Water flow on the Wisconsin River is intensively controlled by dams to generate hydroelectric power or to regulate water levels in lakes, reservoirs, or flood events. These structures likely restrict or eliminate upstream/downstream fish movement, creating artificial barriers that result in habitat fragmentation and localized isolation of fish populations. Many riverine fish species move and/or migrate in search of food, suitable spawning habitat, or to meet seasonal habitat needs. Unfortunately, dams may separate these fish from resources and habitat they need to sustain the population, and can reduce gene flow between the isolated populations, potentially reducing genetic diversity and fitness. Therefore, these artificially fragmented river systems are of increasing concern to fish managers as the seclusion of local and migratory fish populations may contribute to a greater risk of population decline or extirpation.
Downstream fish movement at Prairie du Sac is possible, and therefore some downstream transfer of genetic material from Lake Wisconsin to the lower Wisconsin River does occur. However, by blocking upstream migration of fish from the lower Wisconsin River into Lake Wisconsin, the dam prevents the transfer of genetic material from fish populations that reside below the dam to populations of the same species that reside above the dam. Therefore, conservation of genetic diversity is of great concern to species that remain present above and below Prairie du Sac Dam because a reduction in genetic diversity can lead to a reduction in the ability to adapt to selective pressures.