486 BULLETIN OF THE BUREAU OF FISHERIES 



the oyster larvae present in greatest numbers at stations near the spawning beds and 

 whirh are heavy setting areas. 



The plankton collections of 1925 and 1926, which were made during flood tide at 

 Stations 4, 5, and 6 contained, on the average, less than 2 larva per 50 gallons. 

 From studies of the distribution of the larvae at various stages of the tide in both the 

 harbor and the Sound, it is evident that since the larvse are absent during flood tide 

 they could not be carried into the harbor from Long Island Sound. 



The second possibility (that the larvse were lying on the bottom the greater part 

 of the time) was studied next by collecting thin layers of bottom at Station No. 3, 

 near the spawning bed. The area of bottom covered by each sample was approximately 

 4 square feet, and the thickness of the layer that was removed was about one-fourth 

 inch. The samples were taken during half-flood and half-ebb stages of the tide, when 

 no larvse could be foimd swimming in the water. In all, 12 bottom samples were 

 collected, 4 of which were made in 1925 and the remainder in 1926. The bottom 

 samples were placed in wood tanks filled with sea water and allowed to settle, after 

 which a small stream of filtered sea water was maintained in the tank and the overflow 

 strained through a No. 20 bolting-silk net. Oyster larvae were obtained from all of the 

 samples, the number per sample ranging from 15 to 147. The total number of larvae 

 collected in the 12 samples was 662, the average being 55 larvse per 4 square feet of 

 bottom. This number of larvse on a given area of bottom is small as compared to the 

 number of spat that were found near by on the shells and other collectors, and may 

 be due to the fact that the samples were taken from clean and smooth areas that 

