The ability of rainbow trout (Salmo gairdneri Richardson) to acclimate to sea water or partially diluted sea water has been studied many times (Houston, 1959; Parry, 1960; Eddy and Bath, 1979; Prunet and Boeuf, 1985). However, the reverse transition from sea water (SW) to fresh water (FW) has been less well studied. The transition from a hyperosmotic medium to a hypo-osmotic medium reverses the burden on the osmoregulatory organs, i.e. the gills and the kidney. SW teleost fish normally show net

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... salt (NaCl) efflux, whereas FW fish need to acquire salt from the medium. Recent work on salmon (Salmo salar) transferred from SW to FW (Talbot and Potts, 1989; Talbot et al. 1989; Potts et al. 1989) has described the changes in sodium balance, osmoregulation and renal function in adult and juvenile fish. Potts et al. (1985) showed that salmon are capable of taking up sodium immediately on transfer to FW. Chloride uptake via the gills of FW fish is of equal importance to sodium uptake, and the two ions are thought to be transported independently (Maetz and Garcia-Romeu, 1964). This paper examines the changes in chloride transport that occur during the transition from SW to FW.