Embryos of steelhead trout, Salmo gairdneri gairdneri Richardson, and chinook salmon, Oncorhynchus tshawytscha (Walbaum), were reared from fertilization of the eggs to hatching at different constant oxygen concentrations and water velocities. For this purpose, an apparatus was developed that makes it possible to control oxygen concentration independently of water velocity, which was maintained at levels ranging from 6 to 1,350 centimeters per hour. Measurements of the embryos and hatched fry indicate that water velocities must be high enough not only to transport enough oxygen to the redd for supplying the total requirement of all embryos, but also to deliver sufficient oxygen to the surface of the chorion enveloping the individual embryo. Steelhead embryos held at 9.5 ø C. and chinook salmon embryos held at 11 ø C. all died at an oxygen concentration of 1.6 mg/1. Survival of large percentages of embryos reared at concentration as low as 2.5 mg/1 was apparently made possible by reduction of respiration rates and consequent reduction of growth and development rates. Sac fry from embryos reared at low and intermediate oxygen concentrations were smaller and weaker than sac fry from embryos reared at high concentrations. Although weak sac fry may survive under laboratory conditions, they cannot be expected to do so in nature. The size of steelhead trout and chinook salmon fry at hatching probably was dependent on water velocity even at velocities as high as 740 and 1,350 cm/hr, respectively, and on oxygen concentration even at concentrations near saturation levels. Mean size differences among embryos reared under different conditions at the higher velocity and oxygen-concentration levels were not great, particularly in the case of the steelhead trout. t

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Embryos of coho salmon, Oncorhynchus kisutch (Walbaum), and steelhead trout, Salmo gairdneri gairdneri Richardson, were reared from fertilization of the eggs to hatching, at about 10° C, at different concentrations of dissolved oxygen ranging from about 2.5 to 11.5 mg/liter and at different water velocities ranging from about 3 to 750 cm/hour. Some of the embryos rested on porous plates, while others were buried in glass beads so as to simulate natural conditions more closely. Fry from embryos reared at low and intermediate oxygen concentrations hatched later and were smaller in size at hatching than fry from embryos reared at concentrations near the air‐saturation level. At all oxygen concentrations tested, reduced water velocities resulted in reduced size of hatching fry. This effect of velocity was nearly as pronounced at high oxygen concentrations as at low concentrations. The effect of the difference of water velocities tested was less than the effect of the difference of oxygen concentrations tested. When some embryos were buried in glass beads while others were not, and the discharge rates of water through cylinders containing the embryos were the same, the fry that hatched in the cylinders containing beads were larger in size than those in cylinders without beads. This effect is ascribed to the increase of water velocities around the embryos buried in beads. It was usually most pronounced when a mixture of large and small beads was used.

Experiments are reported on the influence of nearly constant dissolved oxygen concentrations, both below and above the air-saturation level, and of wide diurnal fluctuations of oxygen concentration on the appetite, growth, and food conversion efficiency of juvenile largemouth bass, Micropterus salmoides (Lacépède). The experimental apparatus used was designed to provide constant flows of water at 26 C and with controlled oxygen content through 12-gal (45-liter) bottles each containing 10 test fish. The fish were fed unrestricted rations of small, live earthworms throughout the six experiments, whose duration was usually 15 days.The growth rates and food consumption rates of the bass increased markedly with increase of the constant oxygen concentrations to levels near the air-saturation level, and declined with further increase of oxygen concentrations. Gross food conversion efficiencies were considerably reduced only at concentrations well below 4 mg/liter.The growth of bass subjected alternately to low and higher oxygen concentrations for either equal or unequal portions of each 24-hr day was markedly impaired. It was almost always less than that which presumably would have occurred had the fish been held at a constant concentration equal to the mean, either arithmetic or geometric, of the higher and lower concentrations to which the fish had been exposed.