top of page

Climate Change - Comparing the Effects on Brook and Brown Trout

Updated: Jan 5, 2023

In a rather short and succinct post that I am really proud of - yes, I am more proud of some posts - I have written about climate before. And I almost certainly will write about climate change again assuming I continue to write this blog. That is because there is probably no more important long-term issue that we - and the trout we chase - face than climate change. Rather than rehash the science behind climate change and its effects here, go read that post, if you have not already.

Driftless Brook Trout - on my not so secret fly.
A "stray" Brook Trout caught on a Coulee stream - I catch a couple on "Brown Trout" streams each summer but they are 1 in a few thousand on many of Driftless streams.

If your experiences are similar to mine, you have seen fewer Brook Trout (Salvelinus fontinalis) and more Brown Trout (Salmo trutta) in the last decade or more. Science supports our observations - Maitland and Latzka (2022) recently published an analysis based on data from Wisconsin trout streams collected between 1994 through 2019 (Figure 1) and the University of Wisconsin Center for Limnology posted about an article on their blog (edit to add, the first author was on WPR's The Morning Show after I published this post). But they are not alone in finding these results. Tom Waters, an ecologist of great fame from the University of Minnesota who wrote the book on sediment in streams, documented the replacement of Brook Trout by Brown Trout in a Minnesota stream, Valley Creek (Waters 1983) some years ago. Mitro et al. (2019) modeled the effects of climate change and predict a 68% decline in Brook Trout range and a 32% decrease in Brown Trout range by the middle of this century in Wisconsin. However their estimates were lower than previous studies because of the increase in precipitation, in large part due to climate change, which has increased spring flow. Hoxmeier and Dieterman (2019) found that native Brook Trout replaced non-native Brown Trout in East Indian Creek, Minnesota because, as they note, this rare example was most likely due to increased baseflow, again, an effect of increased precipitation due to climate change. The Hoxmeier and Dieterman paper shows us that the answer is not as simple as it may seen at first blush. Or another way to put it, while temperature and precipitation are both increasing, temperature increases are having a greater impact than are precipitation increases which are mitigating the effects of climate change.

Data from the West Fork of the Kickapoo watershed showing the increase in precipitation and streamflow. Few ecological questions are simple - while we are getting warmer, we are also getting wetter which means that streams have more spring flow. Then add in the effects of floods due to larger storms, longer periods without precipitation, polar vortexes, and other changes and there are no simple answers. But there are data showing us what has happened.

In my part of the Driftless, Brook Trout are mostly holding on in the margins - headwaters and small spring runs and occasionally accessing the larger streams, and through the stocking of "wild hatchery" trout, if you can call that holding on. The only places that Brook Trout seem to be thriving are where Brown Trout numbers are low or they are not present - in restoration projects above PL structures (dams), small streams that flow into warm streams like direct drains to the Mississippi River and tributaries to the Wisconsin and lower Kickapoo Rivers, and in some parts of the northcentral Driftless where there seems to be greater groundwater input and sandier streams where it is possible that Brook Trout have a competitive advantage - or at least less of a competitive disadvantage. Researching this last question is how I plan to spend much of my summer - so expect a bunch of Brook Trout / Brown Trout competition and habitat suitability posts this spring leading up to the summer.

A big Driftless spring
A really big Driftless spring - even the stream this big spring turns into "Brook Trout water" has become largely taken over by Brown Trout. Old photo - scanned from a photo.

For this post, I plan to do something a little different, talk through a recent peer-reviewed paper - Maitland and Latzka (2022) - that uses WDNR fisheries trend data to assess the changes in Brook and Brown Trout populations over time. Because this paper is part of the Creative Commons, the paper is free for all to read and download - which is not true of most scientific research. And the Creative Commons allows myself and others to use this article in any way we see fit so long as we attribute the authors and the Creative Commons license (which I have now done several times). This is how all science should be disseminated, rather than being stuck behind paywalls accessible only to those able to access this information.

The introduction sets the stage for their study and their objectives. They highlight what we know about climate change, these two species of trout, and how Wisconsin with its variability in landscapes and climatic conditions is a perfect "laboratory" to ask questions about the impacts of climate change and how those effects may vary seasonally and spatially. Of particular note is the understanding that trout populations are highly variable and this variability is typically tied to weather conditions. Then they present their objectives, 1) to look for trends in population changes across time and for different parts of the state, and 2) to test if these changes are tied to winter, spring, summer, and/or fall climatic conditions.

Northern Wisconsin Brook Trout
Northern Wisconsin Brook Trout - Chris Collier photo.

