380 U. S. BUREAU OF FISHERIES 



Although at present it is impossible to predict with certainty when; 

 setting will begin, it is known that after the first larvae begin to- 

 attach there is still suiRcient time for the growers to plant cultch. 

 before the most profuse setting takes place. By making constant 

 observations and determining the exact date when setting begins one 

 may be reasonably certain when the peak of the set will arrive, since 

 this is so closely associated with the tidal cycle. By this method in 

 1933 and 1934 definite advice was given to oyster growers as to the 

 time to plant cultch for best results. The seed catch of those who 

 followed the advice has amply demonstrated the adequacy of the 

 method. 



Several years ago, on the basis of observations that larvae attach 

 most abundantly to lower surfaces, a type of cement-coated cardboard 

 spat collector — a modification of the egg-crate filler — was designed to 

 furnish a large amount of horizontal surface. These collectors are 

 now being used commercially in Puget Sound with very satisf actoiy 

 results. 



That the circulatory system of the oyster is more complicated than 

 previously thought was shown by the discovery that an oyster pos- 

 sesses a pair of accessory hearts, or large, rhythmically pulsating 

 blood vessels, within the mantle walls of the cloacal chamber. They 

 appear to pump blood from the kidneys to the mantle and gills, where 

 it is aerated. It is suggested that the organs are homologous to 

 the branchial hearts of cephalopod mollusks. The accessory hearts 

 are found in all three commercial species of oysters in the United 

 States. 



Further experiments were made on the adaptation of the Pacific 

 oyster to changes in salinity. Adaptation of the feeding mechanism 

 is relatively slow following a lowering of salinity, but rapid after a 

 rise. Feeding ceases completely when salinity is reduced to 12-14 

 parts per thousand, and apparently adaptation does not occur, al- 

 though growth of shell continues. It is probable that adaptation to 

 salinities of 15-20 parts per thousand is never such as to permit the 

 oyster to feed as rapidly as it does at 25-35 parts per thousand. 



INVESTIGATIONS ON AQUICULTURE 



Investigations in aquiculture were conducted in 1934 under the 

 direction of Dr. H, S. Davis. These investigations, which were 

 originally undertaken in connection with fish-cultural operations in 

 the hatcheries, have been expanded to include field studies dealing 

 with the various factors which affect fish in their natural environ- 

 ment. Opportunities for such field investigations have been greatly 

 increased during the past year as a result of an allotment from the 

 Public Works Administration for stream surveys and stream im- 

 provement in the national forests and parks. 



The experimental hatcheries at Leetown, W. Va., and Pittsford, 

 Vt., are strategically located for conducting investigations in the 

 waters of the national forests. The Leetown station is within a short 

 driving distance of both the George Washington and Monongahela 

 National Forests, while the Pittsford station is situated within the 

 boundaries of the recently enlarged Green Mountain National Forest. 



