For example, lake waters need to be warm enough in summer to boost young walleye growth, which helps the fish survive their first winter. Biologists determine the relative strength of a year class by seining shallows each summer to capture fish hatched the previous spring. Fall test netting and angler surveys indicate how previous year classes are faring. The information helps biologists predict the abundance of various sizes of fish from year to year. Predictions for Rainy are rosy.
Biologists estimate that a strong year class comes along about every five years or so.
Rainy Lake walleyes reproduce abundantly when springtime water levels rise, as they often did during the 1990s. According to DNR research biologist Paul Radomski, high water in May allows walleyes to spawn on silt-free rock rubble shoals. Unfortunately, spring water levels on Rainy are no longer so favorable for walleye spawning. Since 1996 a Canadian company, Abitibi Consolidated, has operated the water-control structures at the Fort Frances hydropower dam that determine lake elevation. From 1988 until 1996, the Federal Energy Regulatory Commission had required Boise-Cascade, the previous dam operator, to maintain high spring water levels.