Coregonus zenithicus (Jordan and Evermann, 1909)
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Basis for Listing
The shortjaw cisco has a restricted distribution across its range due to overfishing and introduction of the sea lamprey (Petromyzon marinus), alewife (Alosa pseudoharengus), and rainbow smelt (Osmerus mordax). Beginning around 1870, the species supported a valuable commercial fishery and was once the dominant deepwater cisco in Lake Superior. It was subsequently overexploited and between the mid-1930s and late 950s, the shortjaw cisco experienced marked population declines (Becker 1983). Populations have also been adversely affected from predation by the parasitic sea lamprey, and from competition for food and habitat with the introduced alewife and rainbow smelt, which also prey on shortjaw cisco eggs.
The shortjaw cisco has an elongated, elliptically shaped, laterally compressed body. It is olive green dorsally, with large silvery scales on the sides, and white ventrally. The pelvic and anal fins are sometimes pigmented black, especially in older, larger specimens. The shortjaw cisco's small mouth lacks teeth, and its straight snout has a lower jaw shorter than the upper jaw. Males and sometimes females have tubercles (small, hardened breeding growths) during the breeding season. Adults are typically over 30 cm (11.8 in.) in total length and weigh approximately 300 g (10.5 oz.). Females tend to be larger and longer-lived than males.
Shortjaw ciscos occur in deep water, 20-150 m (66-492 ft.) in depth, of large lakes. Spawning probably occurs at 37-73 m (121-239 ft.) deep over clay bottoms (Becker 1983).
Biology / Life History
The life history of the shortjaw cisco is not well known. Their diet includes primarily opossum shrimp (Mysis relicta) and Pontoporeia hoyi, but also planktonic crustaceans, insect larvae, and some aquatic vegetation. Natural predators are lake trout (Salvelinus namaycush) and burbot (Lota lota). Spawning time in Lake Superior is variable. During spawning, shortjaw cisco move to shallower waters, approximately half the depth they usually inhabit. Males arrive first to spawning grounds. Eggs are deposited on the bottom and abandoned. Growth is fairly rapid within the first year and then slows down. Weight increase is not pronounced until after the first 4 years. Males and females grow at about equal rates, but due to their increased longevity, females attain a greater size. Sexual maturity is reached by the fifth or sixth year. In Lake Superior, shortjaw cisco migrate seasonally from 110-144 m (360-472 ft.) deep water in spring, to 55-71 m (180-232 ft.) deep water in summer, and 73-90 m (239-295 ft.) deep water during winter (Scott and Crossman 1973).
Conservation / Management
In addition to overfishing, competition and predation by introduced fishes have adversely affected populations of the shortjaw cisco. Disposal of dredged material and sedimentation may also deposit silt on spawning beds, smothering eggs and reducing the level of dissolved oxygen by introducing organic matter. Many hazardous chemicals present in the Great Lakes may also threaten this species.
Conservation Efforts in Minnesota
The Minnesota DNR Section of Fisheries records any occurrences of shortjaw cisco found while conducting their annual fish surveys. Additionally, documenting new occurrences of this species will be a priority of the Minnesota Biological Survey during future surveys in regions where this species is thought to exist. Surveys in the Border Lakes ecological subsection started in 2009.