Shortjaw cisco

Shortjaw cisco
Conservation status
Vulnerable (IUCN 2.3)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Salmoniformes
Family:	Salmonidae
Subfamily:	Coregoninae
Genus:	Coregonus
Species:	C. zenithicus
Binomial name
Coregonus zenithicus (Jordan and Evermann 1909)

The shortjaw cisco (Coregonus zenithicus) is a deepwater cisco or chub (not to be confused with various species of Cyprinidae called chubs). It has large, smooth scales and is iridescent silver, with a greenish back and white belly. The mouth is small and toothless, and the lower jaw is shorter than or equal in length to the upper jaw. It typically weighs approximately 300 g (11 oz), and ranges from 150 to greater than 300 mm (6 to 12 inches) standard length. Very difficult to differentiate from other cisco species by superficial appearance, this species typically has fewer gillrakers than other ciscoes.

Once common throughout the upper Laurentian Great Lakes it is no longer found in Lake Michigan and populations are much lower in Lake Huron than in the past. The population in Lake Superior is known to be lower than historically, based on commercial catch statistics. Its status in Lake Nipigon is uncertain. It has been reported in at least 22 other lakes outside of the Great Lakes, from the Canadian provinces of Ontario, Manitoba, Saskatchewan, Alberta, and the Northwest Territories. It is not known whether the remaining Great Lakes populations are increasing, decreasing or stable. Population trends in other lakes are unknown.

The shortjaw cisco is most commonly found in the deeper waters of large lakes. It has been found at depths of between 55 to 114 m (180 to 375 feet) in lakes Superior, Michigan, and Huron, and is known to exhibit seasonal changes in depth distribution in Lake Superior.

Spawning occurs in the autumn. Eggs are deposited over the lake bottom in deep water and develop over three to four months, depending on water temperature. This fish reaches sexual maturity at five years of age.

Crustaceans and insect larvae are important food items. Lake trout (Salvelinus namaycush) and burbot (Lota lota) are known to feed on shortjaw cisco. Shortjaw cisco are believed to be vulnerable to the invasive sea lamprey (Petromyzon marinus,) which has presumably played a role in its decline in the Great Lakes, where expansion in the range of sea lamprey has overlapped the decline in shortjaw cisco numbers.

This species was an important component of the chub fishery in the Great Lakes. Competition and predation from rainbow smelt (Osmerus mordax)and alewife (Alosa pseudoharengus), two introduced exotic fish species, have had a negative impact on shortjaw cisco populations. The cumulative effect of these factors, sea lamprey predation and habitat changes associated with urban, agricultural, and industrial activities in the Great Lakes Basin undoubtedly combined to make this species vulnerable to over-exploitation, even at exploitation rates that had once been sustainable.

The systematics of the group of fishes called "ciscoes" is complicated and some scientists propose that the species we now call shortjaw cisco is one form of a species that would include longjaw cisco (Coregonus alpenae), previously considered a distinct species. Longjaw cisco are presumed extinct.

References

• "Coregonus zenithicus". Integrated Taxonomic Information System. http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=161948. Retrieved 18 October 2006. 
• Froese, Rainer, and Daniel Pauly, eds. (2006). "Coregonus zenithicus" in FishBase. January 2006 version.