Many native species spend much of their adult lives in the deeper waters of Lake Michigan, but migrate many miles to spawn in high quality riparian habitats in the upper Milwaukee River watershed.
The watershed downstream of Ozaukee County is highly urbanized and little of the formerly-abundant wetland and riparian habitat remains in its natural state. In-stream habitat has also been significantly altered in many locations for navigation, development and drainage purposes. Therefore, the lower river and estuary have experienced reduced native species abundance and diversity.
In contrast, the upper watershed in Ozaukee County has significant areas of relatively high quality spawning habitat for native species. Until recently, access to these areas was fragmented by large dams and other impediments. Since many freshwater fish move long distances for life-cycle functions, these areas can provide habitat suitable for spawning and juvenile development if hydrologically connected.
Native northern pike are weak prolonged swimmers and jumpers and very susceptible to impediments. Northern pike have difficulty jumping over barriers, cannot swim effectively through fast moving waters greater than two feet per second and require frequent rest areas when traveling through streams. Passage for healthy adults moving upstream and young-of-the-year moving downstream are equally crucial.