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What Controls Maximum Trout Size?

If there is a common complaint about trout fishing in the Driftless Area, it is that the fish do not grow very large. If you travel West, you will run into larger average size trout. The same is generally true of traveling to northern Wisconsin - though maybe not the same as Montana or Wyoming. Generally, the fish in the Driftless generally do not grow to large sizes - though we will talk about some exceptions. The question I plan to take on is why is this the case - and is there anything that can be done about it?

Old scanned photo of Crooked Creek
An old scanned photo from one of the first Driftless trout streams to have too many wild Brown Trout. This image is from the mid-1990s.

I thought to write about statistics and how fish size distribution is measured but I think I will save this for another post (some of that is in the post, "It Depends" - The Answer to Nearly Every Ecological Question. Instead I plan to focus this post just on the largest of the large fish and factors that contribute to relatively high numbers of trophy trout.

This figure from Budy and Gaeta (2017) sort of surprised me. It is quite evident that Brown Trout, in their native environments, generally do not grow very large; certainly not compared to their growth in much of their non-native range. Even the USA Midwest - Wisconsin and the surrounding states - grows them as large or larger than in their native range. For reference, 9 inches is 229 mm, 12 inches is 305 mm, the USA Midwest's average maximum of 400 mm is about 16 inches, and the top of the scale - 650 mm - is about 26 inches. It would have been interesting to have the two countries where most US Brown Trout originally came from - Germany and Scotland - on the figure.

That animals vary in body size across their ranges - both native and non-native ranges - is nothing new. Hell, there have been books written on the subject. Obviously the sizes represented in the figure above are generalizations. Here in Wisconsin, we certainly see a huge range from the Great Lakes "Seeforellen" strain - a German lake-run Brown Trout - and the rather Heinz-57 mix that makes up most of our inland Brown Trout.

Factors Affecting Growth

At first blush, the simple answer is that there are too many fish in most Driftless streams and that reducing their numbers will increase the average size. In ecology / fisheries biology terms, this is an application density-dependence. In which case, reducing the population density will increase the growth of individuals and thus we will get larger trout. While this is a logical assumption, the literature is rather inconclusive about if Brown Trout (Salmo trutta) show density dependent demographic rates (immigration and emigration, birth and death rates; Jenkins et al. 1999) and growth (Dieterman et al. 2012, Lobón‐Cerviá 2007). That is to say, while it seems intuitive that reducing the density would increase the average - and maximum - size of trout; we often do not see this occur in nature. Counter-intuitive, I know but there are so many other factors that limit growth (Dieterman et al. 2012, Lobón‐Cerviá 2007, Budy and Gaeta 2017).

Decent fish out of a small stream
Certainly not a huge fish but it came out of a stream that was 3 feet wide (or less).

This is not to say that reduced densities are not often associated with large trout - Brown and Rainbow Trout in New Zealand and Patagonia, Coaster Brook Trout in places like the Minipi River, and Lahontan Cutthroat Trout of Pyramid Lake - are examples of low density trophy fisheries. What many of these places have in common are access to lakes - like the Great Lakes or the ocean - or significant predators - New Zealand Longfin Eels (Anguilla dieffenbachii) and Northern Pike (Esox lucius) in the Minipi. For example, Longfin Eels in New Zealand prey heavily upon juvenile trout (Jellyman 1996) but those trout that survive face less competition.

Density is only one factor of many that influences fish size. In broad terms, size is a function of their environment and genetics. And, of course, the term environment is a terribly broad term. Probably most specifically for cold blooded fishes is water temperature - a topic I wrote about in a fish bioenergetics post.

How to Grow Big Fish

Ultimately, it comes down to bioenergetics and longevity. For any fish species, give them abundant, energy-rich food in an environment where they need to expend relatively little energy to survive, make that environment as stable as possible and near their thermal optimal for growth. These conditions will allow fishes to grow more quickly and live for longer - generally the two most important factors for growing large fish. This explains why a really big Largemouth Bass (Micropterus salmoides) in Wisconsin weighs 4 to 6 pounds and in Texas, California, Florida, and other southern states, a four pound fish is commonplace. And why lakes - which vary less in temperature and require less energy to hold position - grow larger trout than do streams - typically.

