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Climate Change and Trout Fishing

I started this post some time in the summer of 2020 just after I started the blog and I swear I have picked it up at least a half dozen times since then. However I did not get much further than copy and pasting a few links (see below). It is NOT an easy post to write but it is important. The "debate" has become political, not scientific. Political debates do not need to be full of facts and evidence, scientific ones do. Climate change is probably the greatest challenge we will face as a society in the coming decades. We are already seeing the impacts of climate change.

I am not going to sugar coat it - it is an honest, up-front conversation about the science behind climate change and what it means for trout in Midwestern streams.

Lake Monona ice coverage
Shorter ice duration - one of many pieces of data that show how temperatures are increasing over time.

I am not about to debate whether or not climate change is an issue. In the scientific community, it is not a debate that scientists are having. Scientists debate around the margins - what are the likely impacts, what effect will X have on temperature or frequency of storms, and other such debates. They do not - in any real or significant way - argue whether or not climate change exists. It does - the data are irrefutable. They accept the fact that it is occurring and it is a real issue we need to deal with. That is the thing about science, we do not have to like the answers but if do not have an explanation that makes more sense and is supported by more and better data, we have no choice but to accept the facts.

The science behind why our climate is changing is really not that complex. The physics behind why greenhouse gases increase Earth's temperature is well established. And it is a fact that greenhouse gases have increased over time and can be linked to the burning of fossil fuels. These are facts - observed phenomena, things we have measured - not opinions or conjecture. Quite honestly, they are rather simple facts as the science is not all that complex, despite the large scales (time and space) which they are occurring. Greenhouse gases - carbon dioxide, methane, water vapor, and others - intersect heat energy reflected from the Earth and re-radiate that energy back to the Earth (for more information) causing a warming. This is basic physical chemistry. The carbon in fossil fuels we have burned were formed during the aptly named Carboniferous period from approximate 360 to 300 million years ago. So the carbon in fossil fuels were sequestered over about a 60 million year period and have mostly been released in a period of decades. That carbon has increased in the atmosphere because of the burning of fossil fuels is rather self evident, is it not?

The temperature is changing - again, a fact, not an opinion - as data collected over decades prove this to be true. Data on the effects this has had on extreme weather events are a little "fuzzier" but we have seen an increase in precipitation, polar vortexes, and extreme precipitation events and floods across Wisconsin. In a previous post about the "re-birth" of the Driftless, I posted about floods on the Rush, Kinnickinnic, Coon Creek, and the West Fork of the Kickapoo. Flooding has not been limited to the Driftless, flooding in 2020 north of Milwaukee, Northwestern Wisconsin, and elsewhere in the state and region have been historic. Our winters have been warmer on average but at the same time, we have reached several all-time low temperatures during polar vortex events created by a greatly altered jet stream. A few years ago we experienced that Fahrenheit and Celsius meet at 40 below zero. The extremes are getting more extreme.

Source: Clean Wisconsin (as a scientist I do not love this figure - give me an equation and how well the line fits the data...and remove those silly gridlines!)

Effects on Wisconsin Streams

Climate change is responsible for warmer air temperatures which mean warmer water temperatures as well. The temperature effects are expected to be somewhat offset by the increases in precipitation that equate to more groundwater and more groundwater means cooler summer and warm winter water temperatures. The effects of climate change are a bit mixed for trout but models (more later) show that trout are expected to decrease in range, particularly our native Brook Trout.

The climate change effect we have seen most clearly affect trout streams is the increase in flooding due to larger and more frequent extreme precipitation events. Nearly everywhere in Wisconsin and the surrounding states have experiences flood events that are "historic". Unfortunately, history is repeating itself too often. Increases in air temperature will have an impact on stream temperatures as well.

These are not "doomsday" predictions but should be - or maybe should have been - a warning to change our ways. We are, however, not so great at changing our ways. Let's face it, the status quo is a hell of a lot easier than making changes, even if those changes might be pretty important. That is a huge part of why we are where we are in dealing with climate change. And why we are largely past a place were we can stop the effects of climate change and are stuck with needing to mitigate against the effects of climate change.

WICCI data and model for extreme rainfall events.
Days per 100 years (can be read as the % likelihood) of a greater than 4 inch rainfall. Source: WICCI

As mentioned, one of the great impacts of climate change is and will be an increase in the kinds of rainfall events that are responsible for flooding. Increased temperatures result in more evaporation and evapotranspiration can then lead to larger precipitation events. Temperature increases are directly caused by increases in greenhouse gas concentrations. However increased frequency and magnitude of storms is one level removed from the direct effects of increased greenhouse gases. Yet, these impacts are much easier to witness than are the relatively small (a 1-2*F average annual temperature increase) yet very climatically and ecologically significant changes in temperature.

Eroding banks, increased stream width, and slower shallower streams as a result of the 2018 flood.

Recent flood events have been devastating not only to streams but more importantly to a number of communities, infrastructure such as road and bridges, and to people and our possessions. August and September of 2018 brought floods to communities along Coon Creek (Timber Coulee), the West Fork and mainstem of the Kickapoo River and its tributaries, among other streams. These floods did hundreds of millions of dollars in damage to the area, displaced people, and altered the lives of everyone in the area. Every person in the area, if not directly affected, has family or friends that saw damage. Certainly many people in the area sleep much less soundly at night when large rain storms are predicted.

