The Future of Montana’s Native Fish: FLBS researcher seeks to better understand impacts of hybridization on native fish populations by analyzing long-term data

By 2024 Ted Smith Environmental Storytelling Intern Najifa Farhat

Montana is acclaimed for its angling allure, drawing enthusiasts from distant corners of the world who seek the thrills of freshwater fishing. On a global scale, Montana stands distinguished for hosting some of the world's premier trout fishing, a distinction that has ignited extensive efforts aimed at preserving and enhancing trout populations within its streams and lakes. But the pursuit of improving angling opportunities has encompassed a spectrum of measures, including the introduction of various non-native trout species, that have yielded unintended results.

Since the early 1900s, approximately 200 million rainbow trout have been introduced into Montana's rivers and lakes, with a notable allocation of 20 million within the Flathead Watershed. Rainbow trout boast immense popularity as they are recognized for their multi-hued coloration and the distinctive horizontal pink-red stripe gracing their tail section. Known as a hard-fighting game fish with tasty meat, these readily cultivated and strategically stocked fish quickly solidified their status as cherished targets for angling enthusiasts.

The introduction of rainbow trout and other non-native fishes in Montana was initially heralded as a way to bolster the fishing economy. But following the passing of the Endangered Species Act in the 1970s, which requires the protection of native species, new scientific research began to emerge showing detrimental ecological impacts of non-natives on native fish populations.

Non-native species not only displace native species from their habitat, but also has a profound impact on the entire ecosystem. The non-native species consume resources, disrupt habitats, displace natives, and negatively affect the balance of the aquatic environment.

One such native fish population is the westslope cutthroat trout, an indigenous species and Montana’s state fish, that resides within the Northern Rocky Mountains and all three forks of the Flathead River. In the Flathead, genetic assessments have unveiled a disconcerting reality: the proliferation of rainbow trout was contributing to the erosion of Montana’s westslope cutthroat trout populations.

Today, fish such as rainbow trout are designated as "invasive species" due to their disruption of native communities and competition with or predation on native species.

But there is also a more insidious consequence of non-native species called hybridization. In this context, hybridization entails the offspring from interbreeding parents of two different species. As an example, the mating of the invasive rainbow trout and the native westslope cutthroat trout, cousins within the Oncorhynchus genus, give rise to fertile hybrid offspring.

Though this phenomenon might not immediately raise concerns to the untrained eye—given the continued availability of fish to catch—it conceals a spectrum of far-reaching repercussions. Hybridization can take place swiftly within watersheds where invasive species coexist in great numbers with the native ones, and the loss of native fish offspring can carry profound consequences.

In an effort to better understand the consequences rainbow-cutthroat hybridization on the Flathead Watershed, Jared Grummer, a postdoctoral research associate at the University of Montana’s Flathead Lake Biological Station (FLBS), is turning to long-term data for answers.

Growing up amidst the woods of a state park in Napa Valley, California, Grummer’s deep connection with nature and early experiences with fishing under his mother's influence shaped his passion for the natural world. This bond goes back as long as he can remember when he used to have an orange-colored Snoopy fishing rod. This preliminary bond with nature influenced him to pursue different aspects of evolutionary genetics, and the previous one was studying rainbow trout in British Columbia, Canada, where they are native.

“It is a privilege to combine my background of being in nature with my skill sets in genetics to help natural populations,” Grummer said. “It all just kind of makes sense that I could be here, in Montana, doing work on the conservation genetics of native fish to help their populations.”

Utilizing a comprehensive time-series genetic dataset collected by Montana Fish, Wildlife & Parks (FWP) from the early 1980s to the present day, Grummer’s research into hybridization delves into the intricate interplay of ecological and evolutionary factors that underlie the swift proliferation of rainbow trout within the confines of five creeks in the Flathead Watershed. Through his analysis, he is examining hybrid trout mating preferences, offspring reproduction, and evaluation of the impacts of climate change on the survival prospects of hybrid trout.

Grummer's investigations rely on the combination of a vast data bank and advanced genetic tools. His analysis employs landscape genetics, where Grummer measures the effects of topographical and environmental attributes in each stream, and assesses each population’s genetic diversity as influenced by mobility challenges encountered in distinct locations.

“We’re using a [computer] program to measure how hybrid individuals move throughout the North Fork of the Flathead River,” said Grummer.

Grummer is also conducting an in-depth analysis of mating behaviors exhibited by pure hybrid trout, a blend of 50% rainbow trout and 50% westslope cutthroat trout. His primary curiosity lies in discerning whether these pure hybrids display a preference for mating with each other, opt for one of their parental species, or engage in random mating.

Mating among different trout species is far from uniform. Successful interbreeding resulting in reproductively viable offspring requires a remarkably close genetic relationships within the trout lineage. Certain hybrids may not possess this capability.

