Zebra Mussel Myth #4: Copper kills fish

Copper is toxic to fish at high concentrations. That much is true. But copper is also a natural element that is approved for use in drinking water. When used efficiently, at concentrations well below the minimum for drinking water, it is not a threat to fish or to the larger ecosystem.

The myth that copper inevitably kills fish stems mainly from some unfortunate instances of overdosing with conventional copper sulfate (granular CuSO₄·5H₂O), an inefficient method of delivering copper. When too high a dose is applied or too large an area of a lake is treated, fish can die from overexposure to copper ions or from oxygen depletion caused by bacterial decomposition of algae.

This is what happened in a Nebraska lake in 2008-09. High doses of copper sulfate were applied to the lake on two separate occasions. Subsequent tests indicated that the treatments were successful at eliminating zebra mussels, but 21 different species of fish died, totaling 40,000 pounds. Mussels were again detected in the lake in 2014, although it is not known whether they survived the treatments or were reintroduced. The project contributed to the perception that treatment with copper requires a willingness to sacrifice fish cohabiting the water body.

Not all copper is created equal, however. Liquid copper in the form of cupric ions (Cu⁺⁺) has emerged as an environmentally friendlier and more surgical tool in the effort to stop zebra mussels. Formulated as the most biologically available form of copper, acid-stabilized ionic copper is effective against invasive mussels at substantially lower concentrations. Additionally, advancements in chemistry and treatment methodology now allow it to be dispersed in precise doses to avoid concentrations that would harm fish populations.

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The Susquehanna River Basin Commission’s experience in Pennsylvania shows that eradication of invasive mussels is possible with copper concentrations that are nontoxic to fish. Copper concentrations remained well below the U.S. EPA limit of 1.0 mg/L in open waters throughout the treatment. Post-eradication sampling showed that the fish and zooplankton populations in the lake continued to thrive, illustrating that ionic copper is a valuable weapon in the fight against invasive mussels.

Don’t Be Myth-Led

Zebra mussels are fearsome creatures, but not as invincible as these myths make them out to be. Prevention is critical. Eradication is possible. Control is achievable.

This article is the last of a five-part series excerpted from “Four Zebra Mussel Myths and Where They Went Wrong.” It is reprinted here with permission. The full article with references is available at Water Online. Copyright 2019.

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About The Author

David Hammond is an environmental chemist who has consulted with private industry on topics including pest management, taste and odor problems, biological wastewater treatment, sustainability, and biomimicry. He holds a master’s degree from the Energy and Resources Group and a Ph.D. in Environmental Chemistry, both from the University of California, Berkeley. Dr. Hammond has four patents and several peer-reviewed publications and currently serves as senior scientist with Earth Science Laboratories, Inc.