SEASONAL MORTALITY OF RAINBOW-TROUT (SALMO-GAIRDNERI) PLANTED IN SMALL EUTROPHIC LAKES OF CENTRAL CANADA

The seasonal mortality pattern and the causes of the mortality of planted rainbow trout (S. gairdneri) were studied in 8 small eutrophic lakes in central Canada used for extensive aquaculture. Two periods of mortality were revealed. The first occurred in all lakes during the first 60 days of residence in the lakes and resulted in the mortality of 60-90% of the numbers of fish stocked. The 2nd period occurred in about 20% of the lakes and was due to low O2 levels following the collapse of a bloom of Aphanizomenon flos-aquae. Rainbow trout are close to their physiological upper tolerance levels in these lakes. Possible environmental and biological causes of mortality during the 1st period are discussed.