498 BULLETIN OF THE BXJKEAU OF FISHERIES 



5. During the past six years oyster setting in Long Island Sound has been suc- 

 cessful when air and water temperatures were above normal and has faUed when 

 they were below normal. 



6. Though some setting occurs each year in Milford Harbor, it was found that 

 over seven times as many spat per bushel were produced in 1925, when early water 

 temperatures were 3.5° C. above normal, than in 1926, when they were 3.3° below 

 normal. ' 



7. Oyster spawning occurred on different dates during each summer and was 

 found to be dependent upon an increase in water temperature to 20° C. and above. 



8. The time of spawning and the greatest increase in water temperature were 

 found to take*place during the "fxill-moon tidal period" in July or the first part of 

 August. As a result of greater range of tide during this period, the water was brought 

 to a spawning temperature by heating on a larger area of tidal flats. 



9. In studying the occurrence and distribution of the oyster larvse, over 315 

 plankton collections were made, which showed that the larvae were relatively scarce 

 in the water in proportion to the number of spat found later in the same areas. 



10. The oyster larvse were found to be most abundant at the time of low slack 

 water and gradually disappeared as the tide began to rim flood. 



11. When the flood current had developed a velocity of 0.6 foot per second, 

 virtually no larvse could be found swimming in the water; while samples of bottom 

 taken at the same time contained an average of 14 larvse per square foot of surface. 



12. Experiments with oyster larvse in a tank showed that they swam while the 

 water was at a standstill but dropped to the bottom when it was put in circulation 

 with a current velocity of 0.3 to 0.5 foot per second. 



13. The majority of oyster larvse produced by the spawning bed in Milford 

 Harbor were found to remain and set within 300 yards of its center. This is accom- 

 plished by the oyster larvse remaining on the bottom during the greater part of the 

 larval period and limiting their swimming activities to the tidal periods, when hori- 

 zontal movement of the water is least. 



14. The dm-ation of the larval period was found to vary from 13 to 16 days. 

 The average of the four periods observed during 1925 and 1926 was 15 days at a mean 

 water temperature of 21.5° C. 



15. Setting of oysters occurs at from 2 feet above low-water mark to the bottom 

 of the channel and is of greatest intensity in a zone lying at from 1 foot above mean 

 low-water mark to 1 foot below it. 



16. Setting or attachment of the larvse was found to take place during low slack 

 water and continued until the flood tide had developed a velocity of 0.33 foot per 

 second or 20 feet per minute. 



17. It was found that the vertical distribution of spat and the intensity of setting 

 in any zone could be correlated with velocity of currents and distribution of oyster 

 larvse. 



18. As a result of the investigation of the physical conditions and the biological 

 studies of the oyster, a method has been developed for determining or predicting, one 

 month in advance, (1) the relative intensity of setting that will occur each year and 

 (2) the time when spawning and setting will take place. 



