Effects of Dewatering on Chinook Salmon Redds: Tolerance of Four Developmental Phases to Daily Dewaterings

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Abstract
Four intergravel developmental phases of chinook salmon O. tshawytscha were dewatered experimentally in artificial redds. The redds consisted of aquaria containing a gravel mix and supplied with 4 l of water/min at 10.degree. C. Cleavage eggs and embryos (the egg phases), and eleutheroembryos and pre-emergent alevins (the alevin phases) were dewatered 20 consecutive times in 22-day tests. The egg phases were considerably more tolerant than the alevins. Some cleavage eggs were killed by 12- and 16-h daily dewaterings, but embryos survived up to 22-h daily dewaterings. Embryos also tolerated extended, multiple dewaterings (over 60% survival for 4 consecutive 118-h periods) and 1-time, continuous dewatering for up to 12 consecutive days (> 80% survival). In contrast, about half the eleutheroembryos were killed by 4-h daily dewaterings, and nearly all pre-emergent alevins were killed by 1-h daily dewaterings. Intergravel temperatures were affected by insolation and air temperature. Intergravel temperatures increased to lethal levels during dewatering of cleavage eggs in early fall and limited their survival. Growth of egg phases from some females was retarded by dewatering, but this phenomenon was not consistent for all egg groups. The size of surviving eleutheroembryos decreased as the length of daily dewatering periods increased.

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