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HFF Monitors New Winter Fishing Seasons

Written by James Chandler, HFF intern winter-spring 2016

Survey Objectives

As of January 1, 2016, Idaho Fish and Game has opened up two new sections of the Henry’s Fork to winter fishing: the section from Vernon Bridge upstream to Ashton Dam and the section from the southern boundary of Harriman State Park downstream to Riverside Campground. The winter seasons in both sections of river allow catch-and-release fishing only. Of course, the Harriman-to-Riverside section is catch-and-release all year. In the Ashton-to-Vernon reach, once the general season opens on May 28, regulations revert to the two-fish limit that now applies to the entire river from Riverside Campground to the South Fork Snake River confluence. For complete 2016 fishing regulations go to:

Because some anglers expressed concern over the potential effect that additional winter angling might have on fish populations, HFF jumped at the chance to monitor these new winter seasons. In cooperation with Idaho Fish and Game, we are conducting an angler use survey to gain an idea of how these portions of river are being used now that they are accessible. The objectives of the survey are twofold. First, we are working to estimate angler effort through angler counts. Angler effort is measured in angler hours. For example, if one fisherman spends one hour on the river then we record one angler hour. If two fishermen spend 30 minutes on the river then this would also count as one angler hour. Second, we are gathering information on catch-and-release rates based on angler observations and interviews. We measure this in fish caught per hour fished. For example, if I interview a fisherman at the end of his or her trip and they tell me that they fished for two hours and caught four fish in that time we would be able to say that they caught fish at a rate of two fish per hour.


To accurately reflect the efforts of fishermen without introducing bias to the study we implemented a stratified random sampling design for the study. This means that the study time, January through May, was broken into four strata. The first is January 16 to February 12 and represents the mid-winter portion of the season. The second is from February 13 to April 1, which represents the late winter and early spring portion of the season. The third is from April 2 to April 29, accounting for the spring season. The fourth is from April 30 to May 27, which will describe the late spring season. Within each stratum we divided days into two subgroups: weekdays and weekends/holidays. For example, we would expect that more people would be fishing on the weekends than during the week; therefore we group these times separately. Also, we expect more people to fish during the month of May than during January. Accordingly, we allocated more sampling effort on weekends than weekdays and more effort later in the spring that during the winter. This allows us to concentrate survey time in proportion to expected angler effort.

The final step in setting up the study is to randomly draw sampling days and time blocks within each stratum and subgroup. By doing this, there is an equal chance that any given sampling date falls on a snowy, windy day as it would on a clear, sunny day. Within each randomly selected sample day, we randomly select a three-hour time block in which to count and observed anglers. Again, the random selection provides equal probability of selecting any given time during the day, avoiding bias.

I begin each sampling block by first getting a simple count of the number of fishermen that are on the river. In the Ashton reach, I drive from Ashton Dam to Vernon Bridge, stopping to get out and observe the river at certain points so that I can have a visual of each section of the river on this stretch. After completing the count, the next thing I do is observe anglers that are there to see if they catch anything. Finally, if possible, I will interview anglers to see how long they fished for, what species of fish they caught and the number of fish caught. I also record access method (wade vs. float) and terminal gear type (e.g., fly vs. lure). Similar procedures are applied in the Pinehaven reach. At the end of the survey time I complete a second count along the full stretch of the sample area. The data collected is then extrapolated to describe angler activity in a time period over the subgroup in each stratum.


To date, I have conducted formal surveys on three different days at Pinehaven and six days in the Ashton-Vernon reach. Additionally, I have spent unofficial time on the river. I have logged several hours fishing below Ashton Dam and spent one day determining how accessible the Pinehaven section is by taking a snowmobile out onto Wood Rd 16. It is not easy to access the river at this point in time up there, which why, so far, I have observed no anglers at Pinehaven and no evidence (e.g., tracks in the snow) that anyone has fished there so far this winter. In fact, during this first portion of the survey the river here has been either covered with ice or full of large ice blocks, further inhibiting any possibility of fishing.

In the Ashton-Vernon section of the river I have observed anglers and spoken with them, confirming that they have caught fish. For the first stratum of the survey we have recorded angler-hours and catch rates for both weekday and weekend/holiday sub groups.

To calculate the angler-hours we averaged the number of angler hours per hour fished and multiplied that number by the total number of possible fishing hours during the entirety of the sampling period, in this case from January 16 to February 12. For example, for the two weekdays that I surveyed the Ashton-Vernon site January 18 and February 9, I observed two anglers over four counts, which equates to 0.5 angler-hours per hour fished. This number is multiplied by the total number of possible fishing hours for weekdays, 163, to get a total of 81.5 anger-hours for the first sample stratum. When we do the math for the weekend, we get 1.25 angler-hours per hour. Multiplied by the total number of possible fishable hours for the stratum, 65 hours, we get a total of 81.25 angler-hours. We can calculate total effort by adding the angler-hours from weekdays and the weekend/holiday hours. Total effort equals 162.75 angler-hours in a total of 228 fishable hours.

