publication states, is the “result of continent-wide degradation of aquatic ecosystems.” In the last 150 years, mussel populations, like all Western freshwater ecosystems, have been negatively impacted by trends like urbanization, logging and agriculture. More recently, climate change has exacerbated ecosystem instability, and it will continue to do so.


A Xerces Society conservation booklet titled “Conserving the Gems of Our Waters” seeks to provide “best management practices for protecting native western freshwater mussels” during projects such as river restoration or construction. Efforts to restore habitats for salmon and other species in the Northwest have overshadowed the need to preserve freshwater mussel populations, the publication stated. If mussels are not included in considerations for aquatic projects, the authors argue, these projects may fail to protect or even actively harm mussel populations, which “may take decades to recover, if at all.”


The booklet gives several words of advice to potential aquatic project planners. They should be aware that mussel populations will remain in a work site all year, because they are largely immobile. If a mussel population is driven out or destroyed, it is unlikely that it will recover for decades—and as more mussel beds are lost, the chances of recovery decrease. According to the authors, it is best to institute freshwater mussel best management practices a year or more in advance of any project. Freshwater mussels have different habitat requirements than other species, and general protective measures may not adequately protect mussels from harm.


The loss of freshwater mussels may entail a loss of living history. “Freshwater mussel beds, like old growth forests, can provide a lesson in persistence,” Blevens stated. “The mussel bed you or I observe in a river is likely the same bed, with perhaps even some of the same individuals, that our parents or grandparents would have encountered had they stood (or swam) from the same vantage point decades before. Yet, at the current rate of decline, our children, and their children, too, may not be able to experience western ridged mussels.”