326 BULLETIN OF THE BUREAU OF FISHERIES. 
oxygen at such depths was about 0.05 c. c. per liter and the carbon dioxide, 5 c. c. per 
liter. The perch were allowed to remain for varying lengths of time in the stagnant 
water and then were pulled quickly (10 seconds) to the surface and placed in a large jar 
of water pumped from the same depth at which they had been submerged. They were 
opened as soon as possible under water and the gas in the bladder siphoned out into a 
mercury-filled collecting tube (fig. 32) of 5 c. c. capacity. The samples were carried into 
the laboratory in the collecting tubes and analyzed with a Haldane apparatus, the 
oxygen being absorbed with 10 per cent alkaline pyrogallol and the carbon dioxide with 
Io per cent potassium hydroxide. Every time samples were taken from perch which had 
been submerged in deep water, two or three control individuals which had been in a fish 
car at the surface were also tested. The details of the results of the analyses are shown 
in Table 23, and a summary is given in Table 24. Though considerable variation is 
shown, the latter table indicates that the oxygen in the bladder was used up while the 
perch were in the stagnant water, but the carbon dioxide did not increase. 
From the studies in Lake Mendota the following facts have been ascertained: 
(1) Perch commonly go into the stagnant water below the thermocline, where there is 
only a fraction of 1 per cent of oxygen per liter; (2) they may remain there for two 
hours or more without suffocating, but it is doubtful if they would feed for more than a 
few minutes; (3) when perch invade water which does not contain sufficient oxygen for 
respiration they apparently draw to some extent on the reserve in the swim bladder; 
(4) if through the action of the bladder as a hydrostatic organ a perch is adjusted to 
pressure conditions above the thermocline, it will, if it invades the lower regions where it 
is overcome by lack of oxygen and excess of carbon dioxide, tend to float up into levels 
where gas conditions are more favorable. 
REPRODUCTION. 
It has already been pointed out that the perch in Lake Wingra are generally of 
smaller size than those in Lake Mendota. This backwardness in growth, however, does 
not appear to retard the attainment of sexual maturity (Table 25). Judging by the 
measurements made on individuals from a school of young perch which remained near 
the base of Picnic Point (fig. 2) during the summer of 1916 (Table 18), and by obser- 
vations on the gonads of half-grown perch at various seasons, the authors believe that 
perch may become sexually mature in Lake Mendota at the end of two years of growth. 
Meek (1916), speaking of conditions in Europe, says: ‘“‘The perch appears to become 
mature when it is three years old.”’ 
After a perch attains sexual maturity the gonads in both sexes pass through a regular 
cycle of seasonal changes. After spawning is completed, the gonads remain small until 
late summer and then increase very rapidly in size for a month or more... By September 
they are almost as large asin thespring. The growth of the gonads, then, takes place for 
the most part in the summer, when food is most abundant, and there is little change in 
size during the winter months. By November, perch caught in deep water (18 to 20 m.) 
will often shed eggs when brought to the surface. Such individuals are, of course, not 
completely “ripe” but emit eggs on account of the decrease in pressure. Prof. C. L. 
Turner (1919) has made a careful study of the volumetric and cytological changes in 
perch gonads at Milwaukee, and his paper gives detailed information concerning the 
annual reproductive cycle. 
