use of destruction rates obtained over a short period of the feeding season in 

 calculating predation rates for the entire season, Pope (1910-11) also mentions 

 that drills continued to feed during the spawning period in his observations. The 

 feeding season m eastern Canada according to Adams (1947) lasts 18 weeks It 

 begins in the spring when the temperature of the water reaches 15° C and terminates 

 in the fall as temperatures diaop to 12.8°C. Engle (1953) using drills collected in 

 the vicinity of Milford, Connecticut, carried out a number of detailed laboratory 

 studies relating the destruction rate of one inch oysters by drills of various sizes 

 to temperature m salinities ranging from 22 to 27 o/oo (see Table 9). He shows 

 clearly that within each size group of drills the rate of destruction of oysters in- 

 creases noticeably with an increase in temperature . This is also reflected in 

 figures for the average destruction of oysters per drill per month for all sizes of 

 drills from 16 to 26 mm. in height over the feeding period from May to December, 

 respectively: 0.9, 3.7, 4 9, 9.9, 4.5, 2.0, 0.2. In all size groups within this 

 range the greatest destruction rate took place during the month of August . 



Andrews and McHugh (pers. com.) at Gloucester Point, Virginia, con- 

 tribute further information on the influence of seasonal temperatures on the rate 

 of destruction of young oysters . One hundred and twenty four drills ranging in 

 height from 5 to 17 mm. were placed with caged oyster spat 10 to 15 mm. m diameter 

 on January 20, 1954. During the following months the average number of oysters 

 destroyed per drill per week was recorded as follows: January 20 -March 10, at an 

 average temperature of 6.5° C, 0.06 oyster; March 10-Apnl 15, atll.5°C, 23 

 oyster; and April 15-May 20, at 18.5°C, 0.19 oyster. That less oysters were 

 consumed during the last period than during the middle period is explained by the 

 fact that the oysters continued to grow larger during the observations and that 

 supplementary food in the form of barnacles and other organisms set in the cage. 



Mackm (1946) has shown that in Virginia the rate of predation of oysters 

 by Urosalpinx also appears to be influenced by the duration of exposure in the 

 intertidal zone . In Finney Creek where oyster spat set abundantly as high as 

 three feet above low water mark on vertical frames, he found that 83% of the spat 

 between -2 and -1 feet below mean low water were drilled; between -1 and feet, 

 75%; between and 1 foot above low water, 35%; and above this level no drilling 

 occurred. In other areas slight drilling was detected up to two feet above low water 

 mark. Chestnut and Fahy (1953) in a comparative study of the vertical distribution 

 of oysters and of oyster setting in five different localities in North Carolina observed 

 that setting below low water occurred far in excess of that above low water and in 

 general increased in intensity bottomward, but that drilling of these spat by 

 Urosalpinx was not intense nearest the bottom and diminished in intensity off the 

 bottom . Chestnut and Fahy conclude that the high rate of mortality of young oysters 

 below low water level may offer a partial explanation for the peculiar distribution 

 of adult oysters principally in the intertidal zone in this area . 



58 



