Smaller sculpins are quite commonly eaten by larger individuals in this highly cannibalistic species. Eggs are also cannibalized during the breeding season. Mottled sculpins breed in early spring. The breeding season begins when adult males occupy cavities beneath rocks on the streambed. Males darken considerably so that their heads become jet black and their dorsal fins become outlined in reddish-orange.
Males are pugnacious during the breeding season, and defend their burrows from any intruders.
During this period, females swell with eggs. Fully gravid females look as though they will burst, and the outline of the individual eggs may be seen through the tightly stretched abdominal wall. Males seldom leave their burrows, but will swim out several inches to meet a female. Occasionally, males will bite a female's fins, or even take a female's head entirely into their mouths. This does not appear to injure the female, though.