2018 Henry's Fork Fish Survey Results

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 for each population. 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 the 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 

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.


Box Canyon Population

The total number of Rainbow Trout >6 inches in the Box Canyon reach 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) (Figure 1). 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 anticipated.

Figure 1. Rainbow Trout per 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 in the Box Canyon reach this year. 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 (Figure 2). Couple that with great winter flows in 2018 (mean flow in Box Canyon during coldest 90-day period was 705 cfs), 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 red square is the predicted abundance of Age-2 Rainbow Trout for 2019.


The size structure of Rainbow Trout in the Box Canyon reach looks similar to last year with multiple peaks around 6 inches, 12 inches, and 16 inches. These peaks correspond roughly to different cohorts within the population and the multiple peaks illustrate good annual recruitment and increased stability of fish size within the population for the next few years (Figure 3). The average size Rainbow Trout for 2018 was smaller than last year primarily because there were increased abundances of juvenile trout. Looking into the future, the increased abundances of young trout bodes well for the population, and for fishing, as these fish continue to grow into the 16+ inch size range that anglers are looking to catch.

Figure 3. Length-frequency for Rainbow Trout in the Box Canyon Reach from 2015-2018. Circles are representative of different Rainbow Trout cohorts moving through the population between years. (figure credit, IDFG)

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 Ashton Dam to Vernon Bridge for winter fishing in 2016 may negatively affect fish populations (Figure 4 & Figure 5). In fact, Vernon to Chester abundances (which are an index for fish populations from Ashton Dam to Chester Dam reach) have increased substantially since 2015.  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 trout habitat. In addition, trends in warming water temperatures make habitat in the lower Henry’s Fork 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.

Figure 4. Number of Rainbow Trout and Brown Trout per mile for the Vernon to Chester reach of the Henry's Fork River, 2005-2018. (figure credit, IDFG)

Figure 5. Number of Rainbow Trout and Brown Trout per mile for the Chester to Fun Farm reach of the Henry's Fork River, 2003-2018. (figure credit, IDFG)


24.5 inch Brown Trout caught during the Vernon to Chester fish population survey , 2018.


Mountain Whitefish Populations

Mountain Whitefish populations appear to have dropped preciptiously since the last time all three of these river reaches were sampled together in 2007. While Mountain Whitefish aren't typically a target species for anglers, they are admirable fighters and catching several of them in a day could save a slow day of trout fishing and turn it into a decent day of fishing overall. It is uncertian why there has been a decline in Mountain Whitefish populations but there are several hypotheses. Before we get to those hypotheses, it should be noted that the apparaent decline may not be as drastic as the data show.  Sampling Mountain Whitefish simultaneously while targeting trout species is difficult. Electrofishing equipment needs to be calibrated for the water's specific conductivity, as well as for the size and species of fish that is being targeted. IDFG calibrates their electrofishing equipment to target trout and these settings often don't provide enough electrical current to immobolize Mountain Whitefish at the bottom of deep pool and run habitats that Mountain Whitefish often prefer. This makes it difficult to sample enough Mountain Whitefish to produce accurate and precise estimates using mark-recapture models so the decline in Mountain Whitefish populations may not be as drastic as they appear (Click this link to see how estimates are calculated). 



Figure 6. Mountain Whitefish per mile for the Box Canyon, Chester to Fun Farm, and Vernon to Chester reaches of the Henry's Fork River. (figure credit, IDFG)

Regardless, it is likely that there has been a real decline in Mountain Whitefish (to what extent is still uncertain) and anecdotal evidence from anglers support this trend as well. The reality is, no one knows exactly what has caused the decline. It's possible that increased inter-species competition with Rainbow Trout and Brown Trout and/or increased predation by highly piscivorous Brown Trout in the lower Henry's Fork have factored into the decline in Chester to Fun Farm and Vernon to Chester reaches. In addition, the increasing water temperatures in the lower Henry's Fork make conditions less suitable for Mountain Whitefish. In Box Canyon, low winter flow conditions may be effecting juvenile survival similar to how winter flows effect juvenile Rainbow Trout. Physical in-stream spawning habitat hasn't changed in either stretch but the warmer late summer water temperatures may be impacting Mountain Whitefish spawning and egg development. The consistant decline over all reaches support the idea that there is a common factor at play and increased trends in water temperature in both the lower and upper Henry's Fork could be the common factor. It could be one, none, or a combination, of these factors that have caused the decline in Mountain Whitefish. We currenlty don't have enough data to answer many of these questions but IDFG intends to increase their annual monitoring efforts for Mountain Whitefish to try to find some answers.

Additional Information

If you have questions, comments, or concerns, feel free to contact me at Bryce@henrysfork.org or 208-652-3567.

NOTES: some of the numbers reported in this blog are different than those in the IDFG brochure. IDFG is correcting those discrepencies and will produce an updated brochure in the next few weeks. Once the updated brochure is available, the links above will be corrected.