Fishes of Wisconsin: School’s in Session for Brook Silversides

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.

Courtesy: John Magnuson, Friends of the Lakeshore Preserve

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!

Their noticeable surface dancing can potentially be explained by multiple hypotheses.

Andy Stevens and his "hand-caught" silverside. Note the large mouth and flattened head. Photo: Andy Stevens — Andy Stevens and his “hand-caught” silverside. Note the large mouth and flattened head. Photo: Andy Stevens

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.

Or, their acrobatics could be feeding related as they have huge mouths proportional to the size of their head making them excellent predators of fallen insects that float on the surface of the water or, more spectacularly, anything flying close enough to the surface to become a meal.

Finally, they could be jumping in an attempt to escape predators. I have often seen walleyes darting in and out of schools of silversides in front of the CFL boat garage as evidenced by their marble-like reflecting eyes when a spotlight meets them. Considering these little fish are food for everything from birds to crayfish to walleye and bass, it makes sense that they’d have developed some “evasive maneuvers.”

The jumping is likely a combination of all three hypotheses and, whatever the reason, is entertaining to watch. We’re not used to seeing much of anything, except maybe the occasional carp, break the surface of the water from below. I can’t help but agree with fisheries biologist, Alvin Robert Cahn, who wrote all the way back in 1927, “When the moon is almost full or full, the shallows become the scene of one of the most startling scenes in the fish world.”

So if you have the time and interest over the next week or so, grab a headlight and head down to a Madison lake and see if you can catch the “dance” of the brook silverside in action!