246 
BULLETIN OF THE BUREAU OF FISHERIES 
the flood. Experiments with drift bottles demonstrated clearly the dominant drift 
of the ebb current and showed that an object floating freely in the water would be 
carried out of the harbor the first day by the tidal currents and would never be trans- 
ported back to it. This condition has an important bearing on the distribution and 
occurrence of the oyster larvae in the harbor which will be discussed later. 
The most important effect of the tide was found to be in regard to its influence 
on water temperature. The movement of the water over the tidal flats increases 
greatly the effect of solar radiation and air temperature on water temperature and 
accelerates the exchange of heat between the air, land, and water. It was found 
that during full moon tidal periods in July the water temperature increased from 
approximately 15° C. to 25° C. in a period of 15 days. During these periods the 
range of tide is greater than the mean range, and a much larger area of tidal flats 
are brought into contact with the water. This condition combined with the intense 
solar radiation and the high air temperature that occurs at this time of year, is respon- 
sible for the heating of the water to a temperature of 20° C. and above, which is 
necessary for oyster spawning. 
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Figure 29. — Current velocity and height of tide, Milford Harbor 
BIOLOGICAL OBSERVATIONS 
RIPENING OF THE GONADS AND SPAWNING 
Oysters in Milford Harbor were first found to be ripe during the period extending 
from July 1 to 15 — the exact date varying in accordance with water temperatures dur- 
ing the preceding months. When the water and air temperatures from April to July 
were above normal, as in 1925, the oysters were found to be ripe much earlier and 
contained a greater quantity of spawn than they did in 1926 or 1927, when the tem- 
perature for the same period was below normal. The layer of reproductive tissue sur- 
rounding the liver was found to vary in thickness from 1.5 centimeter in 1925 to 
0.5 of a centimeter in 1926 and 1927. The gonads of the oysters were found to be 
soft and lipe on July 1 in 1925, while this condition was not reached until the middle 
of the month during the other two years. 
The time of oyster spawning depends largely upon the attainment of a water 
temperature of at least 20° C. In Milford Harbor there are generally two spawnings; 
the first being very Ught and occurring about the middle of July, while the second or 
heaviest spawning takes place about the 1st of August. In 1925, 1926, and 1927, the 
