Restoration of the food web of an alpine lake following fish stocking

Trout stocking in the mid-1960s eliminated the calanoid copepod Hesperodiap tomus arcticus and other large-bodied crustaceans such as Gammarus lacustri s, Daphnia middendorffiana, and Daphnia pulex from many alpine lakes in the Rocky Mountain Parks of Canada. H. arcticus frequently dominates the plank ton communities of fishless lakes, preying on rotifers and nauplius larvae. Following the extirpation of H. arcticus, rotifers and small-bodied cyclop oid copepods dominate the zooplankton assemblages of alpine lakes. We studied the zooplankton community of Snowflake Lake, Banff National Park , from 1966 to 1995. H. arcticus was eliminated following stocking of the l ake with trout in the 1960s. It failed to become reestablished after the di sappearance of the fish population in the mid-1980s. Several species of rot ifers and small-bodied crustaceans, species originally rare or absent from the plankton, became abundant following fish stocking and remained so after the fish population declined. In 1992, we reintroduced H. arcticus to Snowflake Lake. The H. arcticus pop ulation grew exponentially for 4 yr, but had not reached stable densities t ypical of unmanipulated alpine lakes by 1995. By 1994, however, even the sm all population of Hesperodiaptomus was beginning to suppress populations of rotifers, copepod nauplii, and large diatoms. Because H. arcticus is omniv orous, a simple model of cascading trophic interactions did not predict the outcome of trophic manipulations in this alpine lake.