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The Upper Henry’s Fork Project: bottlenecks limiting fish populations

Updated: Mar 7, 2022

This is the second blog in a series about research activity in the Upper Henry’s Fork. Click here for part 1 and stay tuned for the next part!

  • Go early or stay late: 14”+ fish in the Henry’s Fork upstream of Island Park Reservoir are usually spawning migrants in early spring or late fall.

  • Red fish, blue water: Keeping Island Park Reservoir as full as possible as long as possible in consecutive years increases the number of spawning salmon and trout in the Henrys Fork upstream of Island Park Reservoir.

  • Thanks for your help: HFF members helped fund Farms & Fish, Precision Management, and Science and Technology programs that saved nearly 32,000 acre-feet (25% volume) of water in Island Park Reservoir in 2021, potentially increasing future kokanee salmon spawning runs by up to 150%.

Go early or stay late

I’ve stepped off the boat in one of the many bends in the Henry’s Fork upstream of Mack’s Inn. Gripping the anchor line, I shuffle my way to shore to secure the boat at a grassy meadow. My family is along for their first ride down the Big Springs Water Trail—this taking place few years back when I had first started working on my PhD with the Henry’s Fork Foundation. After a morning of floating and fishing, we’re looking forward to a rest and a light lunch.

I heave the anchor on shore and step out. With the summer now long since passed, we’re alone. The air and water have a cool bite through my waders and the once-lush grasses and willows are now stiff, dry, and golden, and smell strongly of rotten fish.

Wait. Rotten fish?

At my feet is a pile of kokanee salmon carcasses—all of them intact save for the heads and guts. The grasses are matted down in a large bed abutting the river. In mud nearby are partial tracks of a grizzly bear. We aren’t alone after all! A quick scan of the willows reveals no signs of immediate danger but we’re not sticking around. We retreat, bear spray in hand, and eat lunch in the boat while on the lookout for our furry angler friend.

Grassy riverbank with dead fish next to a clear river
Bear bed with salmon carcasses. I’m not so sure we should eat lunch here! Image by the author.

We never do see the grizzly, but we do end up catching some of his or her quarry: the kokanee salmon. These fish are a landlocked version of the Sockeye or Red salmon and undergo the same amazing transformation and migration as their oceangoing brethren.

Kokanee aren’t native to Island Park Reservoir, but decades of stocking by the Idaho Department of Fish and Game have established a desirable fishery of hard-fighting, large, and tasty wild and stocked fish. Every fall, adult kokanee swim upstream from Island Park Reservoir to spawn and, inevitably, die. Alongside them swim other fall migrants, including the beautiful brook trout and the often disregarded mountain whitefish.

Migrations aren’t limited to the fall; spring brings the spawning runs of rainbow trout, cutthroat trout, and Utah sucker.

Some of the larger examples of Island Park Reservoir migrants. Left: spawning male kokanee salmon showing the bright red body, humped back, olive head, hooked jaw, and razor-sharp fangs. Right: spawning female brook trout. Both were returned rapidly to the water after being photographed. Image credit: Julie McLaren.

Red Fish, Blue Water

These fall and spring migrants from Island Park Reservoir are the backbone of the fishery in the Henry’s Fork River upstream. However, numbers of these fish fluctuate wildly year to year with little apparent correlation to any obvious annual factor.

My goal as Doctoral researcher with the Henry’s Fork Foundation is to “understand what restricts fisheries production”, so it is my responsibility to study the phenomena of fluctuating kokanee salmon populations. I looked for data that might help us understand what might control kokanee salmon numbers. The USGS Interagency Grizzly Bear Study Team had what we needed: they conduct counts of live and dead spawning kokanee salmon in a short reach of the Henry’s Lake Outlet. With their data, we had an annual index of the health of kokanee salmon populations in Island Park Reservoir. Next, I looked for statistical relationships between the number of kokanee observed by the USGS team in any given year and a suite of variables, including climate, fisheries, and reservoir conditions.

I found compelling evidence that the number of spawning kokanee is primarily related to how much water is left in Island Park Reservoir at the end of the irrigation season (called “carryover”). Critically, it appears two consecutive years of good “carryover” are needed to produce good numbers of migratory adult salmon. Salmon require a few years of life to grow big enough before venturing out of the reservoir to spawn. Therefore, bad carryover during any one of these formative years could end up reducing salmon numbers. The number of salmon stocked into Island Park Reservoir matters too, but those stocked fish only survive if carryover is good. Climate variables like air temperature weren’t found to be important for kokanee salmon numbers.

Scientific graph showing the positive relationship between kokanee numbers and reservoir carryover
The y axis represents the maximum number of kokanee salmon found in a tributary stream upstream from Island Park Reservoir in each year of record. These kokanee salmon are age-3 spawning fish that will die at the end of their spawning run. These age-3 fish lived the entirety of the previous two years in Island Park Reservoir. The x-axis shows the maximum drawdown in Island Park Reservoir during those previous two years of kokanee juvenile development. Thus, we found that low carryover during years 1 and 2 of kokanee life may destroy habitat and result in lower recruitment and future low run-returns.
Thanks for your help!

So, what can we do about it? Well, thanks to members of the Henry’s Fork Foundation and our close collaborative work with our conservation management partners, carryover in Island Park Reservoir—the all-important variable controlling water quality and future fish numbers—was much better in 2021 than it would have been otherwise.

Our precision management, Farms and Fish, and science and technology programs have built a portfolio of small-scale, on-farm and on-river collaborative programs to increase precision of irrigation management, thereby reducing the need for irrigation water delivery and resultant drawdown at Island Park Reservoir. These programs saved nearly 32,000 acre-feet (25% volume) of water in Island Park Reservoir in 2021, potentially increasing future kokanee salmon spawning runs by up to 150%!

There’s a long way to go to make this kind of result sustainable in the face of ever-more-extreme droughts. But for now, I’d like to thank the members of the Henry’s Fork Foundation for helping make our relative success in 2021 possible! If you’re interested in becoming a member—and making a direct, measurable difference in the number of wild salmon and trout in our shared rivers and lakes—then check us out and become a member here:

Figure showing the cumulative impact of precision management, which reduces drawdown even in years with remarkably similar water supply. This image compares 2012 with 2018, but we observed a similar effect in 2021. Understanding the fish habitat effect of this reduced drawdown will be critical for optimizing management and quantifying the benefits of precision management. Image credit: Dr. Rob Van Kirk
Next time...

We know that saving water in Island Park Reservoir helps improve fish numbers, but what’s going on in Island Park Reservoir that’s so bad for kokanee and rainbow trout? The Henry’s Fork Foundation and the Reservoir Fish Habitat Partnership funded a research project to find out what happens to water temperature and dissolved oxygen in Island Park Reservoir when carryover is low.

With the help of AJ Mabaka, an undergraduate intern from Washington and Lee University, we monitored water temperature and dissolved oxygen conditions in Island Park Reservoir in five different locations representing different habitat units during the 2021 drought. The results of AJ’s hard work are coming up in another blog where I’ll detail the nitty gritty of water quality in Island Park Reservoir, why drawdown reduces salmon and trout habitat, and what we might be able to do about it.


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