To ensure the re-introduction of walleye we had to decrease the sources of walleye mortality. More importantly an ecosystem shift needed to occur. This shift was created by increasing walleye numbers in the lake through stocking, and dramatically reducing the cormorant population to decrease their direct and indirect impacts to the walleye.
Cormorants have direct impacts to walleye, pike, and whitefish populations through predation upon juveniles. They have indirect effects through their influence on the ecosystem. As cormorants prey on the smaller fish in the lake it puts pressure on them. In the case of perch, this pressure caused these fish to put all their energy into becoming mature at very young ages. This early maturity allowed them to reproduce at very high rates thereby increasing their numbers in the lake to near unprecedented levels. These yellow perch feed on the small walleye fry and then compete with the surviving larger juvenile walleye for limited food resources.
Managing cormorants allowed us to reduce their numbers to levels considered more reasonable and closer to historic for the area. This resulted in less direct predation on juvenile walleye and triggered life history shifts in their other main prey items (ie perch) causing them to mature later and reduce their numbers. The reduction in cormorants initiated necessary ecological shifts to make room for the newly stocked walleye.