Cottage Q&A: Are clams under the dock a good thing or a bad thing?


This article was originally published in the Winter 2016 issue of Cottage Life magazine.

When I was fixing the waterline at our cottage this summer, I spotted hundreds of clams under the dock. We haven’t noticed this before. Are they a good thing or a bad thing?


Without seeing the clams in question, it’s hard to say. (They weren’t all just playing Pokémon Go, right?) Freshwater clams are filter feeders, says Dan Fox, a technician with the Fisheries and Aquaculture department at Vancouver Island University. “Their increase could be due to an increase in food supply—algae—caused by the fertilization of the lake from property drainage or input from septic fields.” While it’s good that all these clams will eat the extra algae in your lake, it’s bad because, well, there’s extra algae in your lake.

It’s also possible that the clams are new to your lake, transferred, for example, from watercraft entering the lake without proper decontamination. “The sudden appearance of large numbers of organisms can sometimes result from the introduction of a non-native species,” says Timothy Pearce, the assistant curator and section head of mollusks at the Carnegie Museum of Natural History in Pittsburgh. “They’re often free from the predators and diseases that kept their numbers in check elsewhere, so they can proliferate into large populations.” (Um, we mean you, zebra mussels.)

Or maybe the number of clams in your lake hasn’t changed—you just happened to see them this summer. A group of clams congregated in one area can look like a large, blossoming population, but freshwater clams in a freshwater lake are not unusual. “It’s like having dandelions,” says Pearce. “It always appears as though they’re aggregating. But that’s just because they’re in your yard.”

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