452 BULLETIN OF THE BUREAU OF FISHERIES 



a biological standpoint both intimately related features of the tidal movement — the 

 rise and fall of the water and the horizontal movement or tidal current — are of con- 

 siderable importance. 



According to Marmer (1925), in most bays and rivers of the Atlantic coast the 

 tide enters as a progressive wave, the characteristics of which are that the strength 

 of the tidal current is greatest at the times of high and low water while the slack of 

 the current comes midway between the times of high and low water. However, in 

 Long Island Sound and its inshore waters we find a different kind of movement, 

 which is of the stationary-wave type. With this type of wave movement the strength 

 of the current comes midway between high and low water, while the slack of the 

 current comes near the times of high and low water. This difference is illustrated 

 by Figure 14, in which the tide and current curves are plotted for Milford Harbor 

 entrance, where the movement is of the stationary-wave type, and for New York 

 Harbor (Marmer, 1925), as determined by the Coast and Geodetic Survey, where 

 we have progressive wave movement. 



In each locality the tidal currents vary in strength from day to day in accordance 

 with the changes in the range of tide. The strongest currents come with the spring 

 tides of full and new moon, and the weakest currents with the neap tides of the moon's 

 first and third quarters. During the spring tides the flood current in Milford Harbor 

 attains a maximum velocity of 1.1 feet per second, and the ebb current, 1.5 feet per 

 second. With neap tides considerably less water passes in and out of the harbor, 

 and the velocity of the flood current at strength is 0.8 per second and the ebb current 

 1.3 feet per second. The tidal current in Milford Harbor is of the rectilinear or 

 reversing type — that is, the flood current runs in for a period of approximately 5 

 hours and 30 minutes and the ebb current runs out for a period of 6 hours. 



In studying the tidal current and its possible effect on the occurrence and dis- 

 tribution of the oyster larvae a complete understanding of the direction as well as the 

 velocity of the current at each stage of the tide is necessary. For this purpose the 

 Eckman current meter and several sets of drift bottles were used. The direction of 

 flow during flood and ebb is shown in Figure 15 for Milford and vicinity. During a 

 complete tidal cycle the distance traveled by an object floating in the water or by a 

 buoyant microorganism is equal to the product of time multiplied by the average 

 velocity during this interval. For a normal flood or ebb period of 6.2 hours, the 

 approximate distance a tidal current with a velocity, at strength, of 1 knot will carry 

 a floating object is 3.95 nautical miles or 24,000 feet. (Marmer, 1925.) 



As a result of river discharge the ebb current has a greater velocity and duration 

 than the flood, and currents of such strength as those at the entrance of Milford 

 Harbor would transport a floating object approximately 21,100 feet during the ebb 

 flow and return it but 15,600 feet during the flood. The currents inside and outside 

 of the harbor are not as strong as those at the narrow entrance, so, in order to deter- 

 mine the actual direction and drift of the currents, several observations were made 

 with floats. In each case two pairs of floats were used and were released at Station 

 2, in the harbor, and their course charted for several hours. When the floats were 

 released, at the beginning of ebb tide, they followed the channel to Station 5 and 

 then swung east, passing the red buoy near Station 6 in approximately^23^ hours, and 

 finally came to a stop about 1,000 yards oft'shore at Pond Point. The distance the 



