salinity of only 5 o/oo, Loosanoff s lower salinity tolerance figure of 7 5 o/oo 

 for oysters normally living in a salinity of 27 o/oo, when compared to the i eported 

 lower limit of 16 at summer temperatures for drills from Long Island Sound 

 (Eagle, 1953), suggests a relatively wide salinity range here tolerable to oysters 

 but intolerable to drills. It is quite probable, as Loosanoff's data suggest, that 

 oysters, like drills, normally living in lower salinities possess lower limits of 

 salinity tolerance. These should be determined routinely for both drills and 

 oysters so that the range of salinity, if present, within which oysters may be 

 cultured free of drills may be known for specific oyster growing regions. 



Mud and sand bottom devoid of hard objects 



After a ground has been cleaned of drills and planted with seed oysters the 

 new stock may be partially protected against the invasion of drills by the presence 

 around the planting of a clean unplanted zone (Lindsay & McMillin, 1950; Cole, 

 1951; H. B. Flower, 1948). Andrews (pers . com.) notes that this may be effective 

 simply because food organisms are far enough removed that drills do not detect 

 them, rather than that drills are unable to cross such waste bottom , This is 

 possible except where drills are located immediately up or down stream from food 

 organisms. The addition of a continuous line of drill traps around the outer edge 

 of the clean zone is recommended for further protection (Galtsoff et al ., 1937) 

 The use of a zone of bare ground in the control of drills by means of the hydraulic 

 suction dredge by H. B Flower (1948) has already been described in a previous 

 section . 



Temporary abandonment of bottom 



Stauber (1943) has suggested that the procedure of permitting oyster grounds 

 to "lie fallow" should be seriously considered as a part of commercial oyster 

 management He notes that bottom becomes relatively barren soon after oysters 

 are dredged for market, and that the commoner organisms such as drills and boring 

 sponge are greatly reduced in numbers; that colonization of a bottom with oysters 

 appears to produce favorable conditions for the attachment and emigration of other 

 animals, thus intensifying pest problems By way of example he cites the case of a 

 drill infested oyster bottom on which a moderate population of oysters had been 

 raised from a natural spatfall. Without the application of drill control measures 

 the owner next planted clean cultch, and obtained a good concentration of spat, but 

 by fall all of these had been destroyed by drills . By contrast the same year a similar 

 spatfall struck on an old vacant ground about 1-1/2 miles distant and by fall only 4% 

 of the spat had been drilled. Stauber recommends that a bottom should remain un- 

 used for at least one year . T. C Nelson (pers com), however, believes that the 

 success of ground left "fallow" for a year or more is the exception rather than the 

 rule And Andrews (pers , com .) observes that if drills live four or five or more 



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