30-year study shows how warmer stream temperatures are linked with hybridization
FRISCO — Scientists studying trout in the northern Rockies say warmer stream temperatures and lower spring flows in mountain streams is increasing the pace at which introduced rainbow trout and native cutthroats interbreed. Hybridization has already contributed to the decline and extinction of many native fishes worldwide, including all subspecies of cutthroat trout in western North America.
The long-term research shows that, while hybridization used to be limited to lower stream reaches, during the past 30 years, it has quickly spread upstream as temperatures warm. According to the scientists, the process is irreversibly reducing the genetic integrity of native westslope cutthroat trout populations. Genetically pure populations of westslope cutthroat trout are known to occupy less than 10 percent of their historical range.
Compared to cutthroat trout, rainbow trout prefer conditions under climate-induced changes, and tolerate greater environmental disturbance. These conditions have likely enhanced rainbow trout spawning and population numbers, leading to massive expansion of hybridization with westslope cutthroat trout.
Historical genetic samples revealed that hybridization between the two fish species was largely confined to one downstream Flathead River population. However, the study noted, over the past 30 years, hybridization rapidly spread upstream, irreversibly reducing the genetic integrity of native westslope cutthroat trout populations. Genetically pure populations of westslope cutthroat trout are known to occupy less than 10 percent of their historical range.
The new study, published in the journal Nature Climate Change reinforce suggestions that global warming will decrease biodiversity by enabling more cross-breeding between invasive and native species. The findings are based on 30 years of research by scientists with the U.S. Geological Survey, the University of Montana and Montana Fish, Wildlife & Parks. The results are based on long-term genetic monitoring and climate and stream temperature models.
“Climatic changes are threatening highly prized native trout as introduced rainbow trout continue to expand their range and hybridize with native populations through climate-induced ‘windows of opportunity,’ putting many populations and species at greater risk than previously thought,” said project leader and USGS scientist Clint Muhlfeld. “The study illustrates that protecting genetic integrity and diversity of native species will be incredibly challenging when species are threatened with climate-induced invasive hybridization.”
Westslope cutthroat trout and rainbow trout both spawn in the spring and can produce fertile offspring when they interbreed. But that eventually results in a population of mainly hybrid fish that aren’t ideally adapted to the local environment. Protecting and maintaining the genetic integrity of native species is important for a species’ ability to be resilient and better adapt to a rapidly changing climate.
The rapid increase in hybridization was highly associated with climatic changes in the region. From 1978 to 2008 the rate of warming nearly tripled in the Flathead basin, resulting in earlier spring runoff, lower spring flooding and flows, and warming summer stream temperatures. Those locations with the greatest changes in stream flow and temperature experienced the greatest increases in hybridization
“The evolutionary consequences of climate change are one of our greatest areas of uncertainty because empirical data addressing this issue are extraordinarily rare; this study is a tremendous step forward in our understanding of how climate change can influence evolutionary process and ultimately species biodiversity,” said Ryan Kovach, a University of Montana study co-author.
Overall, aquatic ecosystems in western North America are predicted to experience increasingly earlier snowmelt in the spring, reduced late spring and summer flows, warmer and drier summers, and increased water temperatures – all of which spell increased hybridization between these species.
The study is online at this website.