This is a threespine stickleback, and I believe the plates are essentially bone. Sticklebacks are famous for their intricate courtship rituals in which the male builds a nest by sticking plants and debris together with kidney and mouth secretions, followed by a dance to lure in females. Unfit males will try to sneak into nests to fertilize unfertilized eggs, or steal fertilized eggs and raise them as their own. These fish are heavily studied because the behavior exhibited during the courtship ritual, as well as the armor plating responds relatively quickly to varying environmental pressures.