Biological Responses of Sockeye Salmon to the Fertilization of Chilko Lake, a Large Lake in the Interior of British Columbia

We evaluated the response of sockeye salmon Oncorhynchus nerka to a 5-year experimental fertilization of Chilko Lake, a large oligotrophic lake of the Fraser River, British Columbia watershed. To evaluate the effects of nutrient addition on smolt production, smolt size, and adult returns, we compared data from the treated years to 39 years of pretreatment data for Chilko Lake, as well as data from seven other untreated sockeye salmon populations that served as controls. By using the other populations to control for variation in survival outside of Chilko Lake, we found that the mean productivity (recruits per spawner) of fertilized broods was 73% higher than unfertilized broods, but the uncertainty surrounding this estimate was substantial: 90% confidence interval of−2% to 174%. Total phosphorus, primary productivity, and to a lesser extent, zooplankton biomass were greater during the fertilized years. Nutrient additions increased the mean size of age-1 smolts by 34% and that of age-2 smolts by 58%. We found a weak positive relation between the size of age-1 smolts leaving Chilko Lake and their subsequent survival. Thus fertilization appeared to increase adult production by improving the survival of smolts in the ocean.