While rainbow and cutthroat trout interbreed, another invasive fish in the Flathead, the lake trout, exclusively mate with their own species. Native bull trout and invasive brook trout, on the other hand, both stem from the Salvelinus genus and can mate and hybridize. But unlike the westlope and rainbow trout, the resultant offspring bull and brook trout are incapable of reproduction.

But the definitive impact of hybridization between rainbow and cutthroat remains elusive, and Grummer is examining potential implications of the mating pattern between these two species in different tributaries of the North Fork Flathead River.

One critical area for Grummer’s research was finding out whether hybridization affects the reproductive output of hybrid offspring. To address this issue, Grummer looked at previous research from another FLBS scientist, Clint Muhlfeld who works as an Aquatic Ecologist for the US Geological Survey. In 2009, Muhlfeld demonstrated that a hybrid individual with as little as 20% rainbow trout genes experienced a 50% reduction in reproductive output compared to a pure westslope cutthroat.

“When you are a westslope cutthroat that is 20% hybrid, you now produce half as many eggs,” Grummer said.

Ultimately, Grummer hopes to better understand how hybridization influences the number of offspring each individual can produce and the ultimate consequences of reduced reproductive success.

“From a population perspective,” Grummer said, “this declining reproductive output could lead to smaller populations and maybe finally either go to extinction or just to pure rainbow trout.”

Hence, the prevention of hybridization between rainbow and cutthroat trout has emerged as a critical concern. To address this, FWP strategically erected five dams in the Abbott, Cyclone, Langford, and McGee Creeks, which are tributaries of the North Fork Flathead River. With these dams, FWP is aiming to maintain strict segregation of these two species in those creeks.

It is worth noting that dams usually impact fish movement negatively. Dams impede natural migration between vital feeding and spawning habitats. Such obstructions disrupt their life cycles, thereby limiting their reproductive capacities.

However, in this scenario dams serve a beneficial purpose—functioning as tools for "genetic rescue”. Genetic rescue entails deliberate human intervention to enhance favorable genetic traits or curtail undesirable ones within a species' population in a given area. FWP has created a barrier between the two fish species, strategically aimed at curbing further proliferation of interbreeding. According to Grummer, these structures have proven quite effective in preventing the two species from interacting.

Apart from the construction of dams, FWP employs an additional genetic conservation method known as "direct stream intervention." This strategy entails the selective removal of hybrid trout that possess 30% or more of their genetic makeup derived from rainbow trout.

In Abbot Creek, nearly 99% of the fish population is composed of rainbow trout and hybrids, despite this creek once serving as habitat for native westslope cutthroat trout. FWP is actively extracting rainbow trout from Abbot Creek to prevent further breeding that would result in an even higher proportion of rainbow trout hybrids in the population.

In three other creeks included in the study, the level of hybridization varies from 1% to 90%. This range in hybrid percentages has led to some creeks being predominantly populated by rainbow trout hybrids.

For Grummer, the importance of this intervention is significant, but it isn’t without its own set of consequences.

"They're trying to remove as much of the rainbow trout genetics as they can from the system,” said Grummer. “But the barriers that work for preventing the further spread of hybridization can bring up other problems.”

One such problem resulting from intervention could be a lack of genetic diversity for the native species. When small, isolated populations are full of closely related individuals, inbreeding becomes common. This can lead to a decline in genetic diversity and reproductive success. This genetic uniformity can ultimately threaten the survival of whole populations or an entire species through a phenomenon known as the extinction vortex.

Hybridization isn’t the only threat to native fish populations in Montana, either. Climate change has also recently emerged as another significant contributing factor impacting Montana’s native species.

Water temperatures and stream elevations both play pivotal roles in shaping the location and habitat preferences of different species. Indigenous westslope cutthroat prefer higher, cooler waters, while rainbow trout tend to inhabit the lower regions of the watershed, where temperatures are comparatively warmer.

A study published by Muhlfeld in 2014 in Nature Climate Change titled "Invasive hybridization in a threatened species is accelerated by climate change," underscores that rapid climate warming can amplify interactions between native and non-native species, thereby accelerating the occurrence of hybridization. This process holds the potential to lead to the extinction of native genes.

Due to climate change, rivers are warming, potentially creating comfortable zones for rainbow trout to move higher up into stream headwaters, where they interact with native cutthroat. Grummer's research raises concerns about the potential consequences of increased competition between the two species, as rainbow trout expand to higher elevations.

Eventually, Grummer believes a decline in native fish diversity could potentially diminish Montana's allure as a globally acclaimed fishing destination. His prediction for the future of fishing in Montana is murky at best for any hopeful young angler with an orange-colored Snoopy fishing rod in tow.

“There's a chance that the rainbow trout are going to push the native species towards extinction because westslope cutthroat trout move any higher in the watershed than they already are,” said Grummer. “There just isn’t anywhere else Montana’s westslope cutthroat trout populations can go.”