SPAWNING AND SETTING OP OYSTERS 
75 
This gradient of salinity in the bayou shows that the water least subject to change, 
as a result of the inflow of West Bay water, is that at the head of the bayou. These 
tests were made more than a week after the brief setting period on the middle ground 
reef took place, but yet show the presence of water of higher salinity at the inner end 
of the bayou. 
The significance of this salinity gradient with regard to the light setting period 
which occurred at the end of May on the middle-ground reef is possibly that water 
of high salinity left in the inner end of the bayou moved outward along the channel 
at ebb tide and furnished adequate setting conditions along the main channel, in 
which the middle ground is located. Such water would not be expected to touch 
the main reef to the side, where no set took place. Such a condition would be expected 
when the salinity in West Bay is falling, and the bayou water is of a higher salinity. 
When the situation is reversed, however, and the salinity of West Bay water is 
rising while that of bayou water follows, the water flowing into the channel at flood 
would be of higher salinity than that at ebb tide. This would also give opportunity 
for setting to occur on the middle-ground reef. This condition obtained early in 
July, when a fairly heavy set was obtained, at the Deer Islands, amounting to between 
400 and 500 spat per day per bag of shells. At the same time a set of 15 to 25 spat 
per day per bag of shells was obtained on the middle-ground reef, but not on the 
main reef. After this no further set took place in the bayou until the week following 
August 19, when the bags of shells on both reefs received a fairly heavy set. The 
graph (fig. 10) shows that at this time the salinity was rising gradually from 20 to 
about 24 parts per thousand. 
No set was obtained on the Deer Islands Reef (fig. 11) until early in July, and 
this occurred on a rapidly rising salinity, which, during this time according to the 
samples taken, rose from about 14 to over 20 parts per thousand. These measure- 
ments are necessarily rough for there is much variation in such a body of water as 
West Bay where the effect of tides and winds is great. The salinity then remained 
at a slightly reduced level until the first of August when it started upward and by the 
end of August was close to 30 parts per thousand. Coinciding with this rise came a 
period of very heavy setting during which up to 3,000 spat were obtained daily per 
half bushel bag of shells. Figure 17 shows a sample of one of the clumps of shells 
bearing spat up to about two weeks old. Setting was still continuing when 
observations were discontinued. 
In East Bay at Hanna Reef (fig. 12) only a few salinity determinations were made, 
but the results are in harmony with those above described. A period of setting took 
place early in May, extending over about two weeks, with its peak a few days earlier 
than that in the bayou. It may be assumed that at this time the salinity was high 
in East Bay as well as in the bayou. On the 23d it was down to 6 parts per thousand, 
even lower than that in the bayou. The infrequent determinations, as shown in the 
figure, indicate that after June 24, when the salinity was less than 2 parts per thou- 
sand, it rose to 12 parts per thousand on July 14 and possibly higher at the end of 
the month, as was the case in West Bay. (Fig. 11.) Samples of shells brought in 
on August 5 showed that a fair set had taken place during the preceding two weeks. 
On this date the salinity was 13 parts per thousand and on the next day was down to 
less than 10 parts per thousand, and no more setting occurred. Previous to the 
setting period the salinity was rising, and afterwards it was falling, but the salinity 
at the time of setting is unknown. Following this the salinity gradually rose to 18 
