10 U. S. BUREAU OF FISHERIES. 



strongly indicates that for the purpose of artificial propagation the 

 latter are more desirable, Tlie tanks receiving less light maintained a 

 moi-B even temperature, kept clean longer, and developed the larvae 

 more rapidly than those placed in the sunlight. Sunlight and high 

 temperatures do not accelerate the development of the larvse but 

 promote the growth of bacteria, infusoria, and minute plants which 

 foul the water and necessitate frequent changes of tanks and filters. 

 The setting of the larvae occurred as heavily on the light side of the 

 collecting shells as it did on the darkened side as both sides were 

 equally clean. 



WATER SUPPLY. 



A pure and uncontaminated supply of salt water is essential and 

 necessaiy for the propagation of any marine animal. By pumping 

 on the flood tide the water in Milford Harbor was found suitable 

 but not ideal for experimental work. Each year a small scattered set 

 occurs in the harbor, which indicates that in general conditions there 

 are favorable for the development, of oyster larvae. The water was 

 pumped to the 500-gallon reservoir at every flood tide when possible, 

 in order to maintain a good quality in storage for distribution to the 

 spawning and rearing tanks. The reservoir was drained and cleaned 

 occasionally though very little sediment was found to have settled in 

 the bottom. 



From the reservoir five glass pipe lines with separate wood valves 

 conveyed the salt water to the various tanks where it was projected 

 through fine sprays in order to create a circulation. The water level 

 in the rearing tanks was controlled by hard-i*nbber overflow valves 

 that were located at one end of the tanks outside of the filters. 



FOOD AND ENEMIES. 



The food of the oyster larvae consists of ultra-minute organisms or 

 nannoplankton, which are found in abundance in ordinary sea water. 

 The plankton was excluded from the water supply by means of a net 

 made of No. 20 bolting silk through which the water was pumped 

 before going into the reservoir. This also removed all large known 

 enemies of the larvae and allowed only the nannoplankton to pass 

 through. 



The presence of infusoria in abundance in the tanks is an indica- 

 tion that conditions are becoming unsatisfactory and foul as a result 

 of the death and decomj)Osition of many of the embryos. To separate 

 the healthy embryos from these organisms, which collected at the bot- 

 tom of the tanks, was at first a problem but by allowing a good over- 

 flow of water to a new tank the more vigorous ones soon swam out 

 to the new quarters. As the larva?, increase in size and pass the 

 straight-hinge stage they can be readily separated from infusoria, 

 sediment, etc., by the use of bolting silk. However, a troublesome 

 growth of infusorians was experienced only in the tanks placed in 

 sunlight and with spawn taken from artificially fertilized eggs, 

 when making various tests with it in comparison with natural spawn 

 under the same conditons. 



