Wisconsin Fly Fishing for Trout

Are Mayflies Declining in Wisconsin?

All over the country fly fishermen are starting to talk about the mayfly hatches are not what they used to be. The historic Hendrickson mayfly hatch that so many love to fish in the Spring has been declining in many Midwest and Eastern areas, including in Wisconsin. On some streams in southern Wisconsin the Hendrickson mayfly nymphs (Ephemerella subvaria) are totally gone, as well as many other mayfly species.

In northern Wisconsin we still enjoy the wonderful Hendrickson hatch on many streams, thanks to all the federal, state, and county public lands. The public lands help protect the northern cold-water streams from the effects of human influence, which keep our streams healthy. Although this sounds great, we cannot take it for granted.

In southern parts of Wisconsin, there are trout streams where mayfly hatches were once prominent and now don't exist. Yet, wild trout are doing fine, currently. What many fly fishermen don't understand is that brown trout are more tolerant to organic pollution than mayfly larvae. In those streams the trout have adapted to other more pollution tolerant aquatic macroinvertebrates for their food. This makes it easy for many fishermen to ignore what is occurring to the health of their cold-water trout streams. Eventually, the declining health of the trout stream and the aquatic insect population will take its toll on the trout as well.

Sad to say, some or our wonderful cold water trout streams flow through farmlands and private property where sediment, silt, pesticides, and other harmful chemicals enter the streams and are slowly deteriorating the health of trout streams, including the mayfly populations.

I am sure many fly fishermen remember the 2016 and 2018 major floods in northwest Wisconsin. Many streams were blown out and bridges were destroyed. In some Northwoods areas the floods did major damage to the stream banks and washed clay, large trees and sediments into our cold-water trout streams. Add to this, warmer seasonal air temperatures and several years of drought which have also negatively impacted our cold-water streams and rivers.

The decline of the mayfly populations is directly connected to the health of our cold water streams and rivers. A new website addition for 2024 is dedicated to this important topic: Protecting the health of our coldwater trout stream ecosystems..

So, what can we do?

Mayfly, caddisfly, and stonefly larvae are the "Canaries in the Coal Mine". Scientists have proven around the World, and in Wisconsin, that aquatic insect's diversity and density can be used to determine the health of a stream. Scientists have studied the aquatic macroinvertebrates living in the benthic zone of our cold-water streams and have identified species that are the least tolerant to organic pollution, and other aquatic macroinvertebrates that are extremely tolerant to organic pollution. Thus, we can use aquatic macroinvertebrate specie identification and samplings to help us monitor the health of our cold-water streams and rivers.

We need to start to collect aquatic macroinvertebrate species and water chemical data on an annual basis, along with noting the riparian zone and other surrounding areas. With this knowledge we can then start to access the different factors causing the decline of the mayfly population and the health of our trout streams. Only then can we begin to try to fix the issues and accurately monitor the results. In some cases, this might mean state, federal and other conservation groups funding. The other option is to sit back and ignore what is happening and let our cold-water streams and watersheds continue to decline. To me, that is not an option.

John Simonson
Feel free to email me your comments at: john@wiflyfisher.com

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