Global warming speeds diversity threats to native fish

Study looks at hybridization of trout in Northern Rocky Mountains

Biologists study trout in the Blue River in Silverthorne, Colorado. @bberwyn photo.

Staff Report

Global warming is intensifying the hybridization of native and non-native trout in the northern Rocky Mountains, according to a new study from the U.S. Geological Survey and other scientists. The trend is a serious threat to the biodiversity of Rocky Mountain aquatic ecosystems, says the study published in the journal Global Change Biology.

As non-native rainbow trout, introduced by early settlers, interbreed with cutthroat trout it leads to a decline in local adaptations that can threaten the long-term survival of species. Preserving the  genetic integrity of native species is important for  resiliency, the scientists said.

“Our results show that climatic changes are interacting with legacies of rainbow trout introduction and habitat degradation to threaten native cutthroat trout, as invasive rainbow trout continue to expand their range across the northern Rockies,” said lead author and USGS scientist Clint Muhlfeld. “The loss of native genomes from hybridization is a scenario being realized currently and one that will only proceed into the future, representing a critical threat to native trout persistence.

The study used data from a  comprehensive global long-term monitoring program with genetic data from 12,878 individual fish from 582 locations in 17 major river basins, climate predictions, and detailed historical stocking records from 1924-1980 for approximately 200 million introduced rainbow trout. Data going back to the 1980s show that 50 percent of sites with long-term data show increases in hybridization, the majority of which were initially genetically pure.

The spread of hybridization across space and time was primarily driven by historical stocking practices and exacerbated by warmer water temperatures, higher road densities and lower spring precipitation.

Although streams with warmer water temperatures were more prone to hybridization, most hybridized sites (approximately 60 percent) had cold mean summer water temperatures. These findings are counter to the notion that cold headwater streams will serve as a refuge that prevent hybridization.

“Our data clearly show that cold headwater streams are not immune to rainbow trout invasion and hybridization with native cutthroat trout, especially those in close proximity to historical stocking sites,” said Muhlfeld. “Conservation strategies based solely on temperature criteria may seriously underestimate the threat of rainbow trout invasion and hybridization with native trout populations in headwater streams.”

In the western U.S., genetically pure cutthroat trout populations represent only a fraction of their historical range and genetic diversity of cutthroat trout, and the spread of hybridization with introduced rainbow trout represents one of the leading threats to remaining populations. The study underscores the value of long-term monitoring data for detecting how historical invasive species introductions interact with contemporary climate warming to promote irreversible evolutionary changes in native species.

“This research shows that human activities can have long-lasting and increasingly synergistic impacts on biodiversity, including species of recreational value like native trout,” said co-author and USGS scientist Ryan Kovach.

Hybridization between native and non-native species is likely to increase in the coming years due to climate-induced expansions of invasive species. Conservation strategies that mitigate existing human stressors, such as translocation of species and habitat modification that interact with climate to promote the expansion of invasive species, will be crucial for conserving biodiversity and preventing genomic extinction of additional population and eventually entire lineages of native cutthroat trout.

This study was supported by the USGS National Climate Change and Wildlife Science Center, Montana Fish, Wildlife & Parks, University of Montana Flathead Lake Biological Station, University of Montana Conservation Genetics Laboratory, and the National Science Foundation. More information about impacts and prevention of invasive species and hybridization can be found on the USGS Northern Rocky Mountain Science Center website.

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