Some Effects of Temporary Exposure to Low Dissolved Oxygen Levels on Pacific Salmon Eggs

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Abstract
Eggs of the chum salmon (Oncorhynchus keta) were exposed to various constant levels of dissolved oxygen for a period of 7 days. The procedure was repeated with fresh egg samples at various developmental stages. Temperatures were constant at 10[degree] C from fertilization to hatching. Estimates of oxygen consumption uninhibited by low dissolved oxygen levels were obtained at various stages of egg development for whole eggs and also on the basis of the weight of larvae, exluding the yolk. Eggs were most sensitive to hypoxia between 100-200 C[degree] days and compensated for reduced oxygen availability by reducing the oxygen demand and rate of development. Very low oxygen levels at early incubation stages resulted in the production of monstrosities. When the circulatory system becomes functional, the compensatory reduction in rate of growth under hypoxial conditions is reduced, but eggs no longer survive extreme hypoxial conditions. Eggs, subjected to low dissolved oxygen levels just prior to hatching, hatch prematurely at a rate dependent on the degree of hypoxia. The maximum premature hatching rate corresponded approximately with the median lethal oxygen level. Estimated median lethal levels rose slowly from fertilization to hatching. Oxygen consumption per egg rose from fertilization to hatching while the consumption per gram of larval tissue declined from a high to a low level at about the time of blasto-pore closure. Subsequently, a slight rise in the rate occurred up to a level which was more or less constant to hatching. "Critical" dissolved oxygen levels were calculated and appear to define the oxygen level above which the respiratory rate is unmodified by oxygen availability. Critical levels ranged from about 1 ppm in early stages to over 7 ppm shortly before hatching.

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