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Barica (1979) for Nebraska and Manitoba, respectively, indicate that wetlands in the Dakotas remain frozen for about 4-5 months each year with maximum ice depths of about 0.7-0.8 m. It is likely that a few temporary and seasonal basins with riparian connections to more permanent wetlands receive an influx of fish in the spring during years of high water levels, but unless water levels are maintained, these fish will soon perish. It is also possible that some temporary or seasonal wetlands are temporarily stocked with fish, fry, or eggs by waterbirds, but this method of dispersal is undocumented (Peterka 1989).

Only in the few semipermanent wetland basins with reliable connections to deepwater habitat or other refugia can one expect to find fish, unless they have been introduced. Lawler et al. (1974) found fathead minnows and brook sticklebacks in 10%-20% of the wetlands in the prairie region of Manitoba, but if the wetlands that contained water >2 m deep had been eliminated from this sample, the percentages would undoubtedly have been much lower.

Wetlands where fish survive in high-water years can lose all fish when low water levels prevail. In some of these wetlands, refugia from winterkill can be provided by ground-water seepage areas where some oxygen is present (Peterka 1989).

Fish survival in a few semipermanent wetlands can be limited by excess dissolved solids, as well as shallowness and low supplies of dissolved oxygen at various times of the year. Total dissolved solids concentrations of 17,000-18,000 mg/1 seem to be the approximate upper limit for survival of native fish in magnesium sulfate and sodium chloride type waters in the prairie region (Rawson and Moore 1944; Burnham and Peterka 1975). For sodium sulfate and sodium bicarbonate type waters, 12,000 mg/1 and 2,000 mg/1 seems to be the upper limits (Held and Peterka 1974; McCarraher 1971).