To calculate catch rates we utilize information obtained from interviews and divide the number of fish caught by the total hours fished. For Ashton-Vernon I conducted five interviews totaling 8 fish caught in 9.5 hours for an overall catch rate of 0.84 fish per hour. Specific catch rates were 0.53 Rainbow Trout per hour, 0.11 Brown Trout per hour and 0.21 Mountain Whitefish per hour.

By combining our angler-hour data and catch-rate information we can estimate total catch. Overall, 137 fish were caught and released from January 16 to February 12. We calculated this by multiplying total effort by catch rate. 162.75 x 0.84 = 137. Of these 137 fish, 86 were Rainbow Trout, 17 were Brown Trout, and 34 were Mountain Whitefish.

Update as of May 2016

Eighteen weeks ago I began monitoring the two new open sections of the Henry’s Fork, from Vernon Bridge to Ashton Dam and from the northern side of Riverside Campground to the southern border of Harriman Sate Park. Since then I have counted, observed, and talked to many anglers who braved harsh weather and enjoyed some of the first warm, sunny days on the river in Ashton. Now that it is May, three of the four monitoring periods within my angler use survey are complete. We have seen a dramatic increase in the number of fishermen out on the river since those cold, short days in January. It was once unusual to see more than four or five people in a day; now it is odd to see less than ten. To better illustrate this progression I will review the numbers from the first periods of the study and present our most recent findings. It is important to note that the following data reflects only the fishing at the section in Ashton and not at Pinehaven.

During the first period, mid-winter, which lasted from January 16th to February 12th, there were twenty possible weekdays and eight possible weekend days that people could have fished. During those days there were 162 fishable hours during the week and 64 on the weekends. Based on our observations, we estimated anglers invested 51 angler-hours during weekdays over the duration of the period. The weekend effort for the period was 40 angler-hours. If we break that down further it equates to 2.5 angler-hours per every week day and 5 angler-hours per each weekend day. For those people who braved the elements and battled frozen line guides to fish in January, they had the luxury of having the river to themselves.

The second period, late winter, began on February 13th and finished on April 1st. This period was not only longer but also saw warmer days, and more fishermen. During this period there were 34 weekdays and 15 weekend days. There were 320 fishable hours during the week and 138 on the weekends. Weekday effort more than tripled during this period, with 175 angler-hours estimated, and quadrupled on the weekends with 162 angler-hours. With the increased effort we saw a large rise in the estimated number of fish caught between the mid- and late-winter periods. During the first period we estimated that 44 Rainbow Trout were caught. In the second period that number jumped to 164. Similar trends were seen with Mountain Whitefish and Brown Trout. At the end of the period we observed a catch rate of 0.8 fish per hour, a rate that includes all species.

The third period, early spring, began on April 2nd and concluded on April 29th. The number of fishable weekdays in this period was 20 and the number of weekend days was 8, just as in the first period. Again, we saw an increase in the angler effort on both the weekdays and the weekend, but most notably during the week. During this period we estimated 427 angler-hours for the weekdays and 261 on the weekends. On a daily basis this equates to 21.4 angler-hours per day on weekdays and 32.6 angler-hours on weekend days. In the third period, fishing pressure was 8.5 times greater on weekdays and 6.5 times greater on weekends when compared to the first period. The overall catch rate for all three periods was 0.7 fish per hour. This is slightly lower than the 0.8 fish per hour at the end of the second period, a result that is likely due to the overall increase of fishing pressure. One reason that increased pressure could decrease the hourly catch rate is because there is more competition for the finite number of fish in the river. With more anglers fishing to the same fish, each angler’s hourly success is bound to decrease. However, the increased effort more than made up for the slight decrease in catch rate. The estimated number of fish caught during the early spring period was substantially greater than in the previous periods, simply because of the increase in fishing pressure. We estimate that 334 Rainbows, 86 Mountain Whitefish, and 50 Brown Trout were caught during the third period.

With two weeks left in the new season we are continuing to see an increase in the number of people fishing on the Vernon Bridge to Ashton Dam section.

In the Pinehaven area, after four and-a-half months, I finally observed and interviewed one angler who was fishing from Wood Road 16. With summer weather and hatches fast approaching, I fully expect to see more people fishing in the Pinehaven section over the next 10 days, but that stretch did not see any fishing pressure for the vast majority of the newly opened season.

The Ashton section, as clearly demonstrated from our findings, received consistent fishing pressure from the beginning of the season, which will likely help to reduce the opening day pressure observed there in previous seasons. Furthermore, the opportunity for winter fishing between Vernon Bridge and Ashton Dam most likely removed some fishing winter fishing pressure that would have otherwise occurred on other reaches of the lower Henry’s Fork.

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