What it Means for Trout

Scientifically, we talk about the catostrophic events as density-independent population regulation meaning that their effect on the population of a species is not related the density of the the population (for more: D-D and D-I population regulation). Density independent events tend to have an immediate - and often drastic - impact on the population. Conversely, the effect of density dependent factors is a function of their density. These are things like predation, competition, and disease that increase mortality as the population density increases.

However, trout are often much less affected by floods than you might think. I remember reading a paper in graduate school about a "catastrophic debris flow" - basically more rock than water in a Pacific Northwest flood - that really intrigued me and made me think differently about how resistant and resilient trout can be. The catastrophic event had an immediate and drastic impact on the age-0 Cutthroat Trout (Oncrohynchus clarkii) but the effect was short-lived as in the second year, age-0 fish surpassed their pre-flood numbers. While we typically talk about death rates being density dependent, birth rates can be as well. When the density of the population is low relative to carrying capacity, there are few constraints on the growth of the population so mortality is low and birth rates are high. As we have seen here in Wisconsin, floods clear sediment from the stream and create new, clean spawning riffles and amazing trout year classes typically follow flood events. For trout, the biggest concerns with floods are that they happen frequently enough that they do not allow a population to rebound. Maybe a more significant concern is that floods decrease habitat complexity and make streams wider and shallower, causing them to warm. These two factors - loss of instream habitat and warmer temperatures reduces the carrying capacity of the stream for trout.

What I found most interesting following the 2018 floods was how drastically changed some stream reaches were and how nearly untouched others were. On the same stream, there are places where holes were "hat floaters" after the flood and other reaches are wide and average less than ankle deep. Some reaches have dug new channels and other reaches are nearly unaltered after the floods. Sometimes these drastic differences were evident in 100 yards of less of stream. Floods are funny that way.

In the longer term, climate change will warm our streams. There has been a bit of a balance because more precipitation means more groundwater recharge and springs are the life-blood of our streams. However many predictions show that over the next 30 to 80 years (by 2050 or 2100), the range of trout will contract and many streams and Brook Trout will be more affected than Brown Trout.

Source and link for the original paper: Mitro et al. 2019 - full citation at the end of this post.

If summer temperatures do rise as expected, scientists project that by 2050 trout habitat in streams will decline across the state by 32 percent for Brown Trout and by 68 percent for Brook Trout.

Certainly the effects are going to be significant but there are adaptation efforts that we can - and NEED - to take to combat climate change's impacts. My recommendations for how to combat the effects of climate change and help reduce the effects on trout:

  • Stream improvement projects narrow and deepen stream channels which helps reduce warming. The effects of stream improvements have been evident not only in the improved reach but they transport colder water downstream and have extended the downstream length of trout streams.

  • Restoration of connectivity - such as improving culverts and bridges for fish passage. Fishes either relegated to areas above barriers (such as dams) or downstream of barriers (dams, perched culverts) without access to cold water refuges during summer are more susceptible to low population densities and extirpation (local extinction). Additionally, access to spawning habitats once above barriers can increase the age-0 fishes which can be a limiting factor in some systems.

  • Riparian buffers - our pasture streams and others with few trees are particularly susceptible to warming due to a lack of shading. Research has shown that in the Driftless Area, streams 2.5 meters or less in width can be effectively shaded by grassy buffers. In larger streams, we may need to consider planting trees as an important adaptation strategy. (See The Science Behind the Need for Riparian Buffer Protection for details).

  • Flood prevention strategies. Flood control structures have had a rather mixed effect on flood prevention. Conventional wisdom is that they protect against smaller, more frequent floods but have exacerbated larger floods when they have catastrophically failed (link). More effective strategies seem to be those aimed and slowing the rate at which water reaches streams such as grassed waterways, terracing, contour farming, Conservation Reserve Program (CRP) set-asides, and other land use practices. The problem is as we have seen agricultural commodity prices increase, we have seen CRP and other conservation practices decline. For more, read my post on the Rebirth of the Driftless Area.

  • Mitigation - at some point, if you do not turn off the faucet, there is not much you can do to prevent the tub from overfilling. The organization was founded in 2008 and the idea was to prevent carbon dioxide concentrations from exceeding 350 parts per million in the atmosphere. Today they stand at about 415 ppm and rising. Adaptation strategies above are required but as some point, you have to turn off the faucet. It may be one of the greatest challenges we will face in the coming decades.

For more about the likely effects of climate change on trout, watch John Lyons' presentation to the American Fisheries Society.

Climate Change Resources for Wisconsin

Mitro, Matthew & Lyons, John & Stewart, Jana & Cunningham, Paul & Griffin, Joanna. (2019). Projected changes in Brook Trout and Brown Trout distribution in Wisconsin streams in the mid-twenty-first century in response to climate change. Hydrobiologia. 10.1007/s10750-019-04020-3. (

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First SHORT narrative that I have read on the subject that ties it all together with Wisconsin climate data. The subject is slowly becoming has become less political. Would really make a nice meeting presentation. Thoroughly enjoyed the read.

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