Non-Native Trout Removal as a Management Tool for Native Trout Restoration

Jason G. Freund
4 hours ago
11 min read

Kirk Olson, Wisconsin Department of Natural Resources Senior Fisheries Biologist in La Crosse, and colleagues recent paper "Brook Trout population response to Brown Trout removal by electrofishing in a Wisconsin Driftless Area stream" (Olson et al. 2024) was published which lead to this post. In summary, this is a paper about the restoration of Brook Trout and the removal of Brown Trout from Maple Dale Creek in the West Fork of the Kickapoo River watershed, similar to the upper Seas Branch restoration project. In short, when there is a barrier to Brown Trout recolonization, Brook Trout populations boom after the removal of Brown Trout. It is, in many ways, a pretty simple story.

Kirk Olson and the rest of the WDNR fisheries crew in a river, wearing waders. Kirk pours water into a container on a boat. Lush greenery surrounds them under bright sunlight. — Kirk Olson and the rest of the WDNR fisheries crew about to weigh and measure trout from a Driftless Area stream.

The paragraph above is the short story. This is a classic example of a problem that looks simple on its face, but is more complicated as you dig deeper. The simple part is that there is little question that non-native trout affect native trout. While we know that effects, often we don't know the magnitude of the effect. The more difficult question is, what, if anything, can we do about it? I say this largely because it is a huge, widespread problem across not only Wisconsin, but more or less anywhere that trout are native.

A person in a hood and sunglasses kneels by a clear stream, releasing a Brook Trout into the water. Dry grassy bank in background. Sunny day, calm mood. — My friend Mike releasing a Brook Trout into a restored Brook Trout stream.

We have a ton of evidence that non-native trout negatively impact native trout. But the magnitude of this effect varies greatly and, like most ecological questions, the "where" has a great effect. I know I keep linking to this post, but it's for good reason. And I could list a ton of papers that demonstrate this (they are in the resources below) but I do not feel like writing a scientific paper, and you probably do not feel like reading one. So we'll keep this at a technical blog level, fairly far short of a technical journal article level.

A tranquil forest scene with a clear, rocky creek surrounded by lush greenery and trees, conveying a peaceful, natural atmosphere. — Another stream that is above a barrier to Brown Trout that is home to only native Brook Trout.

Their results are not in the creative commons or publically available, so I can not share tables and figures, but you can read the abstract and understand much of the article. From this, you can see that it took a lot of elecrofishing effort (33.7 km) over 56 site visits to remove non-native Brown Trout from 7.1 km of stream. This effort decreased Brown Trout density to 1% or less in a few years. You can also see that they removed a lot of Brown Trout - over 20,000 of them.

Kirk and his colleagues are hardly the first to recognize that non-native trout are a detriment to native trout. There are tons of examples of how non-native trout negatively impact native trout (see the references below for many examples). And they are hardly the first to demonstrate that the removal of non-native - dare I call them invasive Brown Trout - have a negative effect on Brook Trout (i.e. Huntsman et al. 2023). In the Western United States and elsewhere, it is Brook Trout that are the non-native species negatively impacting native Cutthroat Trout (i.e. Peterson et al. 2004), Bull Trout (i.e. Howell 2018), and Rainbow Trout (i.e. Carmona-Catot et al. 2010). And in parts of the Appalachians, it is non-native Rainbow Trout negatively effecting native Brook Trout (i.e. Kanno et al. 2016). And to make matters more confusing, non-native Brook Trout seem to have some negative effects on native Brown Trout in Europe (i.e. Korsu et al. 2009). Nature is rarely simple.

Man kneels by a stream holding a Brook Trout. He wears a backpack and sunglasses, surrounded by green foliage, under a clear blue sky. — Brandon Thill, a former graduate student and now WDNR employee, holding a Brook Trout in a Driftless Trout stream.

You may be thinking, "why do we need to spend the time and energy to document the 'obvious'"? I will say there are very good reasons. One of the most important reasons is that we need evidence that Brown Trout negatively impact Brook Trout in the Driftless and that having barriers to Brown Trout recolonization helps protect a native species. While you and I may argue that this is obvious, we live in a world where politicians and those that are not scientists make decisions that require them to understand science. When we can document the effects locally, we stand a much better chance of politicians not being able to ignore or discount these results. By demonstrating how Brown Trout removal led to an increase in Brook Trout, it makes it easier to support keeping a few dams, or providing barriers to Brown Trout after dam removal. This is very important to myself and many that live and fish in the Driftless Area. We want to see native Brook Trout prioritized and protected.

Person holds a Brook Trout in a forest stream, wearing waders. Fallen tree and equipment are visible. Calm and natural setting. — A Brook Trout in a north-central Driftless Area stream being weighed after being measured.

I wrote about this in a post - Demystifying the Dam Decommissioning Process - West Fork and Coon Creek Watersheds - and a WisTrout article about this issue.