Image is from a tweet by Eric Larson, University of Illinois, image source: Schlosser 1991.

One of the most important pieces of the puzzle is connectivity - one of my favorite topics as seen in parts I, II, III, and IV of a series on connectivity and "neighborhoods". And I recently wrote about this in a post about Brook Trout restoration. Access to a variety of different habitats, particularly access to larger, warmer streams where trout can access larger, more energy dense prey like minnows and crayfish are important (Schlosser 1991, Carlson et al. 2016, Huntsman et al. 2016, Armstrong et al. 2021, Al-Chokhachy et al. 2022).

Coon Creek Watershed
Model My Watershed's output for Coon Creek above Coon Valley - it is about 201 square kilometers.

I think we are getting to part of the answer of why we generally do not grow large trout in much of the Driftless or Wisconsin inland streams for that matter. Our trout streams, their watersheds, and the "stream neighborhoods" are relatively small. The Wolf River is the largest trout stream in Wisconsin by a pretty fair bit. It is, of course, rather marginally cold and harbors smallmouth as well as trout, a significant portion of which are stocked.

Below are a number of trout streams and their average annual discharge (cubic feet per second) as discharge is the best, most precise measure of stream size. Data are from the U.S. Geological Survey Stream Stats. Watershed maps below are from Model My


  • Wolf River at Langlade = 423 (all measurements are average annual cfs - cubic feet per second; about 35.3 cfs equate to 1 cubic meter per second (cms))

  • Peshtigo River near Wabeno = 380

  • Waupaca River at Waupaca = 238 (well downstream of trout waters)

  • Kickapoo at La Farge = 184

  • Brule River at Brule = 170

  • Plover River at Stevens Point = 143

  • Namekagon at Leonards = 121

  • Kickapoo River at Ontario = 64

  • Coon Creek at Coon Valley = 50 (based on old data)

  • Black Earth Creek at Black Earth = 38

  • Tomorrow River at Neilsville = 29

  • West Branch White River = 22 (old data)

  • Lawrence Creek above dam = 17 (old data from WDNR studies)

Wolf River watershed map
The Wolf River watershed, delineated at Langlade. It is 1,203 square kilometers - 6 times the drainage area and more than 8 times the average discharge of the Coon Creek watershed at Coon Valley.


  • Manistee River near Wellston =1720

  • Au Sable near mouth = 1390

  • Au Sable at McKinley = 1100

  • Au Sable at Red Oaks = 890

  • Pere Marquette at Scottsville = 722

  • South Branch Au Sable at Luzerne = 217

  • Manistee near Grayling = 184

Manistee River watershed map
Michigan's Manistee River watershed at the Wellston gage has about 4,300 square kilometers of drainage area.

Pennsylvania and West Virginia

  • Elk River (WV) below Webster Springs = 686 (downstream of trout waters)

  • Penns Creek at Penns Creek = 452

  • Little Juniata at Spruce Creek = 376

  • Upper Shavers Fork (WV) at Cheat Bridge = 177

  • Spring Creek at Fisherman's Paradise = 96

  • Le Tort Spring Run near Carlisle = 45

Upper Shavers Fork Watershed map
Upper Shavers Fork Watershed - West Virginia - is about 156 square kilomters in drainage area and an average of 177 cfs of flow - pretty large by Wisconsin standards.


  • Missouri River below Holter Dam = 5310

  • Yellowstone near Livingston = 3710

  • Clark's Fork above Missoula = 2920

  • Madison River near Three Forks = 1900

  • Jefferson River near Twin Bridges = 1820

  • Blackfoot River near Bonner = 1540

  • Big Creek = 63 (Yellowstone River tributary)

Yellowstone River watershed
Yellowstone River watershed at Livingston, MT which is nearly 10,000 square kilometers, nearly 8 times the drainage area of the Wolf River at Langlade.

