The Effects of Nutrient Additions on Kokanee in Dworshak Reservoir, Idaho

ABSTRACT We investigated the effects of whole reservoir nitrogen (N) addition on kokanee growth in Dworshak Reservoir, Idaho. We hypothesized that N addition would improve the growth of kokanee ( Oncorhynchus nerka ) at a given level of abundance by increasing the availability of Daphnia , an important food source. Age‐specific size (length, weight, and relative weight) and biomass were compared for periods with and without N addition. Mixed effects models were used to determine the relative importance of N addition, kokanee abundance, Daphnia availability, reservoir inflow, and temperature on annual growth. Simple linear models were used to assess how the importance of these variables changed with age. Kokanee were longer, heavier, and more abundant during periods with N addition, leading to a 56% increase in mean biomass. Age‐specific abundance was the most important factor influencing the growth of age‐0 kokanee. Daphnia availability replaced age‐specific abundance as the most important factor for age‐1 kokanee and increased in importance for age‐2 kokanee. While Daphnia biomass was the proximal driver of kokanee growth, N addition was previously found to increase Daphnia biomass given kokanee abundance. Therefore, N addition may be a useful tool to mitigate for density‐dependent growth or lost productivity in N limited systems.