Daily fluctuations in flow at Ashton, trout population dynamics and more

Before I leave to speak at the national American Fisheries Society annual meeting in Portland next week, I thought I would provide links to several technical documents to keep you informed entertained for a while.

Daily Flow Fluctuations at Ashton Dam

As detailed in a blog post from last winter, daily cycles of macrophyte (rooted aquatic plants) photsynthesis and respiration cause daily cycles in flow in the river. The plants literally cause temporary damming and release of water on a daily basis. These cycles originate in the Harriman State Park reach of the river and persist all the way to Rexburg. They are not caused by operations at Ashton Dam.

The cycles occur during periods of clear weather during late summer and early fall, when days are hot and bright and nights are cool. This weather pattern maximizes the difference in plant physiology between day and night. During periods of cloudy weather, there is less difference in plant physiology between night and day. Although the cycles are present in the river everywhere downstream of Harriman State Park, they are most noticeable between Ashton Dam and the railroad trestle downstream of St. Anthony for two reasons. First, there are no real-time stream gages between Harriman State Park and the USGS gage at Ashton, so there is no publicly available record of these fluctuations upstream of the Ashton gage. Second, the portion of the cycle that results in the greatest change in river flow per unit time occurs during mid-morning between Ashton and St. Anthony, when people are beginning their fishing trips for the day. The greatest changes in river flow occur during the late evening and through the night in the river reach between Harriman State Park and Ashton.

Since HFF installed water-quality sondes at Pinehaven and the old Marsyville Bridge last summer, we have been able to observe these cycles in the river reach between Harriman State Park and Ashton and describe their properties. The full details are available in this pdf document.

Although we have not seen these cycles begin yet this summer, we will begin to see them if and when we enter an extended period of clear weather.  As of August 14, 2015, the two-week outlook calls for warm and dry weather. However, short-term forecasts keep showing cloudy days and showers at least a few days each week, which has been the general pattern since July 5 and the reason why we have not seen these cycles yet this summer. Just be prepared to see the cycles begin any time over the next few weeks if the weather turns consistently clear. In the river reach between Ashton and St. Anthony, the minimum flow occurs between 5:00 a.m. and 8:00 a.m. and then increases rapidly until late morning. After that, flow decreases gradually for the remainder of the day.

Effects of Angling Mortality on Trout Populations

This spring and summer, I worked with Emily Giles, a mathematics student at Brigham Young University-Idaho to create a mathematical model of rainbow trout populations in the Henry's Fork.  The purpose of the model was to quantify the population-level effects of realistic amounts of mortality from both harvest and catch-and-release angling.

As part of this research effort, Emily reviewed peer-reviewed literature on catch-and-release angling mortality. Her review is available here. The literature shows that mortality from catch-and-release angling is about 4% per fish caught when using artificial flies, increases to about 8% when water temperature is above 68 degrees F, and can exceed 25% when bait is used. However, these "nominal" mortality rates are not the same as the actual annual mortality rate in the population for several reasons. First, not all of the fish in the population are caught. In the Henry's Fork, only about two-thirds of catchable-sized fish (fish of ages two and older) in the population are caught and released each season. These fish comprise only about 61% of the total population (the remainder are age-1 fish, which are not caught frequently), so in reality, only about 40% of the total number of fish in the population are caught and released each year. At a 4%-per fish mortality rate, about 2.44% of the total number of fish in the population would die from catch-and-release mortality. Second, natural mortality rates are high in the Henry's Fork, ranging from 35% to 80%. Even at naturality mortality rates at the lower end of this range, many of the fish that die from catch-and-release angling mortality would have died due to natural causes sometime during the year anyway. Our models show that in the Box Canyon population, for example, a nominal 4% catch-and-release mortality rate results in additional mortality of only 1.6% above and beyond natural mortality. When compared to high natural variability in trout population sizes due to factors such as winter flow, the population-level effects of increasing catch-and-release mortality from 4% to 8% is very unlikely to be statistically detectable.

As for harvest, in the river reaches of the Henry's Fork where harvest is allowed, anglers harvest around 3% of the catchable-sized fish, or less than 2% of the total population. When accounting for fish that would have died due to natural causes or catch-and-release mortality anyway, the 3% harvest rate adds only an additional 1.2% mortality over and above these other sources. If harvest was increased from 3% to 9%, there is about a 50% chance that the effect on the population would be statistically dectable.

For details, see Emily's research poster or my presentation to the HFF board of directors.

2014-2015 HFF Research Summary

If the information above is not enough to whet your appetite, here is a summary of the major HFF research projects we have been doing over the past year. I presented this summary at HFF's annual meeting back in June.