What are those little dead fish in Lake Michigan?

Recently, I spent lunch break fishing North Pier in Sheboygan.

I had the good fortune of speaking with a lot of very nice folks, many from out of state, who were out and about enjoying a beautiful day.

The most frequent question I got: what are all those dead fish in the water?

The short answer? They’re alewives: an invasive species of herring, native to the Atlantic Ocean that got into Lake Michigan via the Welland Canal. They were first observed in the lake in 1949.

As a lifelong Sheboygan County resident, I’ve never thought much of the dead alewives. Though not as prominent as they once were, alewife die-off events are historically pretty common during the summer months.

But, from the visitors’ perspectives, I completely understand the question. It’s jarring to see hundreds, if not thousands, of dead fish strewn about the beaches and floating in the water, particularly in the era of heavy pollution and climate change.

However, shockingly, our current understanding of alewife die-offs is that climate change and pollution are not direct contributors to this phenomenon. While we haven’t been able to pin down a precise cause , there are likely a few factors at play here:

Spawning, which occurs June-August, can be particularly hard on these fish.

Sudden water temperature changes capable of causing thermal shock frequently happen in pockets of the lake.

The presence of invasive mussels in Lake Michigan also leads to food scarcity for many of the lake’s inhabitants, including alewives.

Finally, these fish don’t actually belong here. So some alewives develop thyroid deficiencies due to the diet required to survive in Lake Michigan. Typically, alewives are diadromus, meaning they ideally migrate between saltwater and freshwater. This doesn’t happen with our population in Wisconsin and true evolution often takes centuries, not decades.

The alewives’ relationship with the big pond is complex. Many of the salmon and trout species that are now frequently stocked in the lake were originally brought here as a means of alewife control. The pursuit of these sport fish is now an industry in and of itself that the Great Lakes Fishery Commission values at over $7 billion annually.

The alewife population in Lake Michigan has significantly declined in the last few decades, likely due to a combination of the factors above and the fact that the planted salmon and trout are, indeed, doing their jobs. This was an intended result, after all. We just came to this end through a multitude of means, not all of which are positive.

Those who manage our fisheries are now trying to balance the demand for trout and salmon fishing opportunities with the dynamics of a shifting forage base for those fish.

In 2016, the Lake Michigan Committee suggested a 62 percent decrease in King Salmon stocking, but Wisconsin kept its numbers steady and instead opted for reducing the number of Brown Trout and Lake Trout.

The overall Lake Michigan forage base has increased slightly in recent years and salmon stocking has slowly increased along with it. In 2023, Wisconsin increased its planned King Salmon stocking numbers to 1.3 million, 100,000 higher than the previous year.