Remember with walleye it isn't just finding a suitable spawning location. Their fry are highly vulnerable to predation and lack of zooplankton. So even if they did successfully spawn and the eggs survived long enough to hatch. You then have to hope there is enough zooplankton for them to feed on and that predators don't eat all the young fry. Same goes for their eggs, which are very vulnerable to predation as well, especially in marginal spawning areas.