Increased Juvenile Salmonid Growth by Whole-River Fertilization

Abstract

Nutrient concentrations, periphyton standing crop and size of steelhead trout (Oncorhynchus mykiss) and coho salmon (O, kisutch) fry increased after the fertilization of a nutrient-deficient stream with inorganic phosphorus (P) and nitrogen (N). Whole-river fertilization of the Keogh River, British Columbia, during 1983–86 to increase summer average nutrient concentrations from < 1 μg P∙L −1 and 25 μg N∙L −1 to 10–15 μg P∙L −1 and 30–100 μg N∙L −1 resulted in five- to 10-fold increases in periphyton standing crops on artificial substrata and 1.4- to 2.0-fold increases in late-September salmonid fry weights. Diatoms and chlorophytes dominated the algal periphyton on artificial substrata at fertilized sites. Cyanophytes were unimportant despite low N:P ratios in some cases. Juvenile salmonids fed primarily on benthic insects. These results suggest that autochthonous primary production can be an important energy source in forested, middle-order streams, and indicate that the manipulation of autochthonous primary production can be a useful management tool to increase salmonid growth in nutrient-poor coastal streams.