Long-term effects of nutrient enrichment on the condition and size-structure of an alpine brown trout population

Long-term effects of nutrient enrichment on a population of brown trout inhabiting a small, alpine lake in north-central Sweden have been studied for nearly 20 years. The study took place between 1981 and 1999 starting up between 1982 and 1987 with full nutrient enrichment of phosphorus and nitrogen in a ratio by weight of 1:8, followed by a period of reduction by half between 1988 and 1994, and thereafter no enrichment at all. Growth of the brown trout population was low before the application of nutrients. Fertilization promoted the development of zooplankton in great abundance, which gave rise to abundant food for the trout. Already during the first year of nutrient addition the average 4+ and 5+ fish had increased in weight by nearly 50% and in length by about 30%. Maximum growth was reached 5–6 years later—weight by about 175% and length by about 50% higher than before application. The slope of the growth curves for fish of ages 2+ to 5+ increased significantly from 1981 to 1987, and so did the size-variation with a high proportion of the fish reaching larger size. After each change in nutrient treatment the mean weights of 5–6-year-old trout were maintained for about 3 years. Five years after termination of fertilization growth was nearly back to the original state. Although badly needed, long-term studies of fish populations like this are few in the literature.