SynopsisUpper loss of equilibrium and death temperatures were determined by the critical thermal maximum (CTM) method for juvenle coho salmon (Oncorhynchus kisutch) acclimated to 5 and 15" C, and for pumpkinseed sunfish (Lepomis gibboa s ) acclimated to 10 and 20' C. Temperature increase rates were 1, 6, 18, 30 and 60" C h-I . Analysis of variance showed significant (P < 0.5) differences due to acclimation level, temperature increase rate, and an interaction between these variables. CTM values for coho, a stenothermal fish, were lower than those for pumpkinseed, an eurythermal fish. Generally, higher acclimations and more rapid mtes of temperature increase produced higher CTM values. Upward acclimation during testing was detected only for 10" C acclimated pumpkinseed at a lo C h-' increase rate.Assessments were made of CTM procedural relationships from test data and the literature. A lo C min-' (60" C h-') temperature increase rate is so rapid that internal body temperatures of fish will usually lag behind media temperatures.A standard increase rate of 18" C h-I (0.3" C min-') is recommended for routine CTM determinations with fish. Other recommendations are: 1) Test fish should be small, and relatively uniform in size and condition; 2) fhal loss of ability to return upright should be the loss of equilibrium criterion; 3) death also should be included as a criterion in CTM determinations, with the term 'lethal thermal maximum' applied for death temperature; and 4) linearity should be maintained in time-temperature distributions of the heating rate. AU procedures are also applicable to determinations of the critical thermal minimum. An amended description of the CTM is given that expands its scope.
Four intergravel developmental phases of chinook salmon Oncorhynchus tshawytscha were dewatered experimentally in artificial redds. The redds consisted of aquaria containing a gravel mix and supplied with 4 liters of water per minute at 10 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‐hour daily dewaterings, but embryos survived up to 22‐hour daily dewaterings. Embryos also tolerated extended, multiple dewaterings (over 60% survival for four consecutive 118‐hour periods) and one‐time, continuous dewatering for up to 12 consecutive days (over 80% survival). In contrast, about half the eleutheroembryos were killed by 4‐hour daily dewaterings, and nearly all pre‐emergent alevins were killed by 1‐hour 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.
Four intergravel development phases of chinook salmon Oncorhynchu$ tshawyt$cha were experimentally dewatered in artificial redds. The redds consisted of aquaria filled with a gravel mix and supplied with 4 liters of water per minute at 10 C. Cleavage eggs and embryos (the egg phases) and eleutheroembryos and pre-emergent alevins (the alevin phases) were each dewatered once for a continuous period. Egg phases were considerably more tolerant than alevin phases. Cleavage eggs tolerated one-time dewaterings up to 12 consecutive days (98% survival)• the maximum exposure period. Embryos had similar tolerance when dewatered 12 consecutive days (92% survival), but survival declined to 64% and 53% when embryos were dewatered 16 and 20 consecutive days, respectively. Loss of embryos after the longer dewaterings was associated with premature hatch. In contrast, eleutheroembryos tolerated only a 6-hour dewatering period (96% survival) and underwent high mortality when dewatered 48 consecutive hours. Pre-emergent alevins underwent near total mortality when dewatered 6 consecutive hours. Loss of alevin phases was associated with development of functional gill structures.Our research objective was to evaluate, in artificial redds, the effect of dewatering on survival of eggs and alevins of chinook salmon Oncorhynchus tshawytscha and development of eggs. The tolerance of four intergravel development phases to daily alewaterings (sequential exposures measured in hours) was described previously (Becker et al. 1982). In the experiments reported here, we examined the tolerance of the same phases to continuous, one-time alewaterings (prolonged exposures measured in hours or days). We also compared our current results with those of our previous daily dewatering studies.Sequential alewaterings represent daily fluc-
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