I wondered about the difference in the number of rod and cones in a crappie eye compared to a trout eye and found this:
Dr. Keith Jones, Pure Fishing Research Director and a world renowned expert on fish behavior, provided this insight into color: “Different fish species differ considerably in what colors they can see, and hence what colors they see best. Fish like bass, bluegill, crappie, walleye, and perch are mostly limited to the spectral range of red-orange and yellow-green. They have very little if any color vision in the range of blue and violet. Their best color vision (i.e. the area where they have the best discrimination between fine differences in color hue) typically peaks in the range of red-orange, with a secondary peak in the yellow-green range. Fish like trout and carp have a broader range of color vision.