1990. Responses of plasma electrolytes, thyroid hormswes, and gill histology in Atlantic salmon (Sa/ms salar) to acid and limed river waters. Can. j. Fish. Aquat. Sci. 47: 2431-2440.Sexually maturing Atlantic salmon, Salrno saiar, were held, in the acidic (pH range 4.7-5.2) Westfield River, Nova Scotia and in the nearby, less acidic (pH range 5.2-5.6) Medway River. Exposure to Westfield River water in 1985 (149 d) and 1986 (1 26 d) reduced plasma osrnolality, Na', Cl-, and Ca' ' (in females only) concentrations of post-spawning fish compared to those in fish held in the Medway River. There were coincidental increases in piasma M', glucose, and unidentified osmotic fraction (UBF). Gill tissue showed hyperplasia of primary Bamellae epithelium. Together, these findings indicate compromised ionoregulatory ability. Decreased plasmaT, (3,5,3'-triiodo-l-thyronine) suggests altered thyroid function. Westfield River water did not affect plasma T,(L-thyroxine) or protein concentrations. An unintentional handling stress caused even more severely depressed plasma ions and more elevated plasma glucose in Westfield fish in 1985 relative to 1986; Medway fish largely recovered from this stress. These observations indicate that acid-exposed fish may be more sensitive to addit'sonal stressors. Limestone treatment of Westfield River water (elevating its pH to Medway values) ameliorated ionoregulatory ability but did not affect plasma T, and Ca' " (female). A high salt diet (3% NaCI) failed to protect salmon from the effects of acidic water.Des saumons de I'Atlantique, Salrno salar, en voie de maturation sexuelle, ont ete plac6s dans I'eau acide (pH 4,7-5,2) de la rivigre Westfieid (~ouvelle-fcosse) et dans la rivigre Medway avoisinante, moins acide (pH 5, 2-5,6).Une exposition A I'eau de la riviere Westfield en 1985 (149 j) et en 1986 (1 26 j) a reduit l'ssmolalit6 du plasma, les concentrations (chez les femelles sesrlement) de Na', Cl-et Ca", apres la fraye, csmparativement 3 celles des poissons places dans la rivigre Medway. Par coTncidence, on a enregistre des augmentations du K', du glucose et de la fraction ssrnotique non identifi 6e du plasma. Le tissu branchial presentait une hyperplasie de If6pith4t ium des lamel les primaires. Tous ces resu ltats revglent une aptitude ionor6gulatrice alt6ree. Une diminution de la T, (3'5'3'-triiodo-%-thyronine) laisse supposer une alt4ration de la fonction tkyroTdienne. L'eau de la rivi6re Westfield n'a pas affect6e la concentration de la T, (L-thyroxine) ni les teneurs en prot6iwes dans le plasma. Une mauvaise manipulation involontaire a cause une baisse encore beaucoup plus importante des ions plasmatiques et 4levation plus forte dsr glucose plasrnatique chez les poissons de la rivi$re Westfield en 1985 par rapport 3 1986; les poissons de la rivi&re Westfield se sont sensiblement biew rktablis de cette agression. Drapr&s ces observations, les poissows en milieu acide peuvent etre plus sensibles A d'autres agents de stress. Le traitement h la chaux de la rivi&re Westfield (en 6levant son ...