Death of Salmon and Trout with High Temperature

Deaths in Moser river, N.S., in 1939 were at time of day, on days, and in locality with highest water temperature. Symptoms were loss of sensitivity to light and very rapid breathing with recovery therefrom in cool water; otherwise followed by loss of equilibrium and cessation of breathing with death. Freshly run grilse died at about 29.5 °C. and acclimated grilse at about 30.5°. Large salmon died first and parr survived. In experiment, the latter died between 32.9 and 33.8°, the smallest at the highest point. Death is considered due directly to temperature.Rainfall after long period of drought brought the salmon from the sea into the river to die where water was shallowest and without cascades, therefore warmest. The very high temperatures and deaths occurred in afternoon from sun's heat and at ends of periods of days with moist air from the ocean and mounting water temperatures, dew point temperatures and mimimal air temperatures.Prevention is seen possible by bringing fish in early with artificial freshets, by constructing low dams to create deep pools and cascades, by making a deep channel or by raising level at critical periods with stored water.