Fingerling brook trout, Salvelinusfontinalis (Mitchill), were subjected to acute and chronic pH stress for maximum periods of 10 000 min in a series of continuous-flow dilutions of sulphuric acid or sodium hydroxide from pH 2.2 to 10.8 at 10 and 20 °C after acclimation at 15 °C and pH 6.8. Various tissues, gill, eye, naris, integument, and alimentary tract, were examined histologically and compared with control samples kept at pH 6.8. No differences in the degree or form of tissue injury were detected between series for corresponding pH levels at 10 and 20 °C. Thresholds for tissue and cellular derangements were pH 5.2 and 9.0. Mucous cells of gills, nares, and integument exhibited progressive degrees of hypertrophy and excessive secretion of mucus with increased pH stress. Epithelial necrosis and sloughing occurred extensively on gills, corneae, and integument. At the lethal levels (pH 3.5 and 9.8), epithelial necrosis also occurred in the esophagus. No cellular injury was detected in the stomach or any part of the intestine.

Hatching of Atlantic salmon (Salmo salar) eggs was delayed or prevented if they were exposed to water of lowered pH (4.0–5.5) after eye pigmentation had developed. Hatching subsequently could be induced by returning eggs to normal pH levels (6.6–6.8). Perivitelline pH fell rapidly to near ambient levels when eggs were exposed to low pH. It is suggested that the observed effects on hatching were due to inhibition of the hatching enzyme, chorionase.Key words: Atlantic salmon, eggs, pH, perivitelline fluid, chorionase

Embryos and alevins of Atlantic salmon, Salmo salar L., at various stages of development were subjected to several levels of low pH ranging from 4.3 to 2.7, for maximum exposures of 10 000 min in static bioassays at 5 or 6 °C, after incubation from fertilization at pH 6.9–6.6 and 5.0–6.7 °C. Sensitivity of the embryos and alevins to pH stress was dependent on the stage of development. Embryos in early cleavage were more sensitive to low pH than older encapsulated embryos. Alevins were more sensitive to low pH than embryos. The lower lethal limit for embryos during early cleavage was about pH 3.6, while for older embryos in two sequential stages just before hatch it was about pH 3.0 and pH 3.1, successively. Alevins subjected to low pH levels at 7 days and at 28 days after median hatch had a lower lethal limit of about pH 4.0.

Embryos and subsequent alevins of the Atlantic salmon, Salmo salar L., were incubated continuously from fertilization to a time 40 days after hatching at six levels of pH, 6.8 (control), 5.0, 4.5, 4.2, 4.0, and 3.7, at 5–6 °C. Embryos had an LL50 of pH 3.9. The rate of embryonal development was not retarded by these levels. The LL50 for alevins was pH 4.3. Externally deformed alevins were few and not related in frequency to particular levels of pH. Populations of Atlantic salmon will decline and ultimately disappear from freshwater habitats whenever the level of pH approaches 4.5 for an extended period.

Fingerling brook trout, Salvelinus fontinalis (Mitchill), were subjected to acute and chronic pH stress for maximum periods of 10 000 min in a series of dilutions of sulfuric acid or sodium hydroxide at 10 and 20C after acclimation at 15C and pH 6.8. Lethal limits of pH were estimated to be pH 3.5 and 9.8 at both temperatures. There were no observable differences in the resistance times at lethal pH stresses between test temperatures. The order of death was not significant in relation to length or weight of the fish.