PROGRESS IN BIOLOGICAL INQUIRIES, 1926 551 
Larval period—In Milford Harbor oysters usually spawn be- 
tween July 15 and August 1, the exact date varying somewhat each 
year. Then follows a light set about the 1st of August and a heavy 
set around the middle of the month. During the interval between 
spawning and setting the oyster larve supposedly swim about in 
the harbor. A large number of plankton collections were made, 
therefore, to determine their abundance, distribution, and growth. 
However, on examining these collections, made at various depths 
and different stages of the tide, it was found (1) that very few 
oyster larvee were swimming about; (2) that of the few larve col- 
lected the majority were either a day or two old or were about 10 
days old and nearly ready to set; (3) that when the tide was at low 
slack water the larvee were most abundant; and (4) that the total 
number of larve found in daily collections, extending over a month, 
scarcely reached 100, while millions of them were found attached in 
the same place later. 
In a series of bottom samples taken in the vicinity of the spawning 
beds many larve in the intermediate and late stages of development 
were found. This is but natural, for the oyster larve are adapted to 
either a free-swimming or a bottom existence. 
These studies revealed the fact that the strong tidal currents cause 
the larvee to settle to the bottom, thus avoiding their being carried 
out of the harbor. In other bodies of water, such as Great South 
Bay, Long Island, where the tidal currents are insignificant, the 
oyster larvee were found to swim actively throughout the larval 
period. This investigation shows that the oyster larve, by settling 
to the bottom, are able to remain and set in the vicinity of the 
spawning beds, affording a sound basis for the development of in- 
shore areas for the production of seed oysters. 
Attachment or setting of oyster larve.—The location and distri- 
bution of the oyster beds in any locality is determined largely by 
the zone in which the oyster Jarvee attach. This zone varies con- 
siderably in different regions, as, for example, in Long Island Sound 
and Milford Harbor setting occurs from the bottom to a point about 
2 feet above low-water mark; in Great South Bay, Long Island, it 
occurs from the bottom to nearly high-water mark; while in South 
Carolina setting occurs between low and high water marks in the 
lower portions of a stream and only below low-water mark in the 
adjacent upper portions. Attempts to attribute the possible causes 
of this phenomenon directly to distribution of salinity, temperature, 
food content, and amount of sediment in the water have failed. In 
Milford Harbor, by means of floating and stationary spat collectors, 
tide gauge, and current meter, it has been found that the predomi- 
nating factor controlling and limiting setting is the velocity of the 
tidal current; that heaviest setting occur during the period of low 
slack water, and continues as the tide begins to run flood, gradually 
becoming less intense as the velocity of the current increases, and 
finally ceases altogether when the current attains a velocity of 
approximately 10 centimeters, or one-third foot, per second. The 
upper limit of setting varies somewhat according to the tidal condi- 
tions, and especially the difference in levels of slack water at the 
time when setting occurs each year. 
Drift-bottle experiments in Long Island Sound.—On September 18 
to 21, 1926, 500 drift bottles, with drags attached, were released off 
