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BULLETIN OF THE BUREAU OF FISHERIES 
very little salinity difference from surface to bottom it may be sufficient to account 
for the results on the basis of Prytherch’s conclusion. More difficult to understand, 
however, is the fact that on the oyster grounds most spat are caught at relatively high 
tide, when the water is deep and of the maximum salinity, while at low tide, when the 
salinity is lowest and the amount of mineral from land drainage presumably in highest 
concentration, almost no larvae set. At this time oilier factors, such as low pH, may 
inhibit setting. It is clear, also, that during a period of extreme tides the fresher 
water entering the upper end of a bay goes farther down the bay and is most 
thoroughly mixed with the sea water. 
These results appear to permit interpretation in the light of Prytherch’s conclu- 
sion, though the specific factor involved is not definitely known. Although copper 
may be the controlling factor in the bays studied it is not difficult to conceive that 
other substances may act in a similar manner. That is, copper may be only one of a 
number of factors which may control the setting process. As a result of field observa- 
tions near Galveston, Tex., (Hopkins, 1931b), it was concluded that setting occurred 
only when the salinity was relatively high, in the neighborhood of 20 p.p.m., for in 
that place the salinity was frequently very low. Prytherch (1934) disagreed with this 
conclusion, although he demonstrated experimentally that the setting process pro- 
ceeds most rapidly at a salinity of 15 to 25 p.p.m. Very slow completion of attach- 
ment may be of considerable disadvantage to the larvae and thereby constitute the 
reason for the writer’s observation that spat were caught chiefly when the salinity was 
high. In addition to salinity and copper there may be other factors which determine 
the time and frequency of setting under different conditions. 
It is not possible to give an exact statement of the number of days required for 
larvae to reach the setting stage, though it was demonstrated that they develop for 
about 10 days within the maternal branchial chamber before being discharged. The 
free-swimming period appears to be 30 to 40 or more days, depending largely, perhaps, 
on water temperature, so that the total larval life is at least 40 days. This is about 
tliree times as long as that of Ostrea virginica (Prytherch, 1929). The long larval life 
permits wide dispersal but also subjects the larvae to various plankton-feeding 
organisms as well as to the effects of tides and storms. 
Mortality of larvae is necessarily large in any species. It may be estimated that 
oyster growers catch and grow not more than about one out of a million larvae pro- 
duced, when it is considered that the 4-year-old oysters discharge about 300,000 eggs 
and all of the younger individuals also propagate on a smaller scale. Mortality of 
spat is also tremendous. It was shown that during a period of profuse setting as many 
as 12,000 spat per day might be caught on the shells in one bag. Since there were 
generally only about 125 shells in a bag, each shell caught several hundred spat within 
a few days. Yet, after 1 year it is remarkable to find a shell with as many as 50 spat. 
Most of the mortality appears to take place within the first few weeks after setting, 
and while some of it is due to overcrowding it cannot all be traced to this cause. 
SUMMARY 
1. Grounds on which Olympia oysters are grown are surrounded by dikes to 
retain a few inches of water over the oysters at low tide. The maximum range of tide 
at this place is about 20 feet, the average about 14 feet, and most grounds are located 
between the minus 2 foot and plus 4 foot tide levels. 
2. Average water temperature varies between a winter low of 6° to 9° C. and a 
summer high of 18° to 20° C. In summer the temperatui'e is highest when the tide is 
