The Westslope Cutthroat Trout is a favourite for many anglers in Southern Alberta, myself being one of them. They aggressively eat dry-flies, are relatively easy to fool, and inhabit some of the prettiest streams in the province. They have a distinctive red-slash under their jaw, a trait which gives them their name. They are a unique fish, and a major attraction for anglers across the west. However, they are presently listed as threatened under the provincial Wildlife act and by COSEWIC. What are the threats facing these trout? What is their status in the province and current management strategies? What are the impacts of angling, and what can you do about it? In this three part series, I am going to do a deep dive into Westslope Cutthroat Trout in Alberta, share some of my personal experiences working with this fish, and provide a framework for limiting your impact as an angler.

This post is intended to provide interesting, relevant information regarding Cutthroat Trout in Alberta, and summarize some of the data and scientific research that is available. It has been compiled using a variety of sources. Where possible, I will provide the source of the information along with links to the source material. If expressing my personal opinions, I will try to make it clear.

Westslope Cutthroat Trout (Onchorynchus Clarkii Lewisi)

A natural place to start when discussing the ecology of cutthroat trout is understanding some of the factors leading to speciation of different trout in Western North America. The story of cutthroat trout begins with the most recent ice age, which peaked around 20,000 years ago and ended around 11,500 years ago. During this time, there were a few places which served as refugia for trout and other species of fish. Rainbow Trout and the closely related species of pacific salmon occupied the oceans before the Ice age, and the pacific ocean served as a refugia for these species during this time. A second refugia was present in the interior of the United States, which also served as refugia to Onchorhynchus species similar to those found in coastal waters. Over the 10,000 years that followed, the isolation between the two populations led to speciation, with the inland Onchorynchus populations evolving into cutthroat trout. Following deglaciation, rainbow trout from the ocean recolonized the entire pacific seaboard, establishing themselves in all waterways without barriers. The newly formed Cutthroat trout began to colonize the Rockies from the interior. Here is the key point when considering this evolutionary history: In all places where the two species converged, rainbow trout became the dominant species. The two species do not coexist naturally in the wild, for rainbow trout have several advantages over cutthroat trout that allow them to outcompete them, except at the outer range of their thermal tolerance. Thus, any body of water in North America that is attached to the ocean and does not have impassable barriers contains rainbow trout. Cutthroat trout were able to successfully colonize many streams in the eastern slopes of the rockies (ie. east of the continental divide), and due to things like glacial lakes, ice dams, and other geomorphological processes during this time, were also able to cross the divide and colonize many reaches west of the divide that were uninhabited by rainbow trout because of barriers. The Cutthroat Trout’s expansion north went as far as the South Saskatchewan River drainage in Canada, including the two watersheds in Alberta where Cutthroat Trout naturally occur- the Bow River and the Oldman river systems. This is how we ended up with the species mosaic we had at the time just prior to European Settlement.

Below is a map of the historic range and densities of cutthroat trout in the province, showing the Fish Sustainability Index score based on density.

Source: Government of Alberta, https://open.alberta.ca/publications/westslope-cutthroat-trout-fish-sustainability-index-maps-2018

The Columbia River system in BC is a great way to exemplify this. Prior to damming, the Columbia River was unimpeded from its headwaters to the ocean; rainbow trout are found throughout the drainage, and cutthroat trout are very uncommon. However, it’s major tributary in Canada, the Kootenay river, contains several impassable barriers, namely Kootenay Falls in Montana. Above this barrier there is only Cutthroat trout, which colonized the stream from the interior.

Making matters more confusing is the two species ability to interbreed with one another and produce viable (aka breeding) offspring. The result of this interbreeding over the long term favor’s the establishment of rainbow trout and the elimination of cutthroat trout.

So naturally, what happened next was European settlement, and the widespread introduction of rainbow trout into cutthroat trout rivers. Knowing what we now know about what happens when these two species coexist, it is not hard to figure out how we’ve now arrived at a point where cutthroat trout are threatened. Some notable examples of Southern Alberta fisheries that shifted from cutthroat trout streams to primarily rainbow trout dominated include the Bow River and the Crowsnest River. However, the loss of cutthroat trout goes far beyond these few streams. In fact, rainbow trout genes are now present in 95% of natural cutthroat populations across the province; meaning only 5 % of cutthroat trout populations in the province remain pure. Essentially, only the populations above waterfalls and barriers remain pure.

What are the implications of this? Essentially, all systems where rainbow trout are present will trend towards the establishment of rainbow trout and the extirpation of cutthroat trout. This is bad news for Alberta fisheries, as that suggests that close to 95 % of our cuttthroat rivers are trending towards extirpation. This puts a great deal of emphasis on the remaining 5 % which contain pure cutthroat, as these will be the strongholds for this species moving forward. To exemplify this, here is the current distribution of cutthroat trout and their ratings on the Fish Sustainability Index.

