The tapetum lucidum in walleye is built from crystals of guanine arranged in the choroid behind the retina. Photons that pass through the photoreceptors without being absorbed are reflected back for a second chance at detection, which roughly increases sensitivity at the cost of a slightly blurrier image. The same structure produces the eyeshine of cats and deer.
The consequence is behavioural. Walleye feed most actively at dawn, dusk and through the night, and in turbid or stained water where light penetration is poor. Yellow perch and other prey species do not see as well under those conditions, so the walleye hunts in a window when its sensory advantage is largest. Anglers describe walleye chop, the surface ripple that scatters light, for the same reason.
The trade-off is discomfort in bright conditions. Walleye tend to move deeper or into shade on clear sunny days, though how much of that reflects light avoidance rather than temperature preference is not always separable.