228 
BULLETIN OF THE BUREAU OF FISHERIES 
TIDES 
The mean range of tide in Onset Harbor is 4.1 feet and the spring range is 4.9 feet. 
The observations of the rise and fall of the tide were made on the tidal gage set up 
at the end of Shell Point. One of the points of interest in a study of tidal phenomena 
is the determination of the time of slack water in relation to the stage of tide. It 
has been shown by Prytherch (1929) that oyster larvae in Milford Harbor were most 
abundant during the low slack water periods when the current velocities were from 
Figure 17. — Horizontal distribution of salinity in Onset Bay, September 4, 1928, at low water. Small figures indicate stations; 
large figures indicate salinities (per mille) 
0 to 18.3 centimeters (0.6 foot) per second, and that they ceased swimming when the 
current velocity exceeded this figure. Hence, the determination of the exact time of 
slack water and of its duration in relation to the stage of tide may have some bearing 
on the understanding of 'the factors which cause the aggregation of the larvae during 
the setting period in a definite zone. The determination of the time of high slack 
water was made on a very calm day, August 3, 1928, when the rise of the water and 
surface tidal currents were observed at very brief intervals by watching the tidal 
gage and noticing the movement of small floats thrown on the surface of the water. 