Source: https://open.alberta.ca/publications/westslope-cutthroat-trout-fish-sustainability-index-maps-2018

Side Note: You may have noticed that the historical range I outlined for cutthroat does not include several of Alberta’s cutthroat trout rivers, such as the North and South Ram, the Red Deer River, the Blackstone, ect. Cutthroat Trout were introduced to these streams and were not found there historically. While these populations are of great significance, especially for recreational angling, they are of lower priority for conservation and not the focus of this report. I will focus on the two systems where cutthroat trout naturally evolved, the Bow River system and the Oldman system.

Ecology

Cutthroat Trout need cold, clean water and pristine environments to thrive. They can be quite long lived- on average around 8-10 years- but lifespans of around 20 years have been documented. They typically reach spawning maturity around age 4 and often spawn several times in their life. Below are some quick facts:

• Cutthroat Trout Spawn throughout the month of June. In the Oldman system, peak spawn is around June 13th.
• Cutthroat Trout primarily spawn in small tributary streams and spring fed creeks, where small gravels are present.
• Eggs develop in these small gravels throughout the summer; fry emerge from the gravels sometime in late summer, depending on stream temps.
• Cutthroat Trout overwinter in deep, slow moving pools. The number of deep overwintering pools is often a limiting factor for population size.

Cutthroat Trout in our province employ three general life strategies; they can either be stream resident (complete entire life cycle in one stream), fluvial (migratory, live most of life in larger stream and reproduce and rear as juveniles in small tributaries), or adfluvial (migratory fish that spend their adult lives in lakes, before returning to tributaries to spawn). While there are now several adfluvial populations in the province due to damming and introductions, this life strategy was quite rare prior to Europeans. In fact, Picklejar lakes are the only alpine lakes in the province where cutthroat trout naturally occurred. All other alpine lakes in our province with Cutthroat Trout were stocked. They were not naturally colonized as barriers mostly prevented Cutthroat Trout from reaching those habitats. However, these stocking efforts create some interesting implications. For example, genetic sequencing from the Highwood System suggest that nearly 100 % of Cutthroat Trout in the river have their genetic origins from fish that were stocked into the regions alpine lakes; the original native cutthroat genes have all but disappeared, yet their presence in the river is maintained by fish that ‘trickle down’ from these naturalized, but introduced populations. This idea will be discussed further when I get into management strategies later in this post.

Cutthroat Trout vs. Rainbow Trout

When comparing cutthroat trout vs. rainbow trout, there a few key differences between them. Firstly, rainbow trout have a faster metabolic rate- that is to say, they are less efficient compared to cutthroat. Furthermore, their metabolism changes more as temperatures change, where as cutthroat have similar metabolic rates throughout their thermal range. Functionally, what this means is that rainbow trout have an advantage in warmer, more productive conditions compared to cutthroat and can grow faster and reach maturity faster. Secondly, they are more aggressive during spawning season, and defend themselves and their territories more rigorously compared to cutthroat. This gives them another advantage over cutthroat, with the later being less likely to defend a redd and successfully mate in the presence of rainbows. There is much nuance when it comes to the impacts of hybridization, however, there is much evidence to show that these interactions evidently lead to rainbow trout dominated systems. However, these advantages disappear in systems that are very cold, as Cutthroat Trout are more metabolically efficient at these temperatures compared to Rainbows. The advance of rainbow trout in Alberta has therefore been contained by these thermal gradients, and many high mountain streams continue to be dominated by ‘mostly pure’ Cutthroat. However, data shows that stream temperatures have been consistently warming in the province due to the effects of climate change, allowing rainbows to advance further and further into cutthroat trout strongholds.

Hybrid Swarms

What commonly happens in hybridized systems is what is known as a ‘hybrid-swarm’. This pattern has been observed in many systems across North America, including in Alberta. In the lower reaches of a stream, there will be primarily rainbow trout as they dominate in warmer conditions. In the upper reaches, there will be mostly pure cutthroat, as they dominate in cold water. In the area in between these two ecozones, a hybrid zone will form. A hybrid-zone is a part of the stream where hybrids have the greatest advantage, as their blended traits give them an advantage over both species. This leads to reaches where there are essentially only hybrids, which then mate with each other, producing more hybrids which continue to push out other species from this area. This process, known as a ‘hybrid swarm’ can happen very quickly, and within the matter of a few generations pure cutthroat can be fully pushed out.

The Oldman River provides a textbook example of a hybrid zone. Downstream of maycroft, it is primarily rainbow trout; from maycroft to the Gap, it is almost exclusively hybrids; upstream of the gap, hybrids become more rare and ‘mostly pure’ cutthroat become dominant. However, I would argue that this hybrid zone is getting bigger very year, and hybrids are penetrating further and further upstream. The two figures below depict this perfectly. The one on the left shows the proportion of cutthroat, rainbow, and hybrids at different elevations in the Oldman River. You can see that between 1300m and 1400m more than 60 % of fish captured were hybrids. The one on the right shows how the proportion of rainbow trout genes in hybrids declines with elevation.

