Ten Years of Nutrient Additions in the Kootenai River, Idaho: What's the Verdict for Fisheries Recovery Efforts?

A large-scale nutrient restoration program (utilizing phosphate fertilizer) was implemented in the Idaho portion of the Kootenai River in 2005 to restore fisheries by increasing primary production.  Lake Koocanusa, the reservoir created by Libby Dam in Montana, retains approximately 63% of total phosphorus and 25% of total nitrogen entering the Kootenai River system.  Declines in fish stocks have long been attributed to this loss of nutrients (along with other factors) via bottom-up trophic cascades.  Annual electrofishing surveys were conducted at multiple sampling sites before and after nutrient additions to evaluate potential responses in fish populations.  All analyses indicated that longitudinal location within the river was the largest factor influencing the fish assemblage.  Total (i.e., all species, combined) catch-per-unit-of-effort (CPUE) and CPUE of Mountain Whitefish Prosopium williamsoni were significantly higher post-treatment compared with pretreatment within the reach of river immediately downstream from the injection site.  A similar trend was observed for CPUE of Rainbow Trout Oncorhynchus mykiss and total biomass-per-unit-of-effort; however, increases were not statistically significant.  Results indicate that this program has largely been successful; however, additional research and analyses are needed to better understand different effect levels and address newly unveiled questions.