Have there been Reports of Dead Fish on the River?

Updated on 8/16:

Yes, anglers have reported several sightings of individual dead fish between August 1-15. HFF's field team has gone out to the river after each reported incident to investigate. All of the fish that we discovered have been juvenile Whitefish (3-5 inches in length). The reports we received were consistent with these findings.

What was the cause of the dead fish?

We believe there are multiple reasons why there have been juvenile whitefish mortalities, and as of right now, we are still investigating the cause(s). We are currently in contact with Idaho Fish and Game and ask that you report any future sightings to Idaho Fish and Game.

Are the reports of dead fish connected to turbidity (water clarity)?

The short answer is that turbidity, while not aesthetically desirable, impacts our fishing day more than it impacts fish and other aquatic life. Turbidity alone is not the cause of the dead fish.

On the Henry's Fork, water clarity is affected by the presence of two things:

1. Inorganic matter (nonliving material, such as mineral sediment)

2. Organic matter (material that comes from a living organism, such as blue-green algae or decaying plants).

HFF monitors turbidity by manually taking water samples and analyzing them in our lab, and through our water quality monitoring network of sondes (instruments). Recently, our equipment registered the highest turbidity in Island Park Dam outflow since the September 2020 turbidity event. However, this event is different from other turbidity events because it was caused from high algae growth in the reservoir which is due to the hot and sunny weather conditions throughout July (providing the perfect environment for the algal bloom to occur).

Photo showing turbidity event - dated 08/05/2024

Looking ahead, our weekly reservoir sampling confirmed a large algae bloom on the reservoir’s West End, and recent rain and cooler weather have cooled the surface of the West End enough that turbidity from the bloom is diving under warmer water on the east end of the reservoir and travling to the dam. We anticipate this turbidity event to pass relatively quickly as the current bloom exhausts available nutrients in the water column. All outflow from the dam is passing through the power plant and will do so for the foreseeable future, which will limit the amount of suspended material delivered to the river.

What about water temperatures?

The warm water temperatures we have experienced this summer are certainly stressful for fish. Despite this, if trout continue to have enough places of refuge, dissolved oxygen, and food available, they can still thrive in warmer water temperatures. The Henry's Fork is special in that it has abundant food and high levels of dissolved oxygen, which offsets the stress that trout might experience at these higher temperatures.

Currently, water temperatures have been improving and are in the good or optimal range for trout.

Comparison of HFF's recent temperature data between Island Park Dam East and Pinehaven

Dissolved Oxygen is crucial!

1

Dissolved oxygen is the amount of oxygen available in the water and available to aquatic life, and it plays a critical role for fish survival. The Henry's Fork is able to maintain high dissolved oxygen levels, regardless of water temperatures, throughout all sections of the river as a result of the photosynthesis of rooted aquatic plants.

HFF’s water quality network is not showing stressfully low levels of dissolved oxygen in the river downstream and is well above stressful thersholds all day throughout the reach.

Comparison of HFF's recent dissolved oxygen data between Island Park Dam East and Pinehaven

What is HFF doing to prevent this from happening in the future?

1. Water Quality Monitoring on the Reservoir: HFF's field team will continue to go out on the reservoir weekly to take water samples and monitor what is happening. This data is being used to determine how to predict these conditions in the future.

2. DIRTT: HFF's plan to Develop Infrastructure to Reduce Temperature and Turbidity is working to create infrastructure at the dam that will minimize the outflow of unclear water into the river, which would improve water clarity on the Henry's Fork long-term.

3. Precision Water Management: HFF's Precisions Water Management program helps to reduce outflow, sending water through the power plant rather than the gates. This is a good thing, because water that goes through the gates tends to be less clear than water that goes through the power plant. This minimizes the amount of suspended material that is exported outside of the reservoir.

What can you do?

Thank you to the anglers who reported this issue to us. If you are out on the river and discover dead fish, please report this to Idaho Fish and Game! Idaho Fish and Game will need the location, time, any close-up photos, and an ID of the species. Thank you for your help in protecting this incredible fishery.

Also, please remember that on the Henry's Fork, peak dissolved oxygen occurs when the sun is at its peak (aquatic plant growth) around 3 or 3:30 pm. Water temperatures will still continue to rise until 7 or 8pm. To limit any negative impact on fish, anglers could considers limiting their fishing until after 4pm, as that is the most stressful time for fish on the Henry's Fork.