436 



BULLETIN OP THE BUREAU OF FISHERIES 



bodies of water, where there is considerable variation according to the day and 

 even the hour. 



In beginning our analysis of the data, we find that the mean temperature for 

 the 2-month period was 20.3° C. for 1925 and 19.7° C. for 1926, showing a slight 

 annual difference of less than 1°. The monthly and daily averages, however, show 

 greater differences between the two years and are of greater importance, as will be 

 shown later because of the effect of temperature on (1) the ripening of the gonads and 

 (2) the spawning reaction of the oyster. In 1925 the mean monthly water tempera- 

 ture during July was 19.7° C, while in 1926 it was 17.8° C, or 1.9° lower. During 

 August, 1925, the monthly mean was 20.8° C. and in 1926, 21.6° C. As can be seen 

 from Figure 3, the water temperature on certain days was above 20° C, and we are 

 interested in discovering the difference between the two years in relation to this point, 

 because it has been found by previous observation that spawning of the oysters 

 occurred after the water had reached this temperature. In July, 1925, there were 



t'C 

 30 



26 



26 



24 



22 



20 



16 



16^ 



14 



/2 



I 3 / y II li n 1/ ly di a dn n a a i ^ a o lu n w it /a <;u zi dt ib ya du 

 JULY AUGUST 



Figure 3.— Mean daily bottom temperature, ° C, at Station 2, Milford Harbor. Computed from thermograph records 



15 days when the temperature was above 20° C and 16 when it was below, while in 

 1926 there were only 7 days when it was above and 24 days when it was below. In 

 August, 1925, there were 19 days when the water temperature was above 20° C. and 

 12 when it was below, while in 1926 it was above this point for 24 days and below only 

 7 days. These data are presented in the following table, together with the highest 

 and lowest water temperatures for each month. 



Table 1. — Fluctuations in daily water temperature from 80° C. 



