Every year the Idaho Department of Fish and Game (IDFG) conducts fish population surveys on various river reaches in the Upper Snake River region. In the spring of 2018, surveys were conducted on the Chester to Fun Farm, Vernon to Chester, and Box Canyon, reaches of the Henry’s Fork River. These surveys provide valuable information on abundance, age-class structure, fish size, and species composition within the fishery. IDFG recently published the results in their annual brochure and this blog will highlight and explain some of those results.

Largest Rainbow Trout sampled during the 2018 population survey in th Box Canyon reach.


Box Canyon

  • Abundance estimate for fish greater than 6 inches in length: 2,796 Rainbow Trout per mile
    • 2017 abundance estimate: 2,913 Rainbow Trout per mile
  • Average length 10.1 inches
    • 2017 average length: 11.5 inches
  • Max length: 22 inches
  • Abundance estimate of age-2 class fish was higher than predicted for a second year in a row (see below for more details)​

Chester to Fun Farm

  • 1,225 trout per mile (727 Rainbow Trout and 498 Brown Trout)
  • Average length
    • Brown Trout: 15 inches
    • Rainbow Trout: 14 inches
  • Max length
    • Brown Trout: 23 inches
    • Rainbow Trout: 20.5 inches

Vernon to Chester

  • 1,612 trout per mile (1,071 Rainbow Trout and 551 Brown Trout)
  • Average length
    • Brown Trout: 11.5 inches
    • Rainbow Trout: 13 inches
  • Max length
    • Brown Trout: 24.5 inches
    • Rainbow Trout: 22 inches

Click on this link for a quick explanation of how these population surveys were conducted.

24.5 inch Brown Trout sampled in the Vernon to Chester reach in 2018.

Box Canyon Pop estimate

The total number of Rainbow Trout >6 inches was 2,796 fish per mile, which is slightly lower than last year (2,913 fish per mile) and the long term average from 1994-2017 (3,034 fish per mile). Considering the majority of Rainbow Trout in the 2018 population are from cohorts who endured less than ideal winter flows during critical juvenile life-stages (cohorts 2014, 2015, and 2016), this population estimate was better than we anticipated.

Figure 1. Rainbow Trout per square mile in the Box Canyon reach of the Henry’s Fork River, 1994 through 2018 (figure credit, IDFG).


Even more encouraging was the estimated number of Age-2 Rainbow Trout. There was an estimated 2,865 Age-2 Rainbow Trout per mile in 2018, which was roughly 700 more Age-2 Rainbow Trout than 2017 and 1,000 more Age-2 Rainbow Trout than what our winter flow model predicted (click on a this link to learn more about our winter flow model). Couple that with great winter flows in 2018 (mean flow in Box Canyon from December 1 through February 28 was 705 cfs) and we anticipate good recruitment, increased abundances, and more large fish over the next 2-3 years.

Figure 2. Mean flow from Island Park and Buffalo River during the coldest 90 day period during a cohorts first winter verse the number of age-2 Rainbow Trout in the cohort.


The size structure for Rainbow Trout in Box Canyon looks similar to last year with multiple peaks around 6 inches, 12 inches, and 16 inches. These peaks correspond roughly to cohorts of fish and having multiple cohorts within a population shows good annual recruitment and increased population stability for the next few years (see example of why in Figure below). The average size Rainbow Trout for 2018 was smaller than last year, primarily because there were higher abundances of young fish entering the population. Looking into the future, the increased abundances of young fish bodes well for fishing over the next several years as these fish continue to grow into the 16+ inch size range that anglers are looking to catch.



Chester to Fun Farm & Vernon to Chester

There are two predominant population trends for the Chester to Fun Farm and Vernon to Chester reaches. First, total trout abundances have increased for both reaches over the last 15 years despite concerns that opening these two reaches to year-round fishing in XXXX may negatively affect spawning and recruitment. Second, Rainbow Trout populations have remained relatively constant while Brown Trout abundances have steadily increased since the early 2000’s. In 2003, Brown Trout made up <15% of the population in both reaches but now compose 41% (Chester to Fun Farm) and 34% (Vernon to Chester) of the total trout populations. The increase in Brown Trout comes as no surprise. Large woody debris, braided channels, and ample amounts of invertebrates make for fantastic Brown Trout habitat. In addition, trends in warming water temperatures make habitat in the lower Henry’s Fork even more suitable for Brown Trout, relative to their Rainbow Trout counterpart that prefer colder water. It is likely that as regional temperatures continue to increase, Brown Trout will become even more prevalent in the lower Henry’s Fork River.