480 BULLETIN OF THE BUREAU OF FISHERIES 



in Milford Harbor was studied in relation to the moon's phases and the range of 

 tide. The oysters were found to have spawned (as indicated by the presence and 

 age of larvae) on July 22, which was also at the end of the "full-moon tidal period," 

 or eight days after the time of full moon. Fifteen days later, setting was found 

 to occur not only in Milford Harbor but also in Southport Harbor, Conn., and several 

 other inshore areas that were examined. The relationships between the moon's 

 phases and time of spawning depends largely upon the range of tide occurring during 

 any particular phase. Whether spawning will take place during or at the end of 

 the periods of greater tidal range is determined by the water temperature at the 

 beginning of the period and the weather conditions accompanying it. The effect of 

 the tide on the trend of water temperatures has been discussed on page 449. 

 ^ In this connection, the studies of J. H. Orton in 1925 at Falmouth, England, 



on the lunar periodicity in the spawning of the European oyster (Ostrea edulis) are 

 very interesting. He found that spawning of the European oyster can be corre- 

 lated with the moon's phases but was unable to determine the direct factors control- 

 ling spawning at such times, though he suggests tide, moonlight, food, temperature, 

 and sunshine. Unfortunately, he did not obtain water-temperature records for the 

 J locality in which the observations were made, and consequently, the relationship 



^ u between increase of water temperature and range of tide could not be determined. 



*-V We know from the tide tables that the spring range of tide at Falmouth, England, is 

 nearly 16 feet, more than twice that in Milford Harbor. Under these conditions, 

 it is likely that the water over the Fahnouth beds would show a marked increase 

 in temperature during spring tide periods because of the warming of the water on a 

 greater area of tidal flats at such times. I^is interesting to note^that he found the 

 m aximum percentag e of rece ntly spaw ned-oyatars, i n the w _eel^^ter^uDr moon , 

 which corresponds very closely to the spawning observations made at Milford on 

 the American oyster. AJso, the first and apparently heaviest spawning at Falmouth 

 occurred on virtuallj^ the same date in July, 1925, as did the heavy spawning at 

 Milford. On the basis of our observations, we can state that oyster spawning in 

 Milford Harbor occurred at the end of the full moon tidal periods because of the 

 increase in water temperature that is produced as a result of the greater range of 

 tide during this period. The effect of the moon on the spawning of the American 

 oyster is only indirect through the changes it produces in the vertical and horizontal 

 movement of the water over the oyster beds. The effect of the tide on water tem- 

 peratures and spawning in any locaUty depends largely upon hydrograghic condi- 

 tions and especially the tidal range and area of tidal flats. 



Another important observation made on oyster spawning in the harbor was 

 that the discharge of spawn occu rs nea r or at the time of high wate r. From July 

 21 to 29, 1926, the water temperature on the last of ebb tide and at low water 

 was often from 20° to 26° C, and yet the oysters failed to spawn. The same oysters, 

 when placed in water pumped at high tide and warmed to the same degree, spawned^ 

 in a very short time. It was observed also that oysters kept in the floats ajwava f ^ 

 s,pawned near the time of high water, at temperatures ranging from 20° to 24° C.,^ * 

 and never at low~water, though it was several degrees warmer. In analyzing the 

 factors of temperature, salinity, and hydrogen-ion concentration at the time when 

 spawming occurred, it was found that the hydrogen-io n concen tration orpH value 

 of the water showed the greatest difference. In all the observations but one, the 



