BRITISH COLUMBIA LAKE ENRICHMENT PROGRAMME: TWO DECADES OF HABITAT ENHANCEMENT FOR SOCKEYE SALMON

The anadromous sockeye salmon (Oncorhynchus nerka) is the most valuable salmonid species on the Pacific coast of North America. Juvenile sockeye rear for 1–2 years in lakes, where they feed primarily on crustacean zooplankton. As a result of overfishing and habitat loss for over a century, most British Columbia nursery lakes are now recruitment limited, with too few adult salmon returning to spawn and provide fry. However, declining adult escapements have also resulted in a significant loss of nitrogen (N) and phosphorus (P) nutrients supplied by decomposing adult carcasses and the lakes have become less productive (oligotrophication) for rearing sockeye fry. To reverse this process and increase sockeye salmon abundance, in 1976 the Canadian Department of Fisheries and Oceans began enriching nursery lakes with nitrogen and phosphorus nutrients to increase their productivity and fry-rearing capacity.  Many of the objectives, techniques and results from the Lake Enrichment Programme are also applicable to the management of regulated watersheds where mitigation for possible long-term reductions in freshwater habitat productivity is a concern. Over the last two decades, 20 lakes were fertilized weekly through the growing season with liquid fertilizers. The lakes have received limnological and fisheries assessments that have focused on pelagic food-webs and production processes.  The lakes have shown a positive response at all trophic levels to nutrient additions, with an increase in activity and doubling of bacteria abundance, increases of 50–60% in autotrophic picoplankton abundance and phytoplankton biomass (chlorophyll) and up to twofold increases in primary production and zooplankton biomass. This enhanced lake production has increased growth and survival of juvenile sockeye and increased smolt weights (>60%) resulting in larger sockeye adult returns worth 10–20 million dollars per year to the Canadian economy. The benefit : cost ratio of lake fertilization is the best of currently available salmonid enhancement techniques.