Cisco

The cisco (Coregonus artedi), also widely known as lake herring, is a slender coregonid fish in the salmon family Salmonidae, closely related to lake whitefish. It is native to the Great Lakes and numerous smaller lakes across the northern United States and Canada. Ciscoes are an important prey species within cold-lake food webs, historically supporting large commercial fisheries in the Great Lakes region before populations declined due to overharvest, habitat change, and invasive species. Several closely related cisco species and forms exist across the region, some now rare or extinct, making accurate identification and conservation monitoring an ongoing priority for freshwater fisheries scientists.

Cisco can be told apart from other coregonids by their slimmer, more herring-like build:

• Slender, laterally compressed silvery body with an iridescent bluish-green to olive back
• Terminal mouth (at the tip of the snout) with the lower jaw often slightly projecting, unlike the underslung mouth of lake whitefish
• Large, easily shed scales typical of coregonids
• Deeply forked tail fin and a single small adipose fin
• Generally smaller than lake whitefish, typically 20-30 cm, with a more streamlined, open-water body form

Cisco inhabit cold, oxygen-rich lakes across the Great Lakes basin and numerous smaller lakes throughout the northern United States and Canada. They are pelagic, open-water fish that spend much of their time suspended in mid-water rather than near the bottom, often in large schools over deep basins. Cisco require cold, well-oxygenated water and are sensitive to warming and low-oxygen conditions caused by nutrient pollution, making them a useful indicator species for lake health. In deeper lakes, some populations descend to considerable depths during summer stratification, returning to shallower water in cooler months to spawn.

Cisco are highly gregarious, forming large, often dense schools in open water, a behavior that historically made them vulnerable to intensive net fisheries. They feed primarily on zooplankton, filtering small crustaceans from the water column, and occasionally take insect larvae near the surface. Spawning occurs in late fall or early winter, when schools move into shallow nearshore areas to broadcast eggs over gravel or sand without building nests. As a key forage species, cisco support populations of larger predatory fish such as lake trout and walleye, making their abundance an important factor in overall lake ecosystem health.