The survival and physiology of rainbow trout in alkaline waters was the focus of this thesis. It is known that salmonids have problems with ammonia excretion in alkaline water (Cameron and Heisler, 1983; Wright and Wood, 1985). The first set of studies attempted to increase the survival rates of rainbow trout planted into alkaline lakes by acclimating them to the alkaline conditions before their release. The first field study acclimated rainbow trout to alkaline waters by acidifying the lake water with C0₂ in order to reduce the magnitude of the pH change experienced by the fish. The second field study acclimated rainbow trout to alkaline waters by increasing the alkalinity of the hatchery water in which the fish were held, over a six day period. In both studies the acclimated fish experienced higher survival rates relative to non-acclimated fish. Plasma sodium concentrations ([Na⁺]p1) of the fish were shown to increase, while plasma chloride concentrations decreased. These changes were attributed to an increase in the exchange of external Na⁺ with endogenous H⁺, and the decrease in the exchange of endogenous HC0₃⁻ with external Cl⁻, respectively. The increased [Na⁺]pl may have also been the result of the exchange of plasma ammonium (NH₄⁺) with external Na⁺. The second set of studies investigated the physiological response of rainbow trout to alkaline waters. The first study, the chronic exposure of rainbow trout to alkaline water, showed that trout in hard alkaline water experienced higher survival rates and regulated plasma ammonia and ion concentrations more competently than trout in soft alkaline water. This increased ability to regulate plasma ammonia and ion concentrations was attributed to the possible "reactivation" of the Na⁺/NH₄⁺ exchange. The purpose of the second study, the acute exposure of rainbow trout to alkaline water, was to further investigate the mechanisms that enable fish in hard alkaline water to survive better than fish in soft alkaline water. The possible activity of the Na⁺/NH₄⁺ exchange was again observed in the hard alkaline water. The addition of amiloride to the alkaline hard treatment water increased plasma total ammonia and stabilized [Na⁺]pl′, which supported the "reactivation" of the Na⁺/NH₄⁺ exchange in hard alkaline water. As a result of these studies, the acclimation of rainbow trout to hard alkaline water before being planted into any alkaline body of water was recommended. / Land and Food Systems, Faculty of / Graduate