migration which a soft mud bottom devoid of shell seems to impose on the drill; 

 (2) the tendency of sand and fine shell fragments to sink into mud and simultaneously 

 of drills to crawl to the surface, thus decreasing the volume of sediment to be 

 transported at each step (for example, the volume of sediment moved during 

 Step 1 was 3,750 cubic yards, and during Step 2, only 375 cubic yards); and (3) 

 the destruction of drills buried deeply in the sediments of the layers of sediment 

 tend to become reversed. It is not known from what depth of sediment drills may 

 emerge and survive . These depths are undoubtedly related to sediment type and 

 particle size, and size of drill. Small drills probably cannot emerge from as great 

 a depth as large drills . The Flower method has the unusual merit of striking with 

 equal, if not greater, severity at the younger drills . It is also evident in this 

 method that egg cases, if dredging is performed during the spawning season, will 

 have to be treated separately on oysters and shell not passing through the screen 

 of the conveyer belt . 



In conclusion it may be suggested that for maximum returns in drill 

 eradication procedures employing suction dredges, careful quantitative checks 

 should be maintained, not only on the rate of removal of drills from grounds being- 

 dredged, but on the trends of drill densities on bottoms between periods of dredging. 

 Since effective operation of the intake nozzle is fundamental to satisfactory opera- 

 tion of the dredge as a whole, periodic tests on its effectiveness are important. In 

 addition, a more complete knowledge of the behavior and life history of the drill 

 would unquestionably contribute much in the development of improved design and 

 operation of these dredges. J. R. Nelson (pers. com.), for example, emphasizes 

 the fact that information on the seasonal habits of the drill would aid materially in 

 the development of improved nozzle design-- considerable information of this kind 

 is now available It is evident that further research on both the dredges and the 

 drill, on a cooperative basis s by dredge operators and marine biologists is to be 

 recommended as an important step toward achieving maximum efficiency and economy 

 in drill control . 



Fate of drills passing through suction dredges 



H. B. Flower (1948) has questioned the survival of drills which pass through 

 a suction dredge, F. B, Flower, working at the Oyster Research Laboratory, New 

 Jersey (pers. com.), has shown that such treatment does not seem to injure them. 

 Forty-eight drills varying in height from 6 .4 to 22 mm. and which passed through 

 the 8 inch centrifugal suction pump of the "Luther Bateman" dredge in October, 1953, 

 weie placed in aquaria in the laboratory for observation. They were maintained in 

 water of a salinity of 24 o/oo, at temperatures varying from 16.8 to 20.8 D C, and 

 1953 oyster set was added for food. Twenty-five drills tanging in height from 15 to 

 25 mm. which had not passed through the dredge were set up as a control. During 



114 





