Six Years of Nutrient Enhancement to Restore Fisheries in the Kootenai River, Idaho

The Kootenai River, Idaho  underwent a reduction in the amount of nutrients available for primary productivity since Libby Dam went into operation in the early 1970s.  The dam acts as a nutrient sink whereby nutrients are bound to the sediments in the reservoir.   Populations of burbot and white sturgeon as well as kokanee and trout were depressed.  In 2005, the Kootenai Tribe of Idaho in conjunction with Idaho Fish and Game began adding liquid, inorganic phosphorous near the Idaho/Montana border in order to stimulate productivity at lower trohpic levels and ultimately improve fisheries.  We added ammonium polyphosphate (10-34-0) to the river to achieve a of total dissolved phosphorus (TDP) concentration of 3.0ug· L^-1, during the growing season from 2005-2010.  Results of nutrient addition were almost immediately visible as algal growth on substrates near the pipeline, previously devoid of growth.  After six years of nutrient addition, we have seen improvements at multiple trophic levels.  Total chlorophyll accrual rates (ug ·cm^-2·day^-1) and mean algal biomass are significantly higher in treatment sites than control sites.  We have seen an increase in diatoms, whereas blue-green algae have been decreased post nutrient addition.  Macroinvertebrate response was similar, showing an increase in abundance and biomass post nutrient addition as well as an increase in species richness.  An increase in EPT species, which are important food sources for fish, was apparent.   Overall fish abundance and biomass in the treatment reach has increased, post nutrient addition.  A population estimate conducted in 2008 found a two-fold increase in mountain whitefish, a three-fold increase in largescale sucker, and an increase of one-and-a-half times in rainbow trout over a 4-year period.  These three species have also shown some increases in relative weight and growth.  We plan to continue to add nutrients to the Kootenai River and in conjunction with habitat improvement projects currently underway, to continue to aid in the restoration of fisheries.