And their methods, which I will also move through quite quickly, are what you would expect. They used a long-term electrofishing data set from class 1 and 2 streams in Wisconsin from 1994 to 2019 (Figure 1). They provide a rationale for what samples are included in the analysis - and which are not. Based on size distributions, they classified trout as young of the year (YOY) or 1+, meaning that they were at least 1 year old. This is because much of what they were interested were changes in recruitment (the number of YOY fishes each year) and because telling YOY from older fish is relatively easy based on length-frequency distributions, determining ages of fishes older than 1 year old is really difficult as there is a lot of variability (i.e. slow growing 2 year old fish and fast growing 1 year old fish overlap in length).

They give us a look into the statewide picture: Brook Trout were found in 89% of sampled stream reaches, Brown Trout in 54%, and their hybrid - Tiger Trout - in 7% of stream reaches. Brook and Brown Trout were sympatric (occurring together) in 44% of classified stream reaches. And then they present their statistical methods - the part where most people's eyes glaze over. Their climate data comes from Daymet which is provided by the US government for free to all. Then they developed a suite of potential models and used a Bayesian model selection criteria to find the most parsimonious model - an application of Occam's razor - the simplest correct model is probably the best model. Models were developed using a priori knowledge, such as that pre-spawn YOY trout densities may be a function of weather conditions in one or more of the previous seasons. A cold-winter, a spring flood, or a summer drought all may be an explanation for how many YOY trout were caught in a late summer sample, for example.

All this leads up to the results and their discussion of those, the real meat of any primary research paper - particularly one like this which is reliant upon a lot of data over a long period of time. There are two really large findings (Figure 2, above). First, quite unsurprisingly, YOY were much more variable than populations of fishes one year or older (Age 1+). But we expected that as each year's spawn is different and even in high YOY years, only a portion of them are likely to become adults next year. I wrote about in posts on density-dependence and independence and mortality - neither of which were specific to YOY but each post provides the background necessary to better understand the Maitland and Latzka (2022) paper. Second, they found that Brown Trout had shown an increase over this time frame and Brook Trout have declined. Again, I do not think this is a surprising finding but it is an important one. Often times, we get the results we expect but what we are really interested in quantifying that effect. How much have Brown Trout increased and how much have Brook Trout declined? And what factors can we attribute to those changes? Another important thing is that these are statewide results, which tell us that these are due to larger scale phenomena - like climate change - and not smaller, local or regional effects - like fisheries management actions, a localized flood, or other smaller scale explanations.

The next few figures go together, explaining at smaller temporal (time) and spatial (geographic) scales what is being the two major findings described in the paragraph above. In examining these figures, remember that temperature and precipitation are from times before that fish sample was taken as those are expected to be the climatic explanations for YOY and adult numbers collected in the each year's fish samples.

Probably the most significant finding is that Brook Trout were negatively affected by summer maximum temperature and Brown Trout responded favorably to the same variable. In other words, summer stream warming leads to increases in Brown Trout and decreased in Brook Trout - this likely explains our own observations over time of Brown Trout replacing Brook Trout. Note that previous year temperatures have much stronger predictive strength than previous year precipitation. Winter precipitation had a strong negative effect on both species - winter rains and/or heavy spring snow melt can scour trout redds, leading to a decline in the next year's recruitment (YOY numbers). Interactive terms in panels c and d of Figure 3 show that many of these variables effects are different from northern to southern Wisconsin - which Figures 4 and 5 will dig deeper into.

First and maybe most importantly, temperature effects were greater in southern Wisconsin than they were further north. This is because we have experienced greater warming in southern Wisconsin and Brook Trout streams were more likely to "be on the edge". Previous summer maximum temperature had a negative effect on Brook Trout recruitment in northern, mid, and southern Wisconsin (panel a - c) but the effects were stronger is southern Wisconsin. And for Brown Trout there was a positive effect of maximum summer temperature in all three regions but the effect was strongest in the southern part of the state (panels m-o). The part of the figure I find more interesting are the effect of maximum spring temperature on Brook (panels j-l) and Brown (panels v-x) Trout differs pretty significantly in southern Wisconsin compared to the rest of the state were deviations from average (0) have a negative effect. That effect is less pronounced for Brook Trout in southern Wisconsin and for Brown Trout, warmer springs result in higher recruitment. There is probably an interaction with precipitation as well. Southern Wisconsin sees less snowfall so snowmelt effects are likely decreased but further north, warm springs probably mean rapid snowmelt, a significant negative on trout redds or recently emerged trout fry.

Precipitation effects also vary by region but their effects are less pronounced than temperature, in part because precipitation varies much more than does temperature.

Management Implications

Despite the inherent variability of trout populations, particular across over four degrees in latitude, the effects of climate change on trout were quite evident. Across the state, Brown Trout are increasing and Brook Trout are decreasing. This replacement is more evident in the south than in the north. Maitland and Latzka (2022) write,

Given the apparent universality of negative effects of high winter and spring precipitation on trout recruitment, actions that mitigate flooding effects through improved infiltration of precipitation, like floodplain or wetland restoration and bank grading, or that increase watershed connectivity, like dam removal, may be suitable in any location where trout are projected to persist.

