Anderson Ranch Reservoir kokanee fishery showing signs of improvement after 2020 lows

The 2020 kokanee season in Anderson Ranch Reservoir proved to be one of the worst on record, with dismal catch rates and poor overall fishing conditions.

The cause of this slump was simple.

The Anderson Ranch Reservoir kokanee population has traditionally been self-sustaining, requiring little hatchery supplementation.  High runoff events in 2017 that flushed many young kokanee out of the Boise River system coupled with higher levels of angler harvest on adult fish limited an entire age-class of young kokanee from entering the fishery.

Fast forward three years to 2020 and the 3-year-old fish that generally make up the fishery in Anderson Ranch Reservoir weren’t there for anglers to target. However, since 2020 the kokanee fishery has shown consistent signs of improvement.

Fishery management targets for the Anderson Ranch Reservoir kokanee fishery are an angler catch rate of 0.5 fish per hour of fishing and for kokanee to average over 12 inches in length.

To assess whether these goals are being met, Idaho Fish and Game uses a variety of sampling methods. For example, Department biologists conduct angler creel surveys every summer during the peak of the kokanee fishing season. These surveys give the opportunity to observe angler catch rates, harvested fish sizes, and overall angler satisfaction.

In addition to these creel surveys, Fish and Game has completed netting surveys since 2015 to determine relative abundance of kokanee in Anderson Ranch Reservoir. Every fall, curtain gill nets are set in standard locations and allowed to fish overnight. These nets are set in the fall after mature kokanee have left the reservoir to spawn in its tributaries. This gives biologists a clear look at the relative abundance of upcoming kokanee year-classes and the ability to forecast what the fishery will look like in the coming years.

Based on survey data collected in 2022 and 2023, the outlook for the 2024 kokanee fishery is improved from previous years. Kokanee angling catch rates in 2023 were double those observed in 2021 and eight-fold higher than the 2020 estimate.  In addition to an increase in catch rates, anglers are harvesting smaller fish with average total fish length decreasing from 17.5” in 2021 to 16.5” in 2023. This is important because kokanee growth is density-dependent, meaning that when kokanee abundance is high, growth rates slow given limited food resources. Similarly, when kokanee density is low, they attain larger sizes due to less limitations for resources. The observed increase in catch rates coupled with a reduction in average length suggests that kokanee abundance is increasing in Anderson Ranch Reservoir.

Netting surveys have also indicated the kokanee population is improving. Netting catch rates were over 20 percent higher in 2022 than 2021 and remained consistent through 2023. Most importantly, however, was the higher catch rates of age-0 and age-1 kokanee. Our data shows that these year-classes are stronger than in previous years. This suggests that natural reproduction is increasing in the system and better fishing may be on the horizon for the 2024 fishing season and beyond.