Effects of regulated lake levels on the reproductive success, distribution and abundance of the aquatic bird community in Voyageurs National Park, Minnesota

Reiser, M; Voyageurs National Park

The effects of regulated lake levels on the reproductive success, distribution and abundance of the aquatic and marsh bird community were studied in Voyageurs National Park, Minnesota. The park's four largest lakes have been regulated by dams since 1909. This regulation has caused less than natural (Rainy Lake), and greater than natural fluctuations in lake levels (Namakan Reservoir lakes); in addition, there are 26 small interior lakes which fluctuate naturally. Park lake designation (by water levels) exerted a significant negative influence on the reproductive success of the Common Loon (Gavia immer) with Rainy Lake having a greater success rate than the Namakan Reservoir lakes. Reproductive success of Voyageurs National Park's Common Loon and Red-necked Grebe (Podiceps grisegena) populations were also significantly lower than populations occurring on other (non-park) naturally fluctuating lake systems. The greatest nest loss on regulated lakes was due to rising water levels. Common Loon nest losses averaged 60% on Rainy Lake, 70.6% on the Namakan Reservoir lakes, and no losses due to flooding on the interior lakes. Red-necked Grebe nest losses averaged 47% on Rainy Lake, and 52% on the Namakan Reservoir lakes, while other marsh nesting species averaged 25.0% on Rainy Lake and 76% on the Namakan Reservoir lakes. A significant proportion of the variation in Common Loon flooded-out nesting attempts could be explained by June lake level changes alone. Using computed natural lake levels for June 1983-1985, predicted percentages of loon nest losses on the Namakan Reservoir lakes would have declined for 37%to 27%, while on Rainy Lake percent nest loss under natural conditions would have increased slightly from actual losses, 20% to 25% . Lake-level change was also highly significant in explaining the percent of flooded Red-necked Grebe nests. Lake-level change was not significant in explaining percentages of flooded-out nests for the other marsh nesting birds. If natural conditions prevailed, Red-necked Grebes on Rainy Lake would have experienced a 21% increase in percent of flooded nests and the other marsh nesting birds, a 58% increase. On the Namakan Reservoir lakes, however, Red-necked Grebes would have seen an 85% decline in the percent of flooded nests. Marsh nesting birds would have had virtually no losses following a 97% decline under computed natural conditions. A modified lake regulation regime is proposed which should enhance the reproductive success of the aquatic avifauna community in Voyageurs National Park and thus increase park visitors' opportunity to enjoy the bird life.