They also highlight that there is a lot we do not know about how these two species interact and compete, particularly under changing climatic conditions. For example, they highlight restoring watershed connectivity but in part of the Driftless that I fish most of the time, Brook Trout above barriers where Brown Trout have been removed provide the best Brook Trout fishing and conceivably, the best chance for long-term Brook Trout sustainability. I am not questioning their conclusions but highlighting the next major idea they write about, how management may have to be tailored to the particular environment.

Fishing a Brook Trout stream
Mike fishing one of the small Brown Trout removal, Brook Trout restoration streams in the Heart of the Driftless.

Maybe one of the most important things to come from this paper are that the effects of climate change vary across the regions of the state and these findings support the idea that different regions need to react differently to mitigate the effects of climate change. Brook and Brown Trout are responding differently in southern Wisconsin than they are elsewhere. The future of our native Brook Trout is uncertain (Dauwalter and Mitro 2019) and brings up questions like, "what do we do to preserve and protect Brook Trout?" - knowing that those answers quite likely are different across the state. Maitland and Latzka (2022) write,

Other actions may be targeted to locations where local factors inhibit resiliency. For example, hot temperatures for brook trout in southern and central Wisconsin may be mitigated through improved groundwater management and channel management for hyporheic exchange, as well as improvements to riparian shading and large wood or log jams.

They highlight the idea that while there is a demonstrated decline due to climate change, there are local bright spots. Identifying and protecting these bright spots will go a long way toward preserving Brook Trout into the future. They specifically mention in-stream or riparian habitat improvement, land acquisition, watershed land-use, groundwater protection, and public outreach as management actions. I think regulations can and should play a role in this as well - in particular, where appropriate, protecting Brook Trout from harvest and promoting the harvest of Brown Trout may be a valuable management strategy - if anglers are willing to do their part.

Improved fish passage at this road crossing
Trout Unlimited in Northern Wisconsin is restoring connectivity and access to spawning grounds which has shown great improvement in Brook Trout. (Chris Collier photo)

There is still lot we do not know but long-term research like this that searches for correlations between fisheries data and climatic conditions are important in understanding the effects of climate change and in providing potential solutions. The problems with these type of studies are that they are informing us about correlations - not causation - and what they tell us may come too late for management actions to prevent against these changes. It does however give us a chance to do something before we no longer can. I would not say it is too late for southern Wisconsin - there are still bright spots - however, the declines are pretty significant. Northern Wisconsin is in better shape, though they too have seen declines. Efforts like Trout Unlimited's Priority Watersheds and the Wisconsin DNR's Brook Trout Reserves are an important part of the identification of watershed where Brook Trout are likely to have brighter futures. Then management actions can be tailored to these watersheds to ensure, or at least improve the chances of, the long-term survival of our native Brook Trout.

Read the Maitland and Latzka (2022) article for yourself and explore the references below. But most importantly, do what you can to help our beautiful native Brook Trout.


Downloaded from, Wiley Online Library on [20/12/2022]. See the Terms and Conditions ( on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License protection, and public outreach.

535 views4 comments

Recent Posts

See All

4 comentarios

As a relatively inexperienced fly fisher, I really appreciate every brookie that I've caught. It will be a big challenge for all of us "Brookie Stakeholders" to ensure their survival. Stream management has evolved and hopefully will continue to do so. Will there be an emphasis on re-planting in riparian zones on selected streams to help cool the water? On a personal level, more TU workdays are in order!

Thanks for another informative article.

Me gusta

Thanks for putting this post together Jason! I had just finished reading their paper and loved having your synthesis of their findings to help digest everything.

I also love your questions about habitat reconnection in areas where barriers are creating brook trout strongholds and if removing those barriers might actually harm brook trout in areas where there might have the best chance to persist. An always difficult and interesting management question.

Me gusta

I prefer the brook trout. One of the streams I fish in NW Wisconsin has a mix of brook trout and browns, while one tributary to the main stem is exclusively brookies and another nearby has only browns. I should ask my local fisheries biologist about that. Or maybe get a little more comprehensive with my use of the thermometer.

I may have mentioned this in comments to earlier posts. One stream I know of has a mix of brookies and browns. A multi-year drought will decrease spring flow, and the browns will come up into the headwaters that were predominately the home of brook trout. A few years after the rains return, the browns are back downstream and the…

Me gusta

Really liked this post. Any post that summarizes the science for us that weren't formally educated in the science or don't keenly follow the science opens my mind wider than normal. That is a good way to start a new year !

Me gusta
bottom of page