D. S. Jordan, 1878
The lake chub, Couesius plumbeus, is a freshwater cyprinid fish found in Canada and in parts of the United States. Of all North American minnows, it is the one with the northernmost distribution. Its genus, Couesius is considered monotypic today. The genus was named after Dr. Elliott Coues, who collected the holotype specimen.
The body is elongate, usually 100 mm (4 in) long, though some individuals twice that length have been found. The back is olive-brown or dark brown, and the sides are leaden silver, hence the word plumbeus, referring to lead, in the scientific name of this fish. The snout is blunt and projects slightly beyond the upper lip. The corners of the mouth each bear a small barbel. The scales are small but well visible, and some may be black and form isolated dark spots on the lower sides. The dorsal, pelvic and anal fins each have 8 rays. The pectoral fins are broad and have 13-18, but more usually 15-16, rays. Breeding males can develop patches of bright orange or red at the base of the pectoral fins and sometimes near the mouth, and small nuptial tubercles on the top of the head, dorsal surface of pectoral rays, and on the belly near the base of the pectorals.
The lake chub is generally found throughout Canada up to the Arctic Circle. Some scattered populations are also present in the northern United States, more precisely in New England, Michigan, Iowa, Wisconsin, Minnesota, the Dakotas, Montana, Wyoming, Colorado, Washington, Idaho and Utah.
As its common name implies, the lake chub is most commonly found in lakes, but it can also live in rivers and streams. In mid-summer it may move to the deeper parts of a lake to avoid the warmer waters of the lake shore.
The diet of the lake chub is varied: zooplankton, insects, aquatic insect larvae, and algae. The largest individuals can capture small fishes. The lake chub itself can be eaten by large predatory fishes and is therefore suitable as bait for fishing.
In and around Catamaran Brook, New Brunswick, Canada, lake chubs over 5 cm long (2 inches) were found to grow by about 0.8 cm (0.3 inch) a month in summer.
Lake chubs normally undergo spawning migrations in early summer. Temperature plays a role in triggering migration onset, as migrations are delayed on colder years. Migrating chubs leave their lakes and rivers to ascend tributary streams, in which they mate (by day or night) and release their eggs over gravel or rocks. There is no parental care. During migration the fish move mostly during dusk and at night.
In the laboratory, lake chubs have expressed free-running circadian rhythms that are among the most precise of the few fish species studied to date.   These fish are usually diurnal in the laboratory, but in the wild they can be diurnal, crepuscular, or nocturnal.
The lake chub has large optic lobes in its brain and is therefore presumed to be a good sight feeder. It can, however, feed at night also. Comparisons with other freshwater fishes such as stickleback, northern pike, sculpin, and burbot have revealed that it has a superior hearing capacity.
The lake chub is sometimes called by other names such as northern chub, lake northern chub, chub minnow, plumbeus minnow, or bottlefish. The species is also found in Quebec and its French name is "mené de lac".
- Reebs, S.G., S. Leblanc, A. Fraser, P. Hardie, & R.A. Cunjak, 2008, Upstream and downstream movements of lake chub, Couesius plumbeus, and white sucker, Catostomus commersoni, at Catamaran Brook, 1990-2004. Canadian Technical Report of Fisheries and Aquatic Sciences 2791.
- Brown, J.H., Hammer, U.T., and Koshinsky, G.D., 1970, Breeding biology of the lake chub, Couesius plumbeus, at Lac la Ronge, Saskatchewan, Journal of the Fisheries Research Board of Canada 27: 1005-1015.
- Kavaliers, M., 1978, Seasonal changes in the circadian period of the lake chub, Couesius plumbeus, Canadian Journal of Zoology 56: 2591-2596.
- Kavaliers, M., 1979, Pineal involvement in the control of circadian rhythmicity in the lake chub, Couesius plumbeus, Journal of Experimental Zoology 209: 33-40.
- Kavaliers, M., 1979, The pineal organ and circadian organization of teleost fish, Revue Canadienne de Biologie 38: 281-292.
- Reebs, S.G., 2002, Plasticity of diel and circadian activity rhythms in fishes, Reviews in Fish Biology and Fisheries 12: 349-371.
- Davis, B.J., and Miller, R.J., 1967, Brain patterns in minnows of the genus Hybopsis in relation to feeding habits and habitats, Copeia 1967: 1-39
- Emery, A.R., 1973, Preliminary comparisons of day and night habits of freshwater fish in Ontario lakes, Journal of the Fisheries Research Board of Canada 30: 761-774.
- Mann, D.A., Cott, P.A., Hanna, B.W., and Popper, A.N., 2007, Hearing in eight species of northern Canadian freshwater fishes, Journal of Fish Biology 70: 109-120.
- "Couesius plumbeus". Integrated Taxonomic Information System. Retrieved 30 January 2006.
- Froese, Rainer and Pauly, Daniel, eds. (2011). "Couesius plumbeus" in FishBase. August 2011 version.
- Wydoski, Richard S.; Whitney, Richard R. (2003). Inland Fishes of Washington (2nd ed.). University of Washington Press. ISBN 978-0-295-98338-7.