INVESTIGATION OF OYSTER SPAWNING, ETC., MILFORD, CONN. 491 



that were put out at low water and taken up after two hours' run of flood tide were 

 the only ones on which newly attached spat had collected. 



In Great South Bay, Long Island, the zone of setting can be correlated with 

 the vertical distribution of the oyster larvse and the current. Here there is virtually 

 no current, and the spat are found attached from the surface to the bottom, which 

 corresponds to the distribution of the larvae. Tidal conditions similar to South 

 Bay were found in Holly Pond on the Connecticut shore, where the water was 

 impoimded for hydraulic power by means of a dam. There are insignificant tidal 

 currents in the pond, as it receives water from Long Island Sound only on the very 

 last of flood tide before liigh water. An examination of the pond showed that seed 

 oysters were attached in a zone extending from the bottom of the pond to within 1 

 foot of high-water mark, while just outside and below the dam the vertical distri- 

 bution of spat was entirely different and the same as found at Milford. In this 

 instance the chief difference in the physical conditions above and below the dam was 

 the velocity of the tidal current, which undoubtedly is an important factor in con- 

 trolhng the zone of setting in both places. 



In South Carolina and Georgia waters, where setting occurs between the tide 

 marks, it may also be interpreted on the basis of current velocity, though no actual meas- 

 urements in this region have yet been made. According to Marnier (1926), the tidal 

 current in the mouths of most bays and rivers is different than Ln Long Island Sound, 

 and slack water occurs when the tide is about halfway between low and high water 

 marks. This means that in these coastal rivers the current will have zero velocity at 

 about half tide level in the lower portions of the streams. In examining the piling 

 and tidal flats at many points in South Carolina it was found that oyster setting was 

 heaviest about halfway between tide marks and gradually decreased in intensity in 

 the zones a few feet above or below this level in accordance with the increase in velocity 

 of the currents. An interesting observation in this connection was made near 

 Beaufort, S. C, where a bridge was being constructed and cofl'erdams had been used 

 in laying the piers. An examination was made of several of the steel plates used in 

 mid-channel in the Beaufort River operations and which had been taken up and 

 placed on the wharf. On the outer surface of the plates, the spat were found attached 

 between high and low water marks, a distance of about 6 feet, the distribution being 

 the same as is found on the piling along the shore. However, on the Inner surface, 

 the attachment of the spat was quite different and was found to extend from the 

 mud line, or bottom of the channel, nearly to high-water mark, a difference of approx- 

 imately 16 feet. The extreme difference in the vertical distribution of setting on the 

 outside and inside of the cofferdam apparently was due to the fact that there were 

 no currents within the dam and consequently the larvse were able to attach there 

 from virtually the surface to the bottom. 



Although the vertical range of setting in Milford Harbor is several feet, the 

 intensity or number of spat per square inch at all points or levels is not the same. 

 The relative intensity of setting throughout the zone of attachment is shown in three 

 different locahties in Figure 32. In Milford Harbor the actual number of spat 

 attached per square inch at all levels was determined for the set of 1925. The setting 

 was found to be most intensive in a narrow strip extending approximately from a 

 20701—29 5 