The Ecology of Non-Native Trout Removals

There are a number of ecological explanations for how the removal of non-native species leads to an increase in native species. One of the ecological concepts that supports non-native trout removal is the Allee effect (Tobin et al. 2011). The Allee effect essentially says that as species become less common, finding a mate becomes more difficult and this leads to further population declines or at least slower than expected population growth. Under density-dependent population regulation, we generally expect populations far below carrying capacity to show the highest growth rates as there is much available "space" for more individuals.

Graph showing density-dependent population regulation with axes labeled "Population Density" and "Population Growth Rate." Negative feedback arrow. — This is our classic view of density dependent population growth. As the density of a population gets closer to carrying capacity, the growth rate of the population decreases.

The Allee effect describes the departure from the typical relationship between population density and the population's growth rate at very low population densities. The Allee effect is shown in the figure below where at low densities, we see a negative population growth rate (i.e. the population decreases). This is a strong Allee effect, under weaker Allee effects, there can be a positive growth rate but it is lower than predicted by density dependent models. The ecological explanation is that at low population densities, the ability / likelihood of finding a mate is low and this leads to low reproductive success. Thus, a larger part of removal projects is about reducing populations to the point that finding mates becomes the limiting factor. This is easier said than done - it requires a lot of effort over long periods of time to get to the point where population densities are low enough for the Allee effect to further decrease populations.

Graph illustrating the Allee effect: a curve on axes labeled "Population Growth Rate" and "Population Size (N) or Density" with a dip and peak. — The Allee effect is shown at the low population densities.

Does it Work?

It can, but it is not easy and it takes a lot of effort or a lot of poison which has its own set of issues. Rotenone is a naturally occurring compound in several plants and has been used in fisheries management as a sampling technique as well as to erradicate non-native species. Rotenone is essentially used to suffocate fish - it affects cellular respiration and fishes ability to use oxygen in this proecess. However, rotenone has non-target effects (Beaulieu et al. 2021), requires more than one application (Lampton et al. 2023), and is not always successful, particularly in larger watersheds (Lampton et al. 2023).

Two people in waders work in a lush, green stream with a white equipment crate. One crouches with hands in water. Overcast sky above. — Ryan and Tommy with the barge electrofisher on a small wadeable stream.

Electrofishing - as in the Olson et al. 2024 study - is often used instead of rotenone because it is more selective and has fewer non-target effects. Whereas rotenone is an indiscriminant piscicide (fish killer), electrofishing allows non-native fishes to be removed and native fishes - Slimy Sculpin and Brook Trout in the case of Maple Dale Creek - to be retained in the stream.

Sign for trout anglers with harvest season dates and rules on catch limits. Features a fish illustration. Background: greenery. — Anglers can also play a role by harvesting non-native fishes in targetted restoration sites.

In either case, the goal is to remove non-native fishes to the point where the Allee effect can "handle the rest of them". The goal is not to remove every non-native individual because that is nearly impossible. As populations of non-native fishes are driven lower, it requires more effort to capture them. However, there are some life history events - overwintering in slow, deep pools and spawning - that concentrates fish and may allow more targetted capture events. The difficulty in this process is getting to a point where the densities are low enough to cause strong Allee effects.

Lush green landscape with rocky hillside, wildflowers, and blue sky with clouds; serene and natural setting. — The failed Jersey Valley Dam - it is temporarly rebuilt and continues to provide a barrier to the upstream movement of Brown Trout.

There are many examples of non-native fish removals not meeting their goals. There are three main reasons why removal may not work. First, if the population is not driven low enough, non-native fishes can rebound quite quickly. This is particularly true if agencies are not monitoring the stream post-removal and not following up with targetted removals. Second, without barriers to recolonization, non-native species are likely to recolonize and unravel the removal efforts. And not all barriers have been shown to be effective, particularly in high flows. I know efforts on one Driftless Area tributary where a barrier was insufficient to prevent Brown Trout recolonization and there are a few examples of Brown Trout moving through warmwater streams in winter to recolonize removal streams. Lastly, there are population and genetic consequences of isolating fishes above barriers. I wrote a pretty extensive series of posts on this topic a couple of years back (Using barriers to restore native fishes).

A Slimy Sculpin rests in a person's open hand. The background shows a green bucket and grass, suggesting an outdoor setting. — Trout are not the only native fish that can be protected above barriers. We lack some information about Brown Trout effects on species like sculpin but annecdotally, I see more sculpin in streams with lower densities of Brown Trout.

In short, non-native fish removal to restore native species. The Olson et al. (2024) paper showed that trout biomass stayed about the same but shifted from being dominated by non-native Brown Trout to being comprised mainly of native Brook Trout. Their populations increased several fold after the removal of Brown Trout. It was a great success story and should remain so with some active management (i.e. continued monitoring) as long as a barrier to Brown Trout remains in place. This is why myself and others care so much about what happens with the Seas Branch and Maple Dam dams and the decommissioning process.