SPAWNING AND SETTING OF OLYMPIA OYSTERS 
473 
exact picture of setting behavior if only the results obtained with the parallel series 
were plotted. However, to do so would hardly alter the curve, since the difference 
in efficiency of the various angles is tremendously greater than that between plates 
of the same angle but in different positions with reference to tide. Also, in its present 
form, the curve is more typical of natural conditions, considered from an ecological 
point of view. 
The results of these experiments have an obvious practical application, for they 
point out the desirability of furnishing cultch which has a large amount of the ideal 
under horizontal surface. It is also of practical importance to know that the more 
freely the current flows along the surfaces the more spat will be caught, presumably 
because more water, bearing larvae, comes into contact with the surfaces. A com- 
parison of the efficiency of two types of manufactured spat collectors serves to illus- 
trate well the commercial application of the results described above. 
Prytherch, in 1929, used the egg crate filler, coated thinly with concrete, as a 
collector of seed oysters on the Atlantic coast. (See Bureau of Fisheries Document 
1076, Improved Methods for the Collection of Seed Oysters.) These fillers, made of 
cardboard, provide a large amount of surface for the attachment of larvae, and the 
rough concrete surface is particularly favorable for setting. Such spat collectors are 
spread on the seed grounds at the correct time and have proven to be highly effective. 
The method is in use on some of the Olympia oyster beds where the fillers remain 
covered at low tide because of the dikes (see fig. 2). 
However, the experiments described above show that the best surface for the 
catching of spat of the Olympia oyster is the under-horizontal, and the egg crate 
filler lies on the bed with all partitions in the vertical position. Also, the filler, lying 
on the oyster ground, does not permit the free flow of water over the surfaces and the 
individual cells contain relatively still water, frequently resulting in their filling with 
silt. In order to develop an efficient collector for Olympia oysters it was, therefore, 
necessary to design a modification of the egg crate filler which would provide a large 
amount of under-horizontal surface and also permit the water to flow freely over all 
the surfaces. 
This special collector was made like the egg crate filler, but the individual parti- 
tions are twice as wide, and consists of two rows of six cells each. The cells are 2 
inches square by 4 inches long (see fig. 22). It lies naturally on the ground in a posi- 
tion which is at an angle of 90° to that of the egg crate filler. Both vertical and hori- 
zontal surfaces are present, and the water flows freely through the cells. As they are 
now used they consist of two rows of seven cells, making the total area almost the 
same as that of the egg crate filler, but the counts of spat which were made for 
comparison refer to the original type. 
Table 19. — • Comparison of efficiency of 2 types of manufactured spat collectors 
Number of spat per 1,000 cm 2 
Total area 
(cm 1 ) 
Total 
number 
of spat 
Average 
number 
of spat 
per 1,000 
cm 1 
Under- 
horizontal 
Vertical 
Upper- 
horizontal 
Egg crate filler __ 
427 
1,066 
4, 724 
3, 464 
2,064 
4, 775 
427 
1, 378 
3,235 
28 
Table 19 gives the results of counts of spat caught on egg crate fillers and special 
collectors which were put on the same grounds at the same time and removed together 
3 months later. Each series is the average obtained by analysis of three collectors. 
