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.

Embryos of Atlantic salmon. Salmo salur L., were incubated from fertilization to completion of hatching at 5 and 10 °C, at 20, 50, and 100% air-saturation, and at several rates of water exchange from 0.2 to 15 ml/s. Developmental rate to various structural stages, expressed as the reciprocal of time units, and hatching time were significantly controlled by temperature, oxygen supply, and rate of water exchange. Survival during embryogenesis and during the hatching period were limited primarily by oxygen supply and secondarily by water exchange, both having highly significant effects. The effect of temperature ranked third but was also statistically significant. Lower temperature reduced the rate of development and enhanced survival through hutching. Developmental rate and survival increased directly with increasing concentration of dissolved oxygen and with increasing rate of water exchange.

Relations among weight (W), surface (S), and volume (V) for freshly fertilized water-hardened Atlantic salmon ova from 5 to 7 mm in diameter are: S = log V2/3, S = 1.21 log W, and V = W/1.2.Fresh weights remained relatively constant from fertilization to the establishment of blood vessels in the yolk sac. Subsequently, there was a rapid increase to 110%, followed by a gradual decline to 105% shortly before hatching. Large blastodiscs weighed 1.2 mg and embryonal weight increased very gradually until late prehatching stages, when growth was accelerated rapidly. Embryos, at hatching, after continuous incubation at 5 and 10°C and three levels of dissolved oxygen, varied from 22.1 mg (5 °C, 100% air saturation) to 11.9 mg (5 °C. 30%) and From 14.5 mg (10 °C. 100%) to 10.0 mg (10 °C, 30%).Cumulative and mean daily yolk depletion declined with lowered oxygen supply, but they were considerably greater at 10 °C than at 5 °C. Gross conversion ratios (including uptake of exogenous materials) indicate that yolk is used more efficiently at 5 °C, and 100% air saturation. Alevins, kept in their original environments, became more efficient than they were in their encapsulated state, and, at 10 °C. 100% air saturation, they overtook those at 5 °C before the completion of yolk absorption. Those in lower oxygen supplies continued to lag, at both temperatures.

Lake trout embryos were exposed to dissolved oxygen levels approximating 2.5, 3.5, 4.5, and 10.5 p.p.m. at each of four temperatures, 2.5 °C, 5.0 °C, 7.5 °C, and 10.0 °C, from fertilization to a late stage of development. The three low oxygen levels caused retardations in the developmental rates resulting in delayed hatching and lengthening of the hatching period, reduction of vitelline circulatory structures, and abnormalities of head and trunk. The low oxygen levels at 10 °C caused total mortality just prior to hatching. These results are discussed and compared with previous findings.

Embryos of the brook trout (Salzteli,nus Jontinalis Mitchill) were incubated in reduced levels of dissolved oxygen of approximately 2.5 ppm, 3.5 ppm, 4.5 ppm, and a control level near air-saturation (10 ppm) at each four temperatures, 2.5, 5.0, 7.5, and 10.0 C, from fertilization to a late stage of development. Embryos of the rainbow trout (Salmo guird.nerd Richardson) were incubated in a similar experiment at levels of oxygen approximating 2.5 and 3.5 ppm and a level near air-saturation at each of the four temperatures, to a similar, late stage of development. A third small experiment was conducted in which ernbryos of rainbow trout were incubated at 12.5, 15.0, and 17.5 C, at a level of dissolved oxygen near air-saturation. In all instances for both species the velocity of embryonic development, measured by the times required to attain a series of selected embryonic stages, was accelerated by increasing temperature. The velocity of development was increasingly retarded by progressively lorver levels of dissolved oxygen. The times required to affect hatching and the lengths of the periods of hatching were similarly inlluenced by temperature and by the level of dissolved oxygen. These findings are discussed in the light of pertinent literature. JouRN.\L FlstruRlns RESttARCtI .BOARD o!-cANAljA.