Update on Efforts to Control the Invasive Asian Clam (Corbicula fluminea) in Lake Pend Oreille
Elizabeth Braker, Environmental Science Program, University of Idaho
Christine M. Moffitt, US Geological Survey, Idaho Cooperative Fish and Wildlife Research
Unit, University of Idaho
Frank M. Wilhelm, Department of Fish and Wildlife Sciences, University of Idaho
Joe Noonan, Department of Fish and Wildlife Sciences, University of Idaho
Bob Kibler, US Fish and Wildlife Service
and
Tom Woolf, Idaho Department of Agriculture
In the summer of 2014, the Pend Oreille Chapter of Idaho Master Naturalists teamed up with
University of Idaho researchers from the College of Natural Resources to learn more about a
recent infestation of Asian clams (Corbicula fluminea) in Ellisport Bay of Lake Pend Oreille.
Left unchecked, these Asian clams could alter water quality by changing the availability of
nutrients resulting in algal blooms, which could affect other trophic levels (fish), and concentrate
calcium (shells) along the shoreline. Algal blooms resulting from the clams would detract from
aesthetics, interfere with swimmers, and could enhance the establishment of other aquatic
invasive mollusks like zebra or quagga mussels (Dreisena spp.).
The optimal bottom type for the Asian clam is sand or gravel along the shoreline of streams,
rivers, and lakes. Asian clams have a life span of between two to five years and can filter up to
five gallons a day per individual. Warm temperatures (>60°F) trigger reproduction. Because they
are hermaphroditic (self-fertilizing male and female), a single clam can cause an infestation.
The clams have a dispersal stage (veligers) that can float in water currents, or adhere to surfaces.
As adults, Asian clams use their foot to bury themselves i and move in the sediment.
Asian clams were introduced into water bodies in the United States in the 1930s, but had not
been detected in northern Idaho lakes until a dive team discovered a small population of Asian
clams in Lake Pend Oreille’s Ellisport Bay in 2012. Idaho Department of Agriculture's Tom
Woolf informed the University of Idaho about the infestation, and urged us to consider studies of
control measures. Since 2012, the population was observed to have expanded to the south along
the shoreline and currently is estimated to cover approximately 4,000 square feet. The water
operations of Albeni Falls Dam’s Flexible Winter Power Operation (11.5’ draw down of water
level between November and April) has likely helped to control the expansion of this population
because it exposes clams to freezing winter conditions. This seasonal draw down has also
allowed researchers easy access to the area of infestation during the fall and winter months.
An infestation of Asian clams in Lake Tahoe, a large (192 sq. miles), high mountain (6225’
above sea level), oligotrophic lake important to California and Nevada, was first ignored and
within five years the clam population in some locations had reached a density of more than 150
clams per square foot. Subsequently, managers have applied non-permeable benthic barriers
(rubber pond liner) to isolate the clams in the substrate and deprive them of oxygen thereby
asphyxiating them. The Lake Tahoe Regional Planning Agency coordinated the placement of
barriers over five acres of lake bottom that resulted in 98% clam mortality.
With funding from a program with the US Fish and Wildlife Service and the US Geological
Survey, our team at the University of Idaho elected to use similar barriers with the addition of
sodium hydroxide (NaOH) in small bags to elevate the pH under the barriers to enhance clam
mortality. The NaOH has been shown to be effective against invasive species in ship bilge and
ballast water, and laboratory trials at the University we observed a pH of 12 killed Asian clam
veligers within an hour and large adults in ten days. Use of sodium hydroxide to raise the pH is
ideal as it is easily neutralized by carbon dioxide to yield a non-toxic salt (2NaOH + CO2 
Na2CO3 + H2O). Our research team placed the NaOH in biodegradable burlap bags underneath
the non-permeable barriers.
Naturalist Mary Haly wades out
carrying the PVC frame that was
used to ensure that NaOH packets
used to raise the pH under the
benthic barriers were evenly
distributed. UI students Joseph
Noonan and Juan Ortiz-Perez
wade out carrying a roll of the
benthic barriers.
-Photo by Steve Neff
To permit our experimental applications, the US Fish and Wildlife Service obtained a pesticide
permit from the Environmental Protection Agency and the Idaho Department of Agriculture
obtained a permit from the U. S. Army Corps of Engineers. In February of 2015, the team
deployed a series of barriers along the shore of Ellisport Bay. The deployment would not have
been possible without the collaboration of the Pend Oreille Chapter of Idaho Master Naturalists
and its volunteers: Naturalists Gail Bolin, Denise Dombrowski, Mary and Dan Haley, and Becky
Reynolds. Other area collaborators whose support was critical were the Pend Oreille
WaterKeepers (Mary Franzel & Steve Neff), IdaH2O (Jim Eikens & staff), the University of
Idaho, the Department of Environmental Quality (Tom Herron, Regional Water Quality
Manager), and Lake Cocolalla Homeowners’ Association (Janet Conklin). Betsy Hull of the US
Corps of Engineers supplied sand and sand bags for our operations. Becky Reynolds and Mary
Franzel were instrumental in helping the Moscow-based team collect weekly pH readings above
and along barriers.
In April, the researchers collected pH readings beneath the barriers and confirmed that the
sodium hydroxide had dissipated. The higher-than-average water levels this spring made it
difficult to deploy the barriers as initially envisioned; however, researchers believe that they
were able to cover enough substrate to test the effectiveness of this tandem technique as a “Rapid
Response Strategy” tool for an invasive species. Researchers will remove the barriers this fall
after lake drawdown to evaluate clam mortality in the sediment.
Naturalist Mary Franzel
helps apply NaOH packets as
UI students unroll the
barriers. Department of
Environmental Quality Water
Quality Manager Tom Herron
helps carry the sandbags that
were used to seal the NaOH
beneath the barriers.
-Photo by Steve Neff
In the meantime, “Clean, Drain and Dry” should be forefront in the mind of all recreationalists.
Asian clam veligers and other aquatic invasive species can be transported in bait buckets,
waders, kayaks and other watercraft, in addition to the bilges of boats.
Researchers ensured the
barrier edges are held
down with rebar and
sandbags (incorporating
a two-foot overlap) to
minimize water flow
underneath.
-Photo Christine Moffitt