INVESTIGATION OF OYSTER SPAWNING, ETC., MILFORD, CONN. 493 



the bottom. This is due to the underrun of flood tide along the bottom while the 

 surface strata of less saline water is still on the ebb or at a standstill. 



It was found that at low water in Milford Harbor the current velocity increases 

 from zero at the surface to 24 feet per minute on the bottom, where the tide has already 

 been running flood for some time. Under such conditions it is apparent that at any 

 point from the surface layer to the bottom the current will be slack or of low velocity 

 for a very short interval. Consequently, the oyster larvte in this zone have a limited 

 time in which to attach, and setting here is of a much lower concentration on a given 

 area than is found at low-water mark when the slack-water interval is much longer. 

 With the change of tide from ebb to flood, it was observed that as the water level 

 rose from low-water mark to approximately 1 foot above it, there was virtually no 

 current at the surface, while during the next rise of 1 foot the current increased rapidly 

 to one-third foot per second. This sudden increase in the horizontal movement of the 

 water as it rises from 1 to 2 feet above low-water mark is due to the water leaving the 

 channel and spreading rapidly over the tidal flats. In the zone from low water to 1 

 foot above, setting is heavy; while at from 1 to 2 feet above, the number of spat per 

 square inch decreases gradually until, at the 2-foot level, very few are found. The 

 upper hmit of setting in Milford Harbor is determined by the height of the tide when 

 the surface current has attained a velocity of one-third foot per second after the period 

 of low slack water. 



Since the current velocity at any level varies from day to day with the changes in 

 the range of tide, it is to be expected that sets occurring at different times would have 

 a different upper limit. This actually was found to be the case with the two sets of 

 1925 and was the first clue that led to the studies of the relationship between current 

 and setting. For the light set on July 20, 1925, the upper limit of setting on the glazed- 

 tile collectors was 1 foot above mean low-water mark, while with the heavy set on July 

 29 it was 2 feet above. The range of tide in the first instance was 7 feet, while at the 

 time of heavy set it was 6 feet. We know that current velocity increases with range 

 of tide, and consequently the current on July 20 was stronger at the 1 or 2 foot level 

 than it was on July 29. Under these conditions the limit of setting naturally would 

 be lower for the light set, when the currents were noticeably stronger, and would 

 prevent attachment of the larvse above the 1-foot level. The tile collectors, under 

 the same conditions, also show clearly the relative intensity of the two sets in 1925. 

 The surface of the tiles was approximately 1,000 square inches, and the average 

 number of spat coUected per tile was 1,500 from the first set and 4,000 from the second 

 or heaviest set. The setting on the tiles was not uniform, of course, but decreased 

 in intensity from the bottom to the upper setting Hmit. 



Observations were made, also, in regard to the distribution of the spat on the 

 collectors in relation to the direction of the tidal currents when setting took place. 

 At low water and on the first of flood tide the horizontal current movement is very 

 weak, so that the current meter was useless for determining its direction or velocity. 

 For this purpose a simple device was used, which consisted of a hollow brass rod 

 ]/i inch in diameter, to which pieces of fine white thread 1 foot long were tied at 

 intervals of 3 inches. By setting up the rod near the collectors the direction of the 

 current could be seen easily at depths up to 3 or 4 feet. The distribution of setting 



