NOTE: This information is for the Box Canyon and greater Ranch reaches (roughly Island Park Dam to Riverside). Trout populations in reaches below Mesa Falls tend to be much more consistent and do not have the same relationship with winter flows from Island Park Reservoir.
Insect decline has been a hot topic across the country, but what about on the Henry’s Fork?
In its 9th year, HFF’s Aquatic Insect Monitoring Program data show good abundances of mayflies, stoneflies and caddisflies at Last Chance in 2017 and 2020, and poor abundances in 2019, 2021, and 2022.
So far, this data indicates that insect abundance is driven by river conditions inherited from previous years, and more frequent drought could be playing a role.
The invertebrate populations on the Henry’s Fork remain the envy of most trout streams.
The Henry’s Fork’s densities of 25,000-50,000 individuals/square meter and %EPT (percent of the population that are mayflies, stoneflies, and caddisflies) of 40-65% are very high.
Further, enormous improvements in insect habitat quality has occurred since the 1992 sediment event, both above and below Island Park Reservoir.
The more we learn about this river’s ecology, the more we can influence insect habitat locally.
The Ranch's insect balancing act:
For trout populations:
The long-established relationship between winter flows and trout recruitment below Island Park Dam is reflected in this year's Rainbow Trout populations there.
This year’s estimate from Idaho Dept of Fish & Game is statistically equivalent to those of 1996, 2003, 2005, 2007, 2015, and 2016, despite 2022 being the lowest water year since 1978.***
Each of those years was preceded by a very dry year two years earlier.
HFF’s work continues to make an impact, improving populations by 20% over and above what would have occurred without our work (i.e., nearly 1,000 fish in the 3-mile Box Canyon reach, all other factors being equal).
Idaho Fish and Game’s data shows a strong number of age-1 fish, which bodes well for recruitment into a catchable size class in coming years.
***This estimate is for the Box Canyon and greater Ranch reaches. Reaches below Mesa Falls tend to be much more consistent and do not have the same relationship with winter flows from Island Park Reservoir.