Trout numbers highest in decades

Trout numbers highest in decades

Rainbow trout in the Box Canyon reach of the Henry’s Fork are more abundant this spring than in any year since the 1970s. Based on sampling conducted in mid-May, Idaho Department of Fish and Game (IDFG) biologists estimate the population at just under 6,200 fish per mile, plus or minus 500. That is over 17,000 fish in the 2.8-mile Box Canyon reach, about twice the number present in May of 2012 and more than twice the average population size over the last 20 years. 

The last time trout were this abundant was in 1978, when the population size was just over 18,000 fish. The number of rainbow trout in Box Canyon helps IDFG estimate the number of fish available to anglers throughout the reach from Island Park Dam to Riverside campground. The fish tend to congregate in Box Canyon and in the canyon downstream of Riverside during the winter and then migrate into the flat water between Last Chance and Pinehaven during the summer, as cover and food become available there.

HFF is working with Grand Valley State University and IDFG to study the details of this migration and use of habitat by adult rainbow trout in the Harriman State Park reach. Research began this spring and will run through the next few years. Learn more about this ground-breaking habitat-use study.

Why the larger numbers?

The increase in trout abundance this year was driven by a very large number of two-year old fish—those spawned in the spring of 2011. These fish were between 9 and 12 inches when collected in May and will grow about another 3-4 inches during the upcoming summer. The average size of fish collected this spring was 11.6 inches, reflecting the abundance of two-year old fish in the population.

Research by HFF, IDFG, and others over the years has identified winter flow below Island Park Dam as the primary factor determining the size of the Box Canyon trout population. The number of two-year old fish in the population this spring was influenced by flow during the winter of 2011-2012, which was the highest since the late 1990s. HFF advocates for higher river flows during critical winter months to improve fish survival.

Is the Buffalo River contributing?

Yet the number of two-year old fish in the population this spring is even higher than would be predicted from winter flow alone. Although there are many other factors that could also contribute to this year’s very large group of two-year old fish, increased fish production in the Buffalo River is one possible contributor. HFF and partners have worked diligently to improve fish passage to and from the Buffalo River, and this year’s numbers may be evidence that the Buffalo River is contributing additional fish to the Henry’s Fork population.

The size of the fish population is estimated with a statistical method called mark-recapture. Learn more about how biologists estimate fish populations using this method.

Contributed by Rob Van Kirk, HFF's Senior Scientist. To learn more about our research, email Rob.