Letters from the West

Climate change helps rainbow cross-breed with cutthroats

Scientists have discovered warming streams and a drop in spring flows has increased cross breeding between native westslope cutthroat trout and introduced rainbow trout.

This latest evidence of the effects of rapid climate change on the Rocky Mountain’s ecosystem shows how much already has changed. The study, published today in Nature Climate Change, was based on 30 years of research by scientists with the U.S. Geological Survey, University of Montana, and Montana Fish, Wildlife & Parks.

Hybridization has contributed to the decline and extinction of many native fishes worldwide, including cutthroat trout. The researchers used long-term genetic monitoring data coupled with high-resolution climate and stream temperature data to make their case.

“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.”

Idaho’s westslope cutthroat trout and rainbow trout both spawn in the spring and can produce fertile offspring when they interbreed. Over time, a mating population of native and non-native fish will result in only hybrids and nearly as important with substantially reduced fitness.

That’s because the hybrids are poorly adapted to the local environment, at least in the early generations. Since managing invasives like rainbow that have been encouraged for so many generations is hard, this will lead to ecosystems that are less resilient and with less ability to adapt to a rapidly changing climate.

Historical genetic samples revealed that hybridization between the two fish species was largely confined to one downstream Flathead River population.

Here’s what the scientists saw: 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. Rainbow trout prefer these changes, and tolerate greater environmental disturbance.

Genetically pure populations of westslope cutthroat trout are known to occupy less than 10 percent of their historical range including places like the Middle Fork of the Salmon River, the Selway, the St. Joe, the North Fork of the Coeur d’Alene and the North Fork of the Clearwater rivers in Idaho.