CSU Study: How Narrow Tributaries Can Rescue Threatened Fish Populations

Habitat fragmentation is a pervasive threat to biodiversity in aquatic habitats where fragmentation creates barriers for species like trout to move easily through a stream network.

A recent study out of Colorado State University examined the dynamics of two Japanese land-locked salmonid populations in the highly fragmented Sabusawa Stream, a mountainous headwater stream in the Fuji River system in central Japan using a 9-year mark-recapture dataset.

Stream networks are particularly vulnerable to the negative effects of human-dominated landscapes, said study co-author Yoichiro Kanno, a fish ecologist and associate professor in the Fish, Wildlife, and Conservation Biology department at the CSU Warner College of Natural Resources.

“The difficulty with habitat fragmentation on fish population is that fish movement is restricted to waterways. With human-dominated landscapes, birds can fly over obstacles with no issue; deer can go around obstacles, but fish simply can’t,” Kanno said.

“Dams have imperiled many native species globally. Our study sought to find out why fish populations persist in a fragmented stream in some cases.”

The study, called “Small giants: Tributaries rescue spatially structured populations from extirpation in a highly fragmented stream” and published in the Journal of Applied Ecology examined the critical role two tributaries played on the survival of two salmonid populations, white-spotted charr and red-spotted masu salmon.

“Small giants” refers to the studied tributaries’ small size and big impact. “The size of these tributaries was deceiving to us; they were physically small, yet they contributed disproportionately to the salmonid population persistence,” Kanno said.

Researchers found that the two tributaries, which were each less than 2 meters wide, were critically important in sustaining the populations that occupy the mainstream, which was fragmented by a series of dams. The study found that the tributaries supported higher survival rates in the species.

“It’s something many of the biologists had assumed might be the case, but nobody had really quantified it,” Kanno said. “Conducting an intensive survey in the third weekend of October annually for 9 years helped us to understand what a big impact these narrow tributaries have.”

Implications for Colorado’s cutthroat trout
The findings have implications for habitat fragmentation and trout populations in Colorado, said Kanno who leads the Stream Fish Ecology and Conservation Lab, a lab that studies study stream fish populations in North America and Japan.

“Trout worldwide suffer from habitat loss fragmentation, invasive species, degraded water quality, and climate change,” Kanno said. “It’s a global issue, but it’s especially applicable to Colorado where small headwaters are the stronghold of native trout populations.”

In the South Platte Basin, in particular, the native greenback cutthroat trout is listed as a “threatened” species under the Endangered Species Act with many efforts to restore their populations in streams, including those affected by the Cameron Peak Fire.

Kanno said wildlife and conservation agencies should not overlook the value of tributaries in growing and maintaining the population of Colorado’s native trout.

“You don’t expect to see big trout in small tributaries; you expect to see them in the bigger river,” Kanno said. “However, a lot of critical stages of spawning and rearing happen in the tributaries, so unless we protect tributaries, we won’t have sustainable trout populations in the mainstream.”