Assessing Risk of Electrofishing Mortality and Sublethal Effects to Freshwater Mussels
Subtitle: Are Mussels Collateral Damage When Biologists are Shocking Fish?
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Researchers at North Carolina State University led by Dr. Mike Holliman recently completed a multiyear study to determine if freshwater mussels were harmed incidental to biologists using electrofishing gear to collect fish. They concluded that evidence from laboratory experiments with four life stages of two mussel species shows electrofishing poses little risk to freshwater mussels. The project was funded by the North Carolina Wildlife Resources Commission using State Wildlife Grant monies and was consistent with the goals and objectives of the North Carolina Wildlife Action Plan.
Why was the study necessary? Many species of freshwater mussels are in trouble in our nation’s rivers and streams. Almost half of the species of freshwater mussels in North Carolina are either state or federally identified as critically endangered, threatened, or vulnerable to qualifying for those categories if population trends aren’t reversed. Of course, mussels share their habitats with fish and biologists interested in fish often must use electrofishing techniques. No one knew for sure if biologists electrofishing a stream to study trout or smallmouth bass were exposing rare mussels present to harm. The same question was relevant to biologists studying fish populations as indicators of stream water quality or simply trying to understand what species were present where in a stream. Until somebody looked at electrofishing effects on mussels, biologists had two choices. They could risk the possibility of injuring mussels to get important fish information; or they could decide to not take the chance and choose not to do fish electrofishing. The absence of fish data might then impede conservation goals.
Dr. Holliman and his collaborators couldn’t test every mussel species present in North Carolina so they choose the thick shelled eastern elliptio and the more thin shelled paper pondshell. Mussels have a complex life history. Adults emit very small offspring called glochidia which are free swimming and must attach themselves to the gills of a fish host to live. Mussels live in this “parasitic” stage, attached to fish gills, until they leave the fish to begin life in the stream bottom as still very small juveniles, eventually growing into adults. Dr. Holliman built a tank in his laboratory where he could expose mussels to amounts of electrical current below, at, and above the levels created by electrofishing. He tested all four life stages of both mussels.
Adult mussel survival was not adversely affected by any of the exposures. Glochidia experienced minimal immediate mortalities after exposure. Exposure of mussels in their “parasitic” form (shocking infected host fish) failed to reduce the rate of change from glochidia to free swimming juveniles. Finally, short term survival of juveniles was unaffected by exposure to test current levels.
The NCSU study results suggest biologists studying fish can use electrofishing in streams populated by freshwater mussels without fear of killing adult mussels and anticipate minimal effects on the other mussel life stages. As always, biologists should continue to exercise good judgment and any knowledge of local species and conditions when deciding to use electrofishing gear.