488 BULLETIN OF THE BUREATJ OF FISHERIES 



those in Milford Harbor. The drift of the water in these streams is decidedly seaward, 

 and the fact that under such conditions the oyster larva remain and set there strongly 

 indicates that they are not actively swimming at all times. The experiments with 

 drift bottles and the current measurements in Milford Harbor (mentioned on pp. 452 to 

 455) have shown that the horizontal movement of the water during ebb was much 

 greater than during the flood, and that floating objects carried out of the harbor by 

 the ebb current never were returned by the flood. A similar excess of ebb over 

 flood in tidal movement can be found in nearly all of the bays and rivers emptying 

 into Long Island Sound, and these are the very places where in the past the most 

 prolific natural beds were found to exist. 



One of the theories advanced by the oystermen to explain the continued failure 

 of setting in Long Island Sound is that the inshore and harbor areas' supply larvae 

 and spat for the deep-water beds in the Sound; and since the depletion of these 

 inshore areas, oyster setting in the Sound has been a failure. Such a theory is con- 

 tradicted by (1) the location of the natural beds, as shown in Figure 2; (2) the general 

 set of 1925, when there were very few oysters on the inshore areas; and (3) the dis- 

 tribution of the oyster larvse and setting in Milford Harbor in relation to the spawn- 

 ing bed. 



In recent years, when oysters were plentiful on the natural beds, setting often- 

 times failed on the leased bottoms lying outside of them in the Sound. The fact 

 that during the past centuries the natural beds were not extended into the deeper 

 waters of the Sound strongly indicates that oyster larvae were not distributed freely 

 by the currents over these areas. In 1925, oysters in sufficient quantity to produce 

 the set that was obtained could be found only on the planted beds in the Sound, 

 and the production of a greater number of larvae by these oysters is the only logical 

 explanation that can be given for successful setting during that year. 



In many cases the failure of setting has been attributed to a mortahty of larvte 

 as a result of sudden changes in temperature or saUnity and to heavy rainstorms. 

 In the studies of physical conditions in Milford Harbor during the larval period it 

 has been found that many changes in temperature of 5 to 11.5 degrees in 24 hours 

 and in sahnity from 5 to 25 per mille produced no noticeable decrease in the numbers 

 of larvae present in the water. During the larval period in 1925 and in 1927 the 

 precipitation was several inches above normal, and a tremendous amount of fresh 

 water was discharged into Milford Harbor. The changes in salinity and the increased 

 velocity of the ebb current following these storms apparently did not kill the larvae 

 or carry them out of the harbor. These studies show that oyster larvae can with- 

 stand extreme changes in temperature, salinity, and hydrogen-ion concentration of 

 the water and are not widely distributed by the tides and currents. 



SETTING 



A most important and significant period in the life history of the oyster is that 

 during which the larva sets or attaches itself to some clean, firm surface, such as 

 shells or stones. The act of setting was observed many times by the author, and 

 it was possible to view this interesting process from several angles by causing the 

 larvae to attach themselves to glass shdes. To accomplish this, the fully developed 

 larva releases a fine, threadhke byssus, or anchor, which adheres to the first suitable 



