494 
BULLETIN OF THE BUREAU OF FISHERIES 
and the entire series hung from a float which was anchored in the channel. The 
baskets were supported so that each was in a horizontal position, occupying 5 inches 
of depth. They were placed so as to be well separated in the series and to maintain 
a constant distance from the surface for periods of from 2 weeks to 2 months, and the 
spat then counted. 
The results of four series are given in table 34. In two series the baskets were 
at depths ranging from the surface down to 104 inches. The others reached only to 
37 and 76 inches, respectively. In each basket 30 unselected shells were gone over 
carefully and every spat counted. The results are relatively uniform, considering 
the impossibility of measuring the exact area of individual shells, and when the 
results of the two longer series are averaged and plotted as number of spat per unit 
of culteh at different depths it becomes evident that the most spat were caught within 
the first 20 to 30 inches from the surface. In all cases the sample at the surface 
(0 to 5 inches) caught fewest spat, possibly because of the scouring action of waves, 
partly, perhaps, because larvae do not set as profusely within that area as they do 
a short distance below. On the bags suspended below this level of maximum setting 
fewer and fewer spat were 
caught. About twice as 
many spat were taken on 
the shells at a depth of 14 
to 19 inches as on those at 
99 to 104 inches. 
It was shown above that 
most of the larvae set when 
the water has a depth of 
about 6 to 10 feet or more 
The best seed grounds are 
feet, thereby placing them 
within or close to the area of maximum setting as shown in figure 38. The deeper 
the water on the oyster ground at the time setting takes place the fewer spat will be 
caught on cultch placed on the bottom. It is logical to conclude that this is one of the 
reasons why the higher grounds are best adapted to the catching of spat. 
Difficult to understand in view of these results is the fact that all natural beds 
in the region are located between low- and high-tide levels, or in shallow channels 
which are almost dry at low tide. The graph shows that the number of spat caught 
diminishes gradually with increasing distance from the surface. Tests were made 
only down to about 8.5 feet and the results suggest that larvae would set to some 
extent at much greater depths. It appeared likely that beds of oysters might be 
found in the deep channels well down the bay but extensive dredging in such places 
failed to disclose a single oyster. It may be that there is little clean cultch to which 
the larvae might attach, but clam shells were found abundantly in some places. The 
factor responsible for this localization of natural oyster beds is not clear, but in 
Yaquina Bay, Oreg., oysters of the same species occur almost exclusively in the deeper 
waters. 
These experiments have served a more immediately practical purpose. Follow- 
ing the original observations in 1931, which demonstrated that a very heavy set of 
spat could be obtained by employing floats, it was suggested to oyster growers that 
they try the method on a commercial scale. One of them tried it with a float made 
of two logs and a wire bottom, filed with Japanese oyster shells, in 1932. He was 
quite successful and during the following years others have started catching seeds in 
DEPTH IN INCHES 
Figuke 38. — Average number of spat caught on baskets of shells suspended from floats 
at different depths. See table 35. 
above the zero tide level, at which the tests were made, 
well above the zero level, frequently as high as 6 or 8 
