490 
BULLETIN OF THE BUREAU OF FISHERIES 
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.” 
Where this investigator worked the range of tide is less than half of that in south" 
ern Puget Sound and it is hardly to be expected that setting habits of larvae would 
be identical in the two places. 
When experiments on this subject were begun it was desired to determine at 
what stages of tide setting is most intense and the possible effect of such factors as 
daylight and darkness, sa- 
linity, temperature, and 
pH. Glass plates, sup- 
ported in wire frames as 
previously described, were 
arranged in units of fifteen 
8- by 10-inch plates, mak- 
total area of under 
mg a total area 
surface of 1,200 square 
inches, almost 1 square 
yard. At low tide the 
frames were just covered 
by the water retained in 
Figure 34. — Number of spat caught hourly per unit of cultch with relation to stage of 
tide, temperature, and salinity. (Oyster Bay, dike 5, June 29 and 30, 1932.) 
the dikes. A set of frames 
was placed upon the ground and allowed to remain for a definite interval, then 
removed, allowed to dry and the number of spat counted. 
In the first series, plates were planted in the dike soon after it was exposed by 
the receding tide, at 6:30 a. m., and allowed to remain until just before the flood-tide 
water came over the dike (fig. 34). During this time the plates caught but 3 spat. 
The next set was in the water for a total time of 30 minutes, from near the end of the 
exposed period until the water was about 1 foot deep over the dike. Throughout the 
rest of the tidal cjmle it was arranged to have a set of plates in the water during each 
major tide except for about 
1 % or 2 hours at the times 
of high and low tides, which 
were separately tested. 
In figure 34 the results 
are given as number of spat 
caught hourly on each of 
the sets of plates. Shown 
also are depth of water 
throughout the period and 
values of temperature and 
salinity obtained each time 
samples were changed. 
The fewest spat were 
caught at low tide when the dike was exposed, the most on the two flood tides, 
although at, or near, the time of the higher low tide spat were also caught. 
During ebb tide few larvae set, in proportion to the activity at flood tide. It is of 
interest that salinity and temperature were quite uniform during the experimental 
period, save that the exposed dike permitted warming of the water at low tide. It 
is also clear that attachment of larvae is not markedly influenced by daylight or 
Figure 35. — Number of spat caught hourly per unit of cultch with relation to stage 
of tide, temperature, salinity, and pH. (Oyster Bay, dike S, July 6 and 7, 1932.) 
