496 



BULLETIN OF THE BUREAU OF FISHERIES 



from normal by reference to the air temperatures at near-by stations. If water 

 temperatures for this period are found to be above normal, the outlook for setting 

 is favorable; when below, it is unfavorable. The mean daily water temperature 

 on the bottom or the temperature at the times of high and low water must be 

 determined for a few days around the 1st of July, as it has direct bearing on the attain- 

 ment of a spawTiing temperature later. 



The third condition, quantity of spawn per oyster and its degree of ripeness, 

 can be determined by the methods discussed previously on page 474. These factors 

 not only reflect water temperatures that occurred previously, but will show how 

 great a quantity of spawn can be discharged by each oyster when water temperatures 

 are suitable. Successful spawning can be expected only at a temperature of 20° C. 

 or above, when the gonads are ripe and when a sufficient amount of spawn is released 

 to insure fertilization. 



The final analysis of the first three conditions mentioned will indicate whether 

 the total amount of spawn available is large or small and whether it is ripe enough 

 to be released by the oysters. It wiU indicate also whether a light, medium, or 

 heavy set can be expected, as this has been found to depend largely upon the com- 

 bination of the first three conditions. 



The fourth condition, range of tide, should be considered for the purpose of 

 determining the time of spawning and setting. The relation of this factor to water 

 temperature and spawning, as discussed previously on page 480, shows us that during 

 the full-moon tidal period, in July or the first part of August, we may expect a rise 

 in water temperature of approximately 10° C. as a result of the warming of the 

 water on a greater area of tidal flats. The time of full-moon tides and the daily 

 range can be found in the tide tables issued by the United States Coast and Geodetic 

 Survey. During this period the date on which the water will reach 20° C. or more 

 certainly depends upon weather conditions and the temperature of the water at the 

 beginning of the period. The records for 1925 and 1926 show that when the tem- 

 perature at the beginning of the full-moon tidal period was 16° to 18° C, spawning 

 occurred about 10 days later in the harbor and inshore areas and about 15 days later, 

 or at the end of this period, in Long Island Sound. In calculating the time of 

 spawning, the ripeness of the oysters and early July water temperatures must be 

 taken into consideration. The time of setting depends, of course, on spawning and 

 will foUow it by an interval of approximately two weeks. 



In the following table a comparison is given of the four conditions as they were ob- 

 served in Milford Harbor on July 1 in 1925, 1926, and 1927, together with the results 

 obtained each year in regard to time of spawning and setting and intensity of setting. 



Table 18. — Comparison of conditions in Milford Harbor on July 1 with inlensiiy and lime of selling 



in 1925, 1926, and 1927 



Conditions observed 



1925 



1926 



1927 



Water temperature variations (Apr. 1 to July 1).. 



Water temperature on July 1 (at Station 2) 



Quautity of spawn per oyster 



Condition of gonads.. _ 



Quantity of adult oysters 



(greatest range of tide 



Time of spawning --. 



Time of set ting , 



Intensity of setting (spat per bushel of shells) 



+3.5° C. above normal. 



15.8° C 



Large 



Ripe. 



—3.3° C. below normal, 



13.5° C .- 



Small 



Unripe. 



700 bushels 1,000 bushels. 



July 7.. 

 July 13.. 

 July 29.. 

 15,000... 



July 26.. 

 Aug. 1... 

 Aug. 16. 

 2,000 



— 1.7° C. below normal. 



14° C. 



Small. 



Umripe. 



1,000 bushels. 



July 17. 



July 14. 



Aug. 8. 



2,450. 



