Redband trout and nonnative trout in the Wood River Basin

Cutthroat trout may be the state fish, but the redband trout is itself a state gem. Redband trout are a subspecies of rainbow trout that are native to the interior portion of western North America. In the U.S., this includes Idaho, Oregon, Washington, California, Montana, and Nevada. 

Redband trout are similar in appearance to rainbow trout such as the familiar rosy pink stripe from their gills to their tail. But there are a few differences too, such as larger, more rounded spots, and parr marks which persist into adulthood. Check out the May 2020 edition of the Wildlife Express magazine to learn more about redband trout.

Wood River Basin 

The Wood River Basin in central Idaho lies on the very eastern edge of the native range of redband trout (Figure 1). Like many other river basins in southern Idaho, the Wood River Basin has been hydrologically isolated from the Snake River for quite a long time. Over 50,000 years ago, glaciation and volcanic activity formed the Malad Gorge Falls, an almost 60-foot-tall waterfall that prevents upstream movement of fish from the Snake River into the Wood River Basin. Due to this isolation, there are several fish species that are unique to this system. 

The Wood River sculpin is found nowhere else in the world. The basin is also home to populations of bridgelip suckers and mountain whitefish, which are genetically unique from all other populations in Idaho. It was also recently discovered that redband trout in this basin are also genetically distinct from other redband trout populations.

In 2003, Idaho Fish and Game staff set out to learn where redband trout were in the basin and found that redband trout occupied less than 20% of the more than 100 stream reaches that were surveyed. That’s not unusual in arid portions of Idaho. Many streams which appear on maps aren’t really streams at all – they are just dry channels which rarely have flowing waters let alone trout populations. Despite their somewhat limited distribution, redband trout were the most widely distributed and abundant trout species in the basin.

In 2021 and 2022, Idaho Fish and Game staff returned to 22 stream reaches where redband trout were found in 2003 (Figure 1). Staff found that redband trout were no longer present at five reaches which were previously occupied by both redband trout and brook trout in 2003 but were now entirely comprised of brook trout. This is concerning as nonnative brook trout in western North America tend to outcompete native trout, often leading to displacement of the native fish.

However, staff also found brook trout were no longer present at two reaches which are now completely occupied by redband trout. This was an unexpected finding and shows that redband trout might be able to withstand brook trout displacement in some instances. At the remaining 15 stream reaches surveyed, both species were observed together (Figure 1). The stream reaches where redband trout and brook trout were displaced were small streams less than 5 meters wide suggesting small streams might be the most vulnerable reaches for displacement of either species. Brown trout were also present in the basin but were only found in low elevation mainstem river sections of the Big Wood and Little Wood rivers. At this time, brown trout are not considered a concern for redband trout.

Trout populations are notorious for fluctuating widely from year to year and thus must be monitored through time. More surveys will be used to assess whether the changes we recently observed continue into the future. That is why you often see Fish and Game staff working in streams around the state, monitoring our fish populations. In locations like the Wood River Basin where genetically distinct fish lineages reside, these surveys help us understand and manage such unique fish populations.