28 NATURAL OYSTER BEDS OF DELAWARE. 



At the upper limit of oyster growth the salinity of the water was 

 low at a time when there had been comparatively little rainfall, and 

 it is probable that it may become practically fresh at this point during 

 periods of freshet. This is without much doubt the cause inhibiting 

 the growth of oysters at places higher up the river. 



At the southern end of the planting grounds the salinity is com- 

 paratively high and in consequence it is to be expected that the drill 

 or borer would be destructive. On the more important of the public 

 beds, those lying between the east line and the mouth of Leipsic 

 Creek, the density is favorable for the welfare of the oysters. It 

 probably never falls so low as seriously to threaten the beds, and, on 

 the other hand, it is hardly high enough, excepting close to the east 

 line, to favor an abundance of drills. 



ENEMIES OF THE OYSTER. 



It is stated that schools of drumfish occasionally appear on the 

 oyster beds of Delaware Bay and cause much damage, but none were 

 observed during the survey. This enemy of the oyster is usually 

 more destructive on planted beds than on the public rocks, probably 

 because the single-culled oysters on the former are easier to crush 

 than are the clustered, sharp-edged specimens more common on the 

 natural beds. The inroads of the drumfish are sporadic and unex- 

 pected in most places, although on the coasts of some of the Southern 

 States they are frequent enough to warrant the inclosure of the planted 

 beds with wire fences. This appears to be the only really adequate 

 protection, though if the presence of a school on the beds or in their 

 vicinity is discovered in time it can often be driven from the neigh- 

 borhood by the use of explosives. 



The principal enemy to the oyster on the Delaware beds is the drill 

 or borer, a small marine snail which drills a hole through the oyster's 

 shell and thus gains access to the contents, which it consumes. The 

 perforation is made by actual drilling with a rasplike organ protruded 

 from the mouth, and so far as is known no acid or other solvent is 

 employed to soften the shell. The drill breeds during late spring and 

 summer, laying its eggs in vase-shaped, leathery capsules attached 

 in clusters to shells and other hard bodies on the bottom. These 

 capsules, each containing several eggs, are readily recognizable, 

 being about one-fourth inch long and usually yellow in color. 



In the few places examined on the planted beds there were con- 

 siderable numbers of drills and many small oysters killed by them. 

 On the public beds near the east line some drills and killed oysters 

 were found, but over most of the area surveyed the salinity of the 

 water is somewhat too low to permit these pests ever to become a 

 serious factor. Below a salinity represented approximately by a 

 mixture of equal parts of salt and fresh water, having a specific 

 gravity of about 1.012 or 1.013, the drill will not thrive. 



