494 BULLETIN OF THE BUREAU OF FISHERIES 



on the collectors was found to depend largely on their shape and the position in 

 which they were placed on the tidal flats in relation to the direction of the flood 

 current. The heaviest setting was found near the bottom and on the lee side of 

 the collectors, presumably because of the eddies created by them. The setting on 

 the brush that was planted on the tidal flats is a good example of the effect of the 

 current on attachment of the larvae. For a distance of about 3 inches above the 

 bottom the spat were found on all sides of the main branches and were most numerous 

 on the lee side. From above the 3-inch level to about the 1-foot level the majority 

 of spat were distributed only on the lee side and decreased gradually in numbers from 

 the bottom upward. Branches having the greatest diameter caught the most spat, 

 while small twigs at the same level caught virtually none. Further indication that 

 current velocity is an important factor controlling setting was found from a com- 

 parison of the distribution of spat in wire baskets filled with oyster shells with that 

 on the brush. The baskets were set out next to the brush and were foimd to have 

 received a much heavier set. The spat were found attached in the baskets up to 

 2 feet above the bottom, whereas on the brush setting stopped 1 foot above the 

 bottom. The intensity of setting in the baskets varied from 150 to 200 spat per 

 shell in the bottom layer to an average of 25 per shell at the top of the basket. The 

 differences in the intensity and upper limit of setting in the baskets as compared 

 with the brush undoubtedly is due to the type or shape of each collector. The 

 baskets were a greater obstruction to the current, and by decreasing its velocity they 

 facilitated the setting of a larger number of larvae. 



In summarizing the studies of the time and distribution of oyster setting in 

 Milford Harbor, it has been found that — 



1. Heaviest setting occurs in the surface layer during the period of low slack 

 water, which is the zone in which the oyster larvae were found to be most abundant. 



2. Setting continues as the tide begins to run flood, gradually becoming less 

 intense as the velocity of the current increases, and finally ceasing altogether when 

 the current attains a velocity of 10 centimeters, or one-third foot, per second. 



3. The intensity and vertical distribution of setting varies according to the 

 current velocity at the times when oyster larvae that are ready to set are found 

 swimming in the water. 



4. The distribution of spat on various types of collectors depends upon their 

 shape and especially on the position Lq which they are placed in relation to low-water 

 mark and the direction of the flood current. 



5. The upper hmit of setting varies according to tidal conditions when each set 

 occurs. The range of tide, level of low slack water, and rate of increase in current 

 velocity can be correlated with the intensity of setting and changes in the upper 

 setting hmit. 



By comparing the tidal conditions in various oyster regions it has been found 

 that the zones in which the oysters are attached and the distribution of the natural 

 beds can be correlated with the velocity of the currents and the distribution of the 

 oyster larvae, the heaviest setting occurring at the levels where the larvae are abundant 

 when the current reaches its minimum velocity. 



