16 



Currents. — Probably the first factor determining successful 

 oyster culture is the condition of the currents, their force, di- 

 rection and source. These serve three purposes in the welfare 

 of the oyster. 1st, they constantly bring fresh supplies of food ; 

 2nd, they provide fresh water for respiration; and 3rd, they 

 carry off waste matter. As has been stated above, the oyste..' 

 lives principally on diatoms or microscopic plants. These are 

 found almost universally in the water, both fresh and salt. The 

 oyster strains the water through its gills and takes out the 

 diatoms together with the sediment that may be in it. Sup- 

 posing that there are many oysters lying close together and 

 that there is no current, one can readily see that the water 

 immediately over the bed may soon become deprived of its 

 supply of diatoms; while a slight current will carry off the 

 strained water and bring fresh in its stead. Dr. Graves of 

 Maryland has estimated that the average oyster will strain 

 through its gills about 1-4 of a quart of water per hour and that 

 it requires approximately 40,000 diatoms to feed one oyster per 

 day. The supply is, according to that statement, abundant 

 on our coast as the estimated count of diatoms in various waters 

 runs from 19,000 to 110,000 per liter (about a quart). The 

 former coimt being made from water taken from a current, and 

 the latter from the bottom water in a shallow lagoon or bay 

 averaging 2 1-2 to 3 feet in depth. 



Although the diatoms may be abundant in water at large, 

 they soon become exhausted from water immediately surround- 

 ing the individual oyster, especially if it lies in a crowded reef, 

 providing, of course, there are no currents to replace the waste 

 water by fresh, bearing more food. This is often the case in 

 small lakes, lagoons, and coves. In such sheltered places, oysters 

 are frequently found, but if they are of any size, they are old, 

 the shells usually thickly bored and do not present the healthy 

 appearance of the young oyster. 



Secondly, the currents supply the oyster with fresh, oxygen- 

 ated water, or water in which air is dissolved. The oyster must 

 breathe oxygen the same as a person. The gills of the oyster 

 are the thin, striated flaps on both sides of the body, and, like 

 those of the fish, serve the same purpose, namely, that of taking 

 up the oxygen that is in the water and giving off carbon dioxide. 



