Written by Nick Voss, University of Idaho
Non-native brook trout were introduced to Idaho more than 100 years ago and since that time they have found a home across many drainages in our state. While having brook trout in some places makes for a great day fishing, in others, they are a threat to an iconic Idaho native, the bull trout. Brook trout have the capability to outcompete and cross-breed with bull trout which is not a good thing. So, understanding brook trout populations in Idaho is an important research topic for bull trout conservation, and one University of Idaho graduate student aims to do just that.
An American classic
Brook trout have been called a lot of things in their time as an American sportfish, but one thing they’ve never been called is ugly. Their beautiful colors and delicious fillets have made them a classic sportfish in the eastern U.S. since colonial times. As the west was settled, homesick easterners grew to miss their favorite sportfish and wanted to be able to catch them in places like Idaho.
A brook trout from the upper Crooked River, captured in an area that once held juvenile bull trout.
A legacy of stocking
Beginning in the late 1800s, brook trout were stocked in streams, rivers, and lakes across the West, where they are not native. Some of these efforts failed, while others created popular fisheries. However, in some places brook trout spread far beyond where they were originally stocked, and into some of the most pristine high-elevation streams in Idaho. This might not have been a big problem, except that another species, bull trout, already called many of these streams home.
Interactions with Bull Trout
Bull trout are famous for their large size and diverse life history strategies, but the one thing they all have in common is they need small, clear, and very cold streams to reproduce. Unfortunately, because of those historic stockings, brook trout occupy many such streams across Idaho. In some streams, they have replaced bull trout populations by cross-breeding with them or outcompeting them for food and habitat. The bottom line is that the more brook trout, the lower the success of bull trout in that stream.
A bull trout × brook trout hybrid from the Little Lost River basin. Light spots that reach the top of a dark dorsal fin are unique to hybrids.
Brook trout and bull trout today
At this point it’s important to mention two things: Firstly, fish managers now account for the effect of stocking on native species, so it’s been a very long time since the Idaho Department of Fish and Game has stocked brook trout in streams containing bull trout. Secondly, bull trout remain widespread in Idaho, and some populations have never even seen a brook trout. However, what puzzles managers today is what happens where the two species occur in the same stream. In some areas, brook trout populations have not increased or moved to nearby waters in decades. In others, brook trout continue to increase in numbers and slowly move farther upstream; and every once in a while, they seem to take over entire bull trout streams in the blink of an eye! It’s unclear why we see such different patterns in different areas, and this makes it hard for management agencies such as IDFG to prioritize actions to protect bull trout streams which may or may not be at risk from brook trout moving into them.
What does the future hold?
Understanding which bull trout populations are most at risk of being taken over by brook trout could help prevent that from happening. The past may offer a guide to the future, but a widespread survey of how and where brook trout have expanded their range in Idaho has never been conducted. As a result, our understanding of where they tend to occur (and not occur) remains uncertain.
The past as a guide to the future
Such a survey is being conducted by Nick Voss, a Ph.D. student with the Idaho Cooperative Fish and Wildlife Research Unit at the University of Idaho. Nick has identified hundreds of locations in bull trout streams that were sampled for various purposes about 20 years ago. These sites were selected because they were important to bull trout historically, but brook trout were also nearby and have had the opportunity to move in over the last two decades. Nick and his field crew are re-sampling these sites to determine where brook trout have moved in, and where they have not. Nick is also collecting detailed habitat data, which may help explain why brook trout have made big movements upstream in some places, but little or none in others.
Fisheries Technicians Ford Budge (left) and Taylor Robnett (right) locate a historical electrofishing site. Many of these sites had not been revisited in two decades.
Early results show that brook trout have rapidly expanded in the Little Lost River Basin during the past two decades.
So what?
Bull trout have important ecological and recreational value. Addressing the threats they face will help conserve that value for future generations. Because fisheries biologists cannot be everywhere at once, it’s important to know where brook trout pose the greatest danger to bull trout populations. This research will help biologists identify areas where brook trout are likely to move in if management tools aren’t used to stop them. At the same time, it will also help identify streams where brook trout won’t move in even if no management actions are taken. This information helps focus efforts where we can have the most positive impact for bull trout.
Idaho has some of the best bull trout habitat around, and many of our bull trout populations are currently secure. Doing work like this and keeping an eye out for threats on the horizon will help keep it that way.