Source: JOSEPH B. RASMUSSEN et al. 2012. Metabolic traits of westslope cutthroat trout, introduced rainbow trout and their hybrids in an ecotonal hybrid zone along an elevation gradient, Biological Journal of the Linnean Society, Volume 105, Issue 1, January 2012, Pages 56–72, https://doi.org/10.1111/j.1095-8312.2011.01768.x

Hybrids vs. Pure Cutthroat

In this discussion, I have revealed to you that 95 % of populations in the province are hybridized. However it is important to point out that hybrids appear on a spectrum. For example, there is a big difference between a true hybrid (50 % rainbow, 50 % cutthroat) and a slightly introgressed cutthroat, who may be 95 % pure. The later fish is undoubtedly a cutthrout trout, and few anglers would classify it as a cuttbow. Yet, when it reproduces, it will still be passing on rainbow trout genes to its offspring. Making matters more confusing is that all fish traits appear on a spectrum, so using visual traits is not always effective. The bottom line is that nearly all rainbow trout and cutthroat trout in the province are actually a combination of both genomes, except in the extreme upper reaches of streams and above barriers. However, its still much easier to classify based on what it mostly is. Cuttbow, therefore, is a term I used to describe a fish that is a true hybrid; its genetic makeup is roughly 50 % cutthroat and 50 % rainbow. They result from a pure cutthroat mating with a pure rainbow, or from a cuttbow mating with another cuttbow.

I have noticed more and more people boasting about very large cutthroat taken from streams like the Oldman and Castle in recent years. Talking to anglers on the river last year, many people told me they thought the fish are bigger than they ever have been. I believe this is another symptom of this hybridization. As these fish mature, Rainbow trout tend to grow longer and skinnier, where as cutthroat trout tend to get thicker and fatter, but once they have achieved a certain length, do not grow much longer. Last year, I sampled over 1200 genetically pure cutthroat trout from the Upper Oldman system. Out of those fish, a grand total of 0 measured in over 20 “. We observed probably 50 or more fish that measured between 17″-19.5″. Essentially, pure cutthroat trout almost never exceed 20″. Several times last year I saw people boasting online about cutthroat trout over 20”. This led me to believe that either they were lying, they didn’t actually measure and just guessed that the fish were over 20″, or they actually caught a hybrid. An example of such a trout from the Oldman is below. This fish measured over 21″ but very clearly had both rainbow and cutthroat traits. Fish like this are becoming more and more common throughout the Oldman and Castle.

The gallery below shows some examples of pure cutthroat, mostly pure cutthroat, full hybrids, mostly pure rainbows, and pure rainbows. As you can see, these fish show a spectrum of traits. In general though, rainbows are silvery in color, whereas cutthroat are yellow/orange. The spots on cutthroat generally are denser above the lateral line and towards the tail, whereas they are more or less evenly distributed on rainbows. Rainbows also have a white leading edge on their anal fins, whereas this is usually (but not always) missing from cutthroat. Gill plates on rainbows are generally pink, whereas a pure cutthroat typically doesn’t have any pink/red in their gill plate or their sides. Take a look at the gallery below for examples.

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Cutthroat/Rainbow Hybridization

Click photos to open. Scroll through to see various degrees of hybridization

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I hope that provides a good overview of the two black spotted Onchorynchus species we have in our province, and helps you to understand how hybridization happens and what it looks like when you are fishing. It is probably the most important factor when discussing cutthroat trout conservation in the province. But as with all things in fisheries, the issues are complex and multifaceted, and no one factor can be looked at in isolation. In part two of this series, I will discuss the issues and threats facing cutthroat trout and some of the current management strategies that are being used to conserve them, including how the province is creating a brood stock to conserve the pure genes of our native cutthroat trout.

I hope you enjoyed learning more about this unique fish we have here in our province! Please share your thoughts, observations, concerns, or reach out to me if you want to discuss further. Thanks for reading and be sure to check out part two, Threats facing Cutthroat Trout, next week.

Continue reading part 2 by clicking this link.

Tight lines,

KR

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Extra Reading and Sources Used:

JOSEPH B. RASMUSSEN et al. 2012. Metabolic traits of westslope cutthroat trout, introduced rainbow trout and their hybrids in an ecotonal hybrid zone along an elevation gradient, Biological Journal of the Linnean Society, Volume 105, Issue 1, January 2012, Pages 56–72, https://doi.org/10.1111/j.1095-8312.2011.01768.x

Megan R. McCusker, Eric Parkinson, Eric B. Taylor. 2003. Mitochondrial DNA variation in rainbow trout (Oncorhynchus mykiss) across its native range: testing biogeographical hypotheses and their relevance to conservation. https://doi-org.prxy.lib.unbc.ca/10.1046/j.1365-294X.2000.01121.x

SARA registry. 2019. Westslope Cutthroat Trout: recovery strategy and action plan, 2019 (proposed). Link: https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/recovery/westslope-cutthroat-trout-2019-proposed.html

Government of Canada. 2021. Report on the Progress of Recovery Strategy Implementation for the Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi) Alberta Population (also known as the Saskatchewan Nelson River Populations) in Canada for the Period 2014 to 2019. Link: https://wildlife-species.canada.ca/species-risk-registry/virtual_sara/files/Pr-WestslopeCutthroatTroutTruiteFardeeOuest-v00-2021Feb-Eng.pdf

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