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Freshwater Atlantic salmon (Salmo salar L.) smolts were abruptly transferred to sea water in May and over 3 days blood plasma ion concentrations were determined together with atrial natriuretic peptide (ANP) and plasma renin activity (PRA) using antibodies raised against human ANP and angiotensin I. Blood plasma Na+ and Cl levels in smolts increased and, between 24 and 72 h, PRA increased significantly to 0.9 ng ml−1 h−1, while there was a gradual rise in ANP levels to 10 pmol l−1 at 72 h. Similar measurements were made on parr transferred to sea water in September; in these fish Na+ and Cl levels increased as in smolts, PRA remained unchanged at about 0.6 ng ml−1 h−1 and ANP levels increased significantly to about 20pmoll−1 at 24 and 72 h. After 2h in sea water parr showed wide variability in ANP levels, in keeping with circulatory stress, hypoxia and increased atrial stretching. Parr transferred to sea water in December showed low drinking rates of 1.95 ml kg−1 h−1, even after 20 days, compared to a high drinking rate of about 7 ml kg−1 h−1 reported for smolts transferred in May.

Rainbow trout acclimated to sea water for 3 weeks showed elevated levels of both ANP and PRA, similar to values reported for marine species. Freshwater rainbow trout fed a high-salt diet (12% NaCl) showed significantly elevated levels of ANP compared to fish fed a normal diet (1.5 % NaCl), while PRA levels were depressed, though not significantly.

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