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1. Two levels of oxygen uptake, (1) the lowest point in the resting metabolism in the daily cycle and (2) the maximum steady rate of oxygen uptake found when the fish were stimulated to activity in a rotating chamber, were measured at temperatures from 5 to 35 C.2. The standard (resting) rate was measured over levels of oxygen high enough

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When goldfish are placed in a rotating annular chamber of which the outer wall is glass, they can be induced to swim steadily at a rate which varies with the temperature when the water is aerated. When the subjects have been in each case previously acclimated to the experimental temperature the best performance is attained at temperatures from 20 to 30 °C., the highest temperature to which goldfish can be acclimated being 41 °C. Subjects acclimated to a variety of experimental temperatures gave curves of performance in relation to temperature which were unique for each level of acclimation.

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The ultimate upper lethal temperature is estimated to be 23.5" C. The temperature for maximum activity is from 15" C.-17" C. Active respiration becomes dependent on the oxygen pressure a t approximately two-thirds air saturation. The minitnun1 oxygen pressure for this species is approximately 40 mm. H g a t 20" C. and 60 mnl. H g a t 20° C. These data pertain t o oneand two-year fish.Salvelinus namaycush ranks in importance with S. fontinalis, the eastern Brook trout, as a game fish in eastern North America and is also a commercial fish in the Great Lakes. However, in contrast to the brook trout for which a substantial body of knowledge exists (e.g. 3), virtually nothing is known of the environmental physiology of the lake trout. The present study is therefore a contribution t o fill this gap. In it we have attempted a laboratory definition of the scope for activity afforded to the lake trout by various levels of oxygen and temperature. Materials and MethodsThe fish used in these tests were hatchery stock, one and two years old, obtained from hatcheries operated by the Province of Ontario. The one-year stocli had an average weight of 27.7 gm. with a range from 16.4-37.9 gm. The two-year stock had an average weight of 82.8 gm. and ranged from 57.6-120.6 gm. While in the laboratory the fish were fed freshly ground liver ad libitum every day and not less than 12 hr. before each experiment. Prior to any test, except in the tests to determine lethal temperatures, the fish were maintained in well aerated water a t the temperature a t which the test was to be carried out for a t least a week. In the lethal temperature experiments the thermal history was similarly controlled but the acclimation and the test temperatures did not of course coincide in these experiments.The general plan of experimentation was that described elsewhere as noted below. Lethal limits of temperature were determined according to the method described by Fry, Hart, and Walker (7), who employed a variant of the method in which the time to death is determined for a sample of fish suddenly exposed t o a constant lethal level. T h e lethal baths were shallow tanks about 22 in. square and 6 in. deep, in which the temperature was controlled to & 0 . lo C. For a recent discussion and application of this method see Brett (1).Two levels of metabolism, a standard rate which was the lowest rate of oxygen uptake measured for a quiescent fish in the die1 cycle, and an active Manz~script receiued Nouember 23, 1953.

Coefficients of temperature change in relation to body size are given for three species of fish native in Lake Huron, subjected to a sudden drop in ambient temperature of about 5C. These observations together with published data provide cooling coefficients for fish and aquatic reptiles over a weight range of 1–5000 g. The slope of the regression-log cooling coefficient on log body weight is about −0.6. The hypothesis offered is that the magnitude of the slope in question results from the temperature measured being close to the center of a body of constant thermal conductivity, retaining essentially the same proportions throughout growth.In addition, various observations of excess deep-body temperatures were collected in relation to weight for poikilothermal fish and aquatic reptiles, all presumed to be in thermal equilibrium with their environment. A double logarithmic plot of deep body temperature on weight in this case has a slope of about 0.4. The relation between these two slopes appears to be satisfactorily explained by the general observation that the slope of a double logarithmic plot of metabolic rate per unit weight on body weight has a slope of about −0.2.