74 
BULLETIN OF THE BUREAU OF FISHERIES 
not in the bayou. Observations were discontinued about August 20 and further 
counts were not made. However, shells put in the water on August 20 in both 
West Bay and the bayou collected spat, showing that setting had started again in the 
bayou and was continuing in West Bay. Up until the 27th there was no further set 
in East Bay. 
These records, as shown in Figures 10, 11, and 12, indicate that whatever factors 
may have influenced the setting of the larvae they were not equally effective in the 
three locations. Such natural variations as sunshine, temperature, and local pre- 
cipitation would be virtually the same in all three places, and it is necessary to look 
further in order to obtain a suggestion of the factors controlling setting. 
Table 3 gives a summary of the results of all counts of spat which were made. 
Except near the end of the season, in the case of the Deer Islands samples, setting 
was never extremely heavy. It was not a continuous process, as appears to have 
been true of spawning, but took place in isolated periods. It looks as if the time of 
major setting, during the season of 1929, was early fall, for at the end of August 
profuse setting was tailing place both in West Bay and in Offatts Bayou, and in both 
places larvae in the umbo stage were constantly found in the plankton collections. 
(Figs. 6 and 7.) According to local oystermen most of the set is often obtained early 
in the fall. 
APPARENT INFLUENCE OF SALINITY ON SETTING 
A comparison of the records of salinity with those of setting (figs. 10, 11, and 12) 
suggests the possibility that there may have been some connection between salinity 
of the water and maturation of the oyster larvae. It has been pointed out that in 
Offatts Bayou the two reefs where bags of shells were planted were close together 
but that at times set was obtained on only one of them. This would indicate a 
difference in some respect between the water at the two places. 
During March and up until April 13, the salinity of the water in the bayou was 
around 22 parts per thousand. However, on the morning of April 13 a heavy rain 
fell, and the next day the salinity was down to below 19 parts per thousand. This 
occurred at the time when the beginning of setting was expected. During the follow- 
ing four weeks the salinity rose very slowly and was about 21 parts per thousand at 
the middle of May for a few days. At this time and for a few days thereafter a fairly 
heavy set took place, amounting to about 1,000 spat per bag of shells. Spat were 
found on both reefs in the bayou, but not at the Deer Islands in the adjoining West 
Bay. The salinity then fell abruptly and setting ceased on the main reef. 
A few days later, however, a light set occurred on the middle ground reef only. 
Since setting was not going on in West Bay this could not have been due to mature 
larvae brought in from that place. Further, the marked lowering in the salinity of 
bayou water was due to the inflow of West Bay water, for the bayou is subject to 
almost no dilution by direct land drainage. It was observed consistently that 
changes in the salinity of West Bay water were only slowly transmitted to the water 
up in the bayou, so it would be expected that, following such a change, a salinity 
gradient would exist from end to end of the bayou. On June 6 a series of samples 
was taken from six places and tested for salinity. The first was at the Deer Islands, 
in West Bay, with a salinity of 3.73 parts per thousand. The others were taken 
from positions in the bayou indicated on the chart (fig. 5) by the letters A to E, 
extending from the laboratory dock to the head of the bayou. The salinity values 
in parts per thousand were as follows: A, 6.75; B, 7.25; C, 7.30; D, 7.43; and E, 8.57. 