I know what you are probably thinking at this point - isn't this a simple answer? It is and it isn't. Embedded in drainage area and discharge (stream size) are a large number of complexities. Most significantly, is that their "neigborhood" is so much larger. For a huge variety of animals, as the "patch size" increases, so does the ability to sustain large, top predators (Hastings 1988). In this case, trout are the top predators - though remember that trout often grow larger in systems where they are preyed upon by larger predators.

Trout in larger, well connected stream networks can move to find better foraging, overwintering, and spawning habitats than can fishes in smaller watersheds. In particular, larger rivers and certainly lakes have more forage fishes - a highly profitable food source for trout. There is a strong relationship between stream size - however you measure it - and species diversity with smaller streams having fewer fish species (Angermeier and Schlosser 1989). As we saw in Shavers Fork (WV), access to the mainstem allowed Brook Trout to grow faster and larger (Huntsman et al. 2016).

Can we Grow Larger Fish in the Driftless?

My answer is a definite maybe. What the Driftless has going for it is that its streams are incredibly productive. That is, per unit area, they produce a lot of biomass and our streams produce a huge biomass of trout. In fact, the numbers are quite staggering compared to many other places around the country and the world. We often see trout numbers reported as the number of trout per mile - a really odd measurement given the diversity of stream sizes (widths). However, we often see number per mile in small Driftless streams being similar to very large western river like the Henry's Fork.

A commonly held belief is that there is "X" amount of biomass that a stream can produce. It could be 2,500 pounds of trout in 5,000 fish that average half a pound or 500, five pound trout. It is an application of density dependence but the reality is not quite so simple. Certainly there needs to be some small trout and a diversity of year classes that can grow into those large trout.

Map showing a Brown Trout captured in 2 locations over 30 miles apart
A map from Kirk Olson, WDNR Fisheries Biologist in the La Crosse office, showing the 30.5 mile movement of a Brown Trout in the Kickapoo River watershed, a large, well connected cold water watershed.

We can learn lessons from research like that of Al-Chokhachy et al. (2022) whose large scale analysis of factors influencing trout body size variability show us that stream size, water temperature, and density all strongly affect trout size. We are certainly on the low end of stream sizes and a bit counterintuitively, trout streams that hover near the thermal maximum for trout are more productive. Density can be controlled by natural density dependent processes (mortality and fecundity) and through angler harvest. However, I have my doubts that anglers can have enough affect on populations of trout in Driftless streams. Anglers would have to be willing to harvest the smaller, more plentiful fishes and release the larger trout that have a much greater likelihood of growing to trophy size. However, there is some evidence we see the exact opposite of this - larger fish are much more likely to be kept in Driftless streams. Unfortunately, maybe one of the most likely ways to grow larger Driftless trout is habitat or water quality degradation and increasing stream temperatures that decrease trout populations and give individuals more room for growth. This, essentially, informs us about where large trout now exist in the Driftless Area.

16 inch Driftless Brown Trout
A better than average Driftess Brown Trout of about 16 inches - not a huge trout but memorable once around here. Particularly when caught from a stream that is less than10 feet wide.

There are large trout in Driftless streams - but most anglers, especially those that like to fish dry flies (me!) are fishing in places they are not likely to find them. To want to go to Timber Coulee and catch a 20+ inch trout is like finding a needle in a haystack. Kirk once sent me a photo of a true 20 incher they electrofished in the watershed - the photo was labeled "Timber Coulee unicorn". If I were serious about chasing big trout in the Driftless, I would 1) fish in large streams with low density trout populations within well connected watersheds - quite possibly below what we consider to be "trout water", 2) offer them a large meal (crayfish, streamers), and 3) probably do it at night or at least dark, overcast days. This would put the odds in your favor to catch a PB trout in Driftless, if that is your thing. I am generally pretty happy catching an occasional 16+ inch Brown Trout and a Brook Trout over 10 inches once in a while and enjoying the action that come with higher densities of trout. Your mileage may vary...

