by Andy Stevens
If you’ve taken an evening stroll anywhere along the shore of Lake Mendota over the last week or so, you will likely have noticed large schools of translucent, pencil-like minnows darting about and jumping at the surface.
These fish are a species called the brook silverside – Wisconsin’s only member of the atherinidae family. Found throughout the South and Midwestern lakes, brook silversides prefer clear, weedy lakes and spend almost their entire life just below the surface of the water. In fact, they’re adapted to living at the surface and will often swim with their flattened head making contact with the surface film of the water.
While not exactly abundant across Wisconsin, in Lake Mendota, brook silversides make up the bulk of fish biomass. According to the annual “fish census” taken each year by our North Temperate Lakes Long-Term Ecological Research project, from 1981 to 2011, more than 50% of all the fish caught in Fyke and seine nets were brook silversides!
On still, moonlit evenings (for example, the nearly perfectly calm nights we’ve recently had with the giant yellow “super moon” shining down), you can see this abundance in action. Watch closely and soon you can see large schools of brook silversides jumping all over the surface of the lake. Scientists (and, I’m sure, anglers) have found that the fish are phototrophic, and if you shine a light in the water you can often direct them towards it at the surface. I’ve done this off the Center for Limnology dock, getting the fish so close that I can catch them by hand!
First, during spring and early summer, it could be driven by spawning activities. Silversides have only one season to live, spawn and (shortly thereafter) die, so this jumping display could be one last hoorah for the little guys.