Restoration of an Interior Lake Ecosystem: the Kootenay Lake Fertilization Experiment

Abstract

Abstract Kootenay Lake, B.C. (395 km2), has experienced a series of major perturbations in the past 50 years, resulting in the 1980s’ collapse of South Arm kokanee salmon stocks and a dramatic decrease in the abundance of North Arm stocks. Historical data indicate the collapse was due to reservoir construction together with subsequent impoundment and nutrient retention on both main inflow tributaries (Kootenai and Duncan rivers) to Kootenay Lake, combined with the introduction of an exotic mysid shrimp which is an efficient competitor with kokanee for zooplankton. Nutrients (47.1 t of P and 206.7 t of N) are now being added annually to the 174 km2 North Arm of Kootenay Lake (271.3 mg-m-2 P and 1,190.5 mg-m-2 N) in a 5-year (1992-96) experiment with the goal of restoring historical kokanee salmon biomass and productivity. The concentration of total phosphorus in the North Arm currently ranges from 4–10 µg-L-1 P, which indicates oligotrophic to mesotrdphic conditions, whereas the concentration of dissolved inorganic nitrogen remains in the oligotrophic range (i.e., <200 mg L-1 N) throughout the year. The seasonal mean zooplankton density observed in 1994 was higher than in 1992 and 1993, and similar to the highest densities observed from 1972 to 1984. Cladocerans comprised about 7.5% of the zooplankton in the North Arm in 1994, 12% in 1993 and 7.5% in 1992 as compared with less than 5% between 1949 and 1991. Combined North Arm kokanee escapement (Lardeau River and Meadow Creek) was 1.25 million fish in 1994 and was the highest observed since 1986, but lower than some escapements observed in the 1970s. Kokanee spawner size and fecundity have also increased. No significant trends have been observed in the Gerrard rainbow trout population. To date, the fertilization experiment has been successful at increasing the abundance of clado-ceran zooplankton and restoring kokanee populations in the North Arm of Kootenay Lake.