Mottled sculpin are small, stout fish with relatively flattened bodies. They have round snouts and have colors that help them to blend in with their habitat. They have brown to black mottling on their backs, sides, and fins and whitish bellies. They have 2 dorsal fins. There is a small black spot on the first part of the first dorsal fin, which helps to distinguish them from round gobies, and a larger spot on the back of the first dorsal fin. They do not have obvious scales, although they have a line of small prickle-like scales below their lateral line. Males are slightly larger than females and during the breeding season males have a dark band on the first dorsal fin and a broad, orange band on the edge of the fin.
Mottled sculpin are widespread in North America, but there are several populations that are isolated from each other. Eastern populations are found throughout the Great Lakes region, north to Hudson Bay and throughout much of eastern Canada and south to northern Alabama and Mississippi. There is a smaller population in Missouri and a large western population in the northern Rocky Mountain states, from British Columbia and Alberta south to southern Nevada and northern New Mexico.
Mottled sculpin are found in gravel bottoms and sandy riffles of small headwaters, streams, and small rivers or in rocky shoreline areas of lakes, including the Great Lakes. The type of bottom may be less important than the presence of cover, which can be gravel, stones, or submerged vegetation. They can be found at up to 16 meters depth. Mottled sculpin seem to prefer depths of 0.1 to 0.5 meters and cold, clear water. (Becker, 1983; Froese, 2008)
Mottled sculpin egg development depends on water temperature, at temperatures of 11 to 13 degrees Celsius eggs hatched in 17 days. Mottled sculpin larvae are about 5.9 mm in length when they hatch and leave the nest when they reach about 6.7 mm long and have used up their yolk sac, at about 14 days after hatching. (Becker, 1983)
Male mottled sculpin use courtship movements to attract females to their nest cavities. They shake their heads, raise their gills, and undulate their bodies to get the attention of females. The color and band on their dorsal fin also helps to attract females. When a female approaches, the male will bite her cheek, side, fins, or tail or else grab her by the head and pull her into the nest cavity. Once inside the nest cavity, the female turns upside down so that she can release her eggs onto the cavity ceiling. The male accompanies the female into the cavity and arranges himself next to her. The male's head and fins then become jet black and his body becomes pale. The male blocks the nest cavity entrance for several days so that the female remains inside. Males eventually attract several females into their nests to mate.
Male mottled sculpin begin to defend nest cavities in the spring. Nest cavities are areas beneath rocks or other debris at depths of about 22 cm and in areas with enough water flow to prevent silt build up. Nest entrances usually face upstream. Males attract females to their nests, where the females remain for a few days and lay their eggs. Males then remain in the nest cavities until the eggs hatch and the young fish leave the nest a few weeks after hatching. In Wisconsin, males are in nests from April to the end of May. Females are recorded to have from 111 to 635 (average 328) eggs at a time. Eggs hatch in about 17 days and young depart from the nest about 14 days after that. Sexual maturity is reached at adult sizes of 59.2 mm in males and 53.1 mm in females, sizes that can be reached within a year of hatching. (Becker, 1983)
Male mottled sculpins guard clusters of eggs that have been laid by different females. They protect the eggs from predators until they hatch. (Froese, 2008)
Average recorded lifespan in mottled sculpin is 2 years. (Froese, 2008)
Mottled sculpin are found in areas with fast water movement, their flattened body shape helps them to take refuge from fast currents among the rocks and debris along the bottom. They take refuge during the day under rocks or vegetation. In the still areas along lake shores they may stir up the sand and let it cover them to hide. They swim in small, darting motions which make it seem as if they are hopping from one spot to the next. Outside of the breeding season mottled sculpin are not aggressive and can often be seen near or next to each other. Mottled sculpin are more active at night, feeding in more open areas.
In a dense population of sculpin in Montana, home range sizes were estimated at less than 50 meters and the longest movements were 180 meters. (Becker, 1983)
Based on courtship behaviors, visual and tactile cues are likely to be used by mottled sculpin in communication. They may also have good chemoreception, as in most fish. Mottled sculpin have a lateral line system that helps them to perceive water movements and pressure changes. (Becker, 1983)
Mottled sculpin eat mainly aquatic insect larvae, such as mayfly nymphs, caddisfly larvae, stonefly larvae and midges, but also eat small crustaceans, such as amphipods, copepods, and ostracods, they also eat leeches, smaller fish, fish eggs, and some aquatic plant material and algae. (Becker, 1983; Froese, 2008)
Mottled sculpin are very important intermediate predators in native aquatic ecosystems. They prey on small aquatic animals, mostly invertebrates, and form an important prey base for larger fish, such as brook trout and northern pike. They may also help trout populations through their predation on stoneflies, which each trout eggs and young. Mottled sculpin are hosts for the larvae of some native clam species, including cylindrical papershells and slippershell mussels. Mottled sculpin may compete directly with round gobies, an invasive species in the Great Lakes.
There are no negative effects of mottled sculpin on humans. Some have regarded them as serious predators of trout eggs, but research suggests that their predation on trout eggs is usually on drifting eggs that won't develop anyway. Healthy mottled sculpin populations are used as an indicator of healthy trout populations because they are important prey of trout.
Healthy populations of mottled sculpin are used as an indicator of healthy trout populations, which are very important gamefish. Mottled sculpin have been demonstrated to have a positive effect on trout populations through preying on stoneflies, which prey on trout young and eggs, and because they are important prey for large trout. (Becker, 1983)
Mottled sculpin populations are not considered threatened currently. This is a widespread species with large populations. However, introduced round gobies may dramatically effect mottled sculpin populations through predation and competition. (Becker, 1983)
Mottled sculpin are also known as common sculpin, northern muddler, muddler, blobs, gudgeons, muffle-jaws, bullheads, springfish, lake sculpin, spoonheads, and miller's thumbs. Their generic name, Cottus, is from an old European name for sculpin. (Becker, 1983)
Tanya Dewey (author), Animal Diversity Web.
Becker, G. 1983. Fishes of Wisconsin. Madison, Wisconsin: The University of Wisconsin Press.
Froese, R. 2008. "fishbase.org" (On-line). Accessed December 10, 2008 at http://fishbase.sinica.edu.tw/summary/SpeciesSummary.php?id=4065.