Previous posts on related topics:

Literature Cited and Additional Resources

Al-Chokhachy, R., Letcher, B.H., Muhlfeld, C.C., Dunham, J.B., Cline, T., Hitt, N.P., Roberts, J.J. and Schmetterling, D., 2022. Stream size, temperature, and density explain body sizes of freshwater salmonids across a range of climate conditions. Canadian Journal of Fisheries and Aquatic Sciences, 79(10), pp.1729-1744.

Angermeier, P. L., & Schlosser, I. J. (1989). Species‐area relationship for stream fishes. Ecology, 70(5), 1450-1462.

Armstrong, J. B., Fullerton, A. H., Jordan, C. E., Ebersole, J. L., Bellmore, J. R., Arismendi, I., ... & Reeves, G. H. (2021). The importance of warm habitat to the growth regime of cold-water fishes. Nature Climate Change, 11(4), 354-361.

Budy, P. and Gaeta, J.W., 2017. Brown trout as an invader: a synthesis of problems and perspectives in North America. Brown trout: Biology, Ecology and Management, pp.523-543.

Budy, P., Thiede, G.P., Lobón-Cerviá, J., Fernandez, G.G., McHugh, P., McIntosh, A., Vøllestad, L.A., Becares, E. and Jellyman, P., 2013. Limitation and facilitation of one of the world's most invasive fish: an intercontinental comparison. Ecology, 94(2), pp.356-367.

Carlson, A.K., French, W.E., Vondracek, B., Ferrington Jr, L.C., Mazack, J.E. and Cochran-Biederman, J.L., 2016. Brown trout growth in Minnesota streams as related to landscape and local factors. Journal of Freshwater Ecology, 31(3), pp.421-429.

Cochran-Biederman, J.L. and Vondracek, B., 2017. Seasonal feeding selectivity of brown trout Salmo trutta in five groundwater-dominated streams. Journal of Freshwater Ecology, 32(1), pp.653-673.

Dieterman, D.J., Hoxmeier, R.J.H. and Staples, D.F., 2012. Factors influencing growth of individual brown trout in three streams of the upper Midwestern United States. Ecology of Freshwater Fish, 21(3), pp.483-493.

Hastings, A. (1988). Food web theory and stability. Ecology, 69(6), 1665-1668.

Holmes, R., Matthaei, C., Gabrielsson, R., Closs, G. and Hayes, J., 2018. A decision support system to diagnose factors limiting stream trout fisheries. River Research and Applications, 34(7), pp.816-823.

Huntsman, B. M., Petty, J. T., Sharma, S., & Merriam, E. R. (2016). More than a corridor: use of a main stem stream as supplemental foraging habitat by a brook trout metapopulation. Oecologia, 182(2), 463-473.

Jellyman, D. J. (1996). Diet of longfinned eels, Anguilla dieffenbachii, in Lake Rotoiti, Nelson Lakes, New Zealand. New Zealand Journal of Marine and Freshwater Research, 30(3), 365-369.

Jenkins Jr, T.M., Diehl, S., Kratz, K.W. and Cooper, S.D., 1999. Effects of population density on individual growth of brown trout in streams. Ecology, 80(3), pp.941-956.

Lobón‐Cerviá, J., 2007. Density‐dependent growth in stream‐living Brown Trout Salmo trutta L. Functional Ecology, 21(1), pp.117-124.

Schlosser, I. J. (1991). Stream fish ecology: a landscape perspective. BioScience, 41(10), 704-712.

Segherloo, I. H., Freyhof, J., Berrebi, P., Ferchaud, A. L., Geiger, M., Laroche, J., ... & Bernatchez, L. (2021). A genomic perspective on an old question: Salmo trouts or Salmo trutta (Teleostei: Salmonidae)?. Molecular Phylogenetics and Evolution, 162, 107204.

Tougard, C. (2022). Will the genomics revolution finally solve the Salmo systematics?. Hydrobiologia, 1-16.

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