SPAWNING AND SETTING OF OLYMPIA OYSTERS 
489 
reached a peak 2 weeks later. The first wave of setting may be considered as having 
ended about July 10, during neap tides, and a new set started immediately afterward, 
not reaching a definite sharp peak. It is to be noted that while this setting period 
continued until the end of the month, for a total time of about 3 weeks, the intervening 
neap tides had a high minimum range of 13 feet. The difference between spring and 
neap tides in this case was small, and little difference in rate of setting is to be observed. 
The next major setting cycle is concentrated during the extreme spring tides, 
having started at the time of the preceding neap tides. The following neap tides were 
not markedly different in range, and the effect is slight, though obvious. During the 
prominent neap tide period at the beginning of August almost no spat were caught. 
Figure 38.— Average number of spat caught daily per bag of shells left in water (dike 5, Oyster Bay) for periods of usually 2 or 3 
days, 1935. Tidal range is also shown to illustrate correlation. Compare with figure 27. 
This series shows on a more exact basis that the previous interpretation of results 
obtained with weeldy bags is generally correct. 
An important source of possible error in reaching an understanding of the sig- 
nificance of counts of spat on the shells in bags is the fact that the shells are clean and 
thoroughly efficient as spat collectors only at the time they are put into the water. 
During the next 7 days they become increasing^ less efficient. The error is overcome 
to some degree by using overlapping series. It is to be noted, for example, in figures 
27 and 33 that the average number of spat caught daily during any time is greater on 
the shells that were in the water 2 or 3 days than on those kept for 7 days. In inter- 
preting weekly series it is necessary to take the factor of fouling into consideration, 
for it would not be correct to say that the exact center of the highest column in any 
case represents the day of most abundant setting. 
STAGES OF TIDE AND SETTING 
Since periods of spring tides were shown above to pnmde most favorable condi- 
tions for the attachment of larvae it is of interest to determine the effect of different 
stages of tide. In Milford Harbor, Conn., Prytherch (1929) found for Ostrea vir- 
ginica that — 
“Heaviest setting occurs in the surface layer during the period of low slack water, which is the 
zone in which the oyster larvae were found to be most abundant. Setting continues as the tide 
