492 
BULLETIN OF THE BUREAU OF FISHERIES 
during the two floods did very many larvae set. By far the most were caught during 
the major flood tide following the extreme low of the first day. 
The sets of plates which were in the water for 3 hours each gave generally similar 
results. Those which caught most spat were exposed during the time when the 
flood tide was most rapid, from 1 % hours after the water came over the dike, in the 
first case, until l}{ hours before high tide. During the first, or small, ebb fewer spat 
were caught than on the major ebb the next morning. The current records are in- 
complete in two places, because seaweed became entangled in the current meter. The 
pH was remarkably constant throughout the period, save that at low tide it dropped 
from 8.4 to 7.8, due to respiratory activity of the oysters in the shallow, warm water. 
Salinity and temperature also varied but slightly. 
Prytherch (1934) made the important finding that larvae of Ostrea virginica may 
vary over a wide range in 
the time required for 
completion of the setting 
process, and stated that: 
The most rapid setting was 
observed at salinities of 16 to 
18.6 per mille and was complet- 
ed in from 12 to 19 minutes. 
He also determined that: 
In solutions that were above 
or below this salt concentration, 
the time for setting increased and 
reached a maximum of 140 and 
144 minutes in salinities of 5.6 
and 32.2 per mille respectively. 
It is of great significance 
that the time required for 
a larva to complete the 
process of setting may vary 
from 12 to 144 minutes, for 
it would therefore appear 
that a number of environ- 
mental conditions might 
become limiting factors. 
That the matter of rate of setting may have influenced the results of the experi- 
ments just described appears to be certain. In one case a set of plates was left in 
the water for only 15 minutes, yet it bore spat. In other cases the plates which were 
in the water for a longer period caught fewer spat than expected. In the last series 
described (fig. 37) the plates which were exposed during the major flood tide for a 
period of 6 hours caught a total of 103 spat. The three sets shown in the upper 
graph, covering the same period of time, caught a total of only 42 spat. It is suggested 
that those larvae which had not completed the setting process released their hold 
when the plates were withdrawn from the water, so that possibly only those that 
began to set soon after the plates were immersed were able to attach permanently. 
More clearly to illustrate the point it may be stated that a set of plates, left in the 
water during the entire 24 hours caught a total of 199 spat, while the four sets of the 
lower graph caught 151, and all of the double series of 3-hour plates caught but 129 spat. 
This information is of assistance in interpreting the results of the four experiments 
for it indicates that the first portion of the time that the plates are in the water must 
Figure 37. — Number of spat caught hourly per unit of cultch with relation to stage of 
tide, rate of current, temperature, salinity, and pH. (Oyster Bay, Gale Ground, 
June 27 and 28, 1935.) 
