234 
BULLETIN OF THE BUREAU OF FISHERIES 
Table 20. — Horizontal and vertical distribution of oyster larvx near Shell Point, Onset Bay 
[Sample of 50 liters] 
The failure to take large numbers of oyster larvae in the plankton during the two 
consecutive summers can be explained by either a faulty method of collecting or the 
behavior of the larvae. Experiments carried out by the bureau in Great South Bay, 
where many thousands of oyster larvae were collected by pump or in plankton tows, 
show that in this body of water, with very small range of tide, larvae are easily obtain- 
able by either method. It is, therefore, permissible to assume that the failure to 
collect oyster larvae in plankton in Onset Bay was due to their behavior. The fact 
that at Shell Point the velocity of flood current is greater than the velocity of the 
ebb current may explain the absence of larvae from plankton during the flood tide. 
It is quite probable, however, that besides the velocity of the current other conditions 
govern the behavior of the larvae. The problem calls for an experimental study that 
should be carried out under controlled laboratory conditions. It is doubtful that it 
could ever be solved by field observations where on account of the complexity of con- 
ditions and the impossibility of eliminating various factors, the results obtained are 
often contradictory and their interpretation is difficult. 
The conclusion can be drawn from the examination of plankton collections that 
the small number of larvse, or even their absence in plankton samples taken in the 
inshore waters with strong tidal currents, does not necessarily show the failure of 
oysters to spawn in this locality and can not be regarded as indicative of ensuing poor 
setting. In the attempt to predict setting in such a region, more weight should be 
given to the conditions of the gonads of oysters and to the temperature of the water 
over the oyster bottoms than to the presence of free swimming larvse. 
After the 15th of July, shells from each bar were examined daily to determine 
the exact time of setting. On July 23 full grown larvse were found in the bottom 
sample taken at Stony Bar. (Station G, fig. 15.) The following day a few spat were 
found on shells at Manoman Bar. (Station C, fig. 15.) On July 25 spat appeared 
on the shells all over the bay, and on July 26 the setting was complete, or at least the 
larvse that attached after this date were not numerous enough to make themselves 
evident. 
EXPERIMENTS WITH SPAT COLLECTORS 
The commercial seed catching areas in Onset Bay are located on flat bars between 
mean low water and 2 feet above that level. The oystermen have learned by practice 
to restrict their shelling operations to these limited sections of the bay and to scatter 
the shells over the bars only in the zone which is exposed at low tide. No cultch is 
planted beyond low-water mark, where, according to the opinion of local oystermen, 
no setting takes place. 
