At our cottage on southern Georgian Bay, we have noticed a significant decline in zebra mussels at our dock and on the rocky shores. I’m wondering what the reasons behind this decline might be.—Tom Baskerville, Port Severn, Ont.
This sounds like an expected and ordinary population fluctuation, says André Martel, a mollusc expert (“malacologist”) and a research scientist in zoology at the Canadian Museum of Nature. “Zebra mussel numbers can go up and down in abundance.”
All kinds of factors can affect “recruitment success,” that is, how many new individuals join the population—either because they’re born or arrive in the lake from somewhere else—and survive: water and air temperature; food availability (zebra mussels feed mostly on phytoplankton); the timing of spawning; fertilization success; the number of ducks and diving bird predators. All mollusc species are known for fluctuating dramatically, says Martel. “The low could simply be due to having a very different kind of summer one year to the next. The environment is changing. Summers are changing.”
If the low numbers continue for several years in a row, on the other hand, that could indicate a more permanent change—for example, another species out-competing the zebras. This has happened in the Great Lakes and the St. Lawrence River, says Martel, where quagga mussels—also an invasive bivalve—have overtaken the zebra mussel population.
This article was originally published in the Mar/Apr ’20 issue of Cottage Life.
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