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Native Brook and Non-Native Brown Trout: Towards a Better Understanding their Distributions

Updated: Mar 28

Remember the first day of school and how the first question of every first class was, "What did you do this summer?" Well, I did field work. And in-between, I watched some softball, camped a bit, and, of course, fished - but less than most years. That said, I maybe learned more about where trout live than I would in many years of trout fishing at this point (more on that later...).

Driftless Brook Trout
Brook trout from a northern Driftless stream - those streams are much different from the steams further south.

I spent most of my summer doing "real science". You know, a ton of time spent in the field collecting a whole lot of data to hopefully boil it down to a couple of main findings and if we are fortunate, maybe two scientific publications. That is the nature of science. In general, it is a lot of effort for not a ton of reward. But putting in the time is the only way we gain new insights. Sometimes those new insights come as huge new ideas and shift the way we think about "our world". More typically, we quantify and better understand how things work under a specific set of conditions. Under different conditions, we may have had different findings. I wrote a much longer post about this - "It Depends - The Answer to Nearly Every Ecological Question".

A sandy Driftless stream
A central Driftless Area stream - these are largely sandbed streams and mostly home to Brook Trout.

Our research objectives this summer were were to sample streams in the Driftless Area near(ish) to La Crosse and try to better understand how geology influences habitat and trout. Ultimately, our hope is to better understand the relationship between Brook Trout (Salvelinus fontinalis) and Brown Trout (Salmo trutta). As you have likely experienced, Brook Trout have largely given way to Brown Trout in the southern and northern Driftless Area. At least in "my" part of the Driftless, chasing Brook Trout largely means fishing a few select streams that generally are either quite small, have barriers that prevent Brown Trout from outcompeting Brook Trout, or both. Understanding where Brook Trout are not only holding on but generally thriving is pretty important.

Typical southern Driftless stream
A typical cobble/gravel stream in the dolostone (dolomite) geology of Vernon County.

What we - we is myself and a graduate student - did this summer is collect detailed microhabitat data from 20 streams. We collected data on stream widths, depths, current velocities, fish habitat, and other physical characteristics. These streams ranged from central Vernon County to northern Monroe County. In the south, dolostone (dolomite) is the cap rock whereas further north, sandstone makes up most of the surfical geology. Going into it, we expected to see some differences - and I'm pretty sure we will - but the data are yet to be analyzed.

Northern Monroe County stream
A northern Monroe County stream in the sandstone geology. Nearly all microhabitats were dominated by sand substrates.

What was a little unexpected was just how different these streams are. I expected the physical differences. Streams in the sandstone geology were, well, sandy. What will be interesting is to see how the microhabitat data fit the habitat suitability models. That is, will we see differences in habitat suitability for dolostone and sandstone streams? I honestly do not know what to expect which means I think they'll probably be equally suitable for Brook Trout in terms of habitat. What will almost certainly be different is that the sandstone streams stay colder. When we go to the streams and take the temperature, large streams - as large as the largest of the Driftless trout streams - are quite a bit colder in the central Driftless. During the hot spell we experienced in mid-August, nearly six degrees Celsius (nearly 12F) cooler. More on that once we process all the temperature logger data.

Checking for gill lice
Checking for gill lice in Brook Trout on a sampling trip to northern Monroe County with the WDNR fisheries crew out of La Crosse.

The other major difference I did not know to expect was the productivity differences between the areas. Looking back, it should have been expected as the streams of the southern Driftless are as productive as just about anywhere on Earth. In the sand streams of that part of the Driftless Area, we would catch a fraction of the trout we catch in Vernon County (with electricity!). As anecdotal evidence, two streams we sampled in northern Monroe county had no Brown Trout and Brook Trout densities were quite low. On one reach that was about 180 meters, we caught 24 Brook Trout and no Brown Trout whereas in a popular reach of a Vernon County stream, in 145 meters, we caught 420 Brown Trout and a couple of Brook Trout. What causes these differences, aside from their bedrock geology is probably quite complex and we will not have the data to say too much in this area. Yet...

Graduate student snorkeling for Brook Trout locations
Brandon snorkeling a northern Monroe County stream to find Brook Trout.

The other major research activity of the summer was snorkeling to locate Brook Trout and collect a number of measurements similar to those we collected in the microhabitat surveys. A graduate student doing the snorkeling would place a rock painted orange at Brook Trout locations and we would record data such as the length of the fish or the largest fish if there was a pod of fish and whether the fish was resting or feeding, and how far off the bottom the Brook Trout was. We would then collect other physical habitat locations at the fishes location and at the edges of the fishes "window" - which we defined as a distance two times the fish's length. For more on the Trout's Window of Vision, visit Wisconsin Fly Fisher's website.

Orange rock marks the spot.
A rock placed at the location of a resting or feeding Brook Trout.

Again, the data are not yet analyzed but some things that were evident were that Brook Trout, not surprisingly, are typically found in deeper water, generally not far from cover. Yes, not very surprising. Any successful fly angler understands that. Our analyses will be multivariate and look at the relationship among the data. My experience tells me that predicting relationships in datasets with a large number of variables and observations is generally a fool's errand. Multivariate statistics are useful for quantifying relationships which are often not readily evident - particularly after the major trends are accounted for. That is, they use habitats that are deeper but the relationship to other habitat features are more difficult to discern - but multivariate stats tend to figure out what the naked eye cannot.

Southern Driftless Brook Trout
Of course, I did some fishing too. The Brook Trout are already gaining some fall colors. And yes, this is a friend's fish.

For me, the real fun of research comes in the analysis of the data and lacking that, it is difficult to say too much. And like any research, starting to address one question opens up several other questions. I still have a ton of questions about Brook and Brown Trout in the Driftless.

And the Rest of the Summer...

While field work was the dominant summer activity, it was not all work and no play.

Waterloo Softball team wins to go to state
My niece's softball team earning their first ever trip to the State Tournament at Goodman Diamond in Madison.

Summer is always a time to get in all the things I have a hard time making time for during the school year - visiting friends and family, watching a lot of my nieces' softball games - including the team's first trip to state, and, of course, fishing. I fished a lot less than most summers. But don't feel to badly for me, I probably got in about 50 days this year including a couple of epic trips. I did more Smallmouth Bass fishing this year than last with a Crash Camp trip and a more recent trip to the Sylvania Wilderness.

Nebraska Sandhills
A work trip to Nebraska's Sandhills - a gorgeous region of theit state.
Lake in Sylvania Wilderness
An August trip to Sylvania Wilderness in Michigan's Upper Peninsula.

My summer was pretty busy - how about yours?

Since this post was published, here are the two Driftless Symposium talks that came from the summer of 2023 research:

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