Comparative Analysis of lonoregulation in Rainbow Trout (Salmo gairdneri) of Different Sizes Following Rapid and Slow Salinity Adaptation

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Abstract
Plasma ionic values and gill (Na+,K+)-ATPase activity were compared for rainbow trout (Salmo gairdneri) of three different fork lengths (< 11, 11–15, and > 15 cm) following transference (1) from freshwater directly into seawater of 28–29‰ (DTT), (2) from freshwater directly into an estuarine environment at Fortune Bay where salinity fluctuated primarily between 17 and 29‰ (DTF), (3) from freshwater into a salinity that slowly increased to 28‰ in 22 d (SAT), and (4) from freshwater into freshwater (FC). Trout of all sizes displayed better growth, lower mortality, and better ionoregulatory capacity as judged by plasma Na+, K+, and Cl−concentrations and (Na+,K+)-ATPase activity in sea cages in Fortune Bay (DTF). The poorest ionoregulatory capacity and growth was in the direct transfer from freshwater to salinity of 28–29‰ (DTT). Failure of small trout to adapt to constantly high salinity of 28–29‰ was due to a slower development of (Na+,K+)-ATPase activity, excessively high plasma Na+and Cl−concentrations, hemoconcentration, and tissue dehydration. Except for trout severely stressed by high salinity, growth in seawater was more favorable than in freshwater. Prince Edward Island estuaries are very favorable as adaptation and rearing areas for small and large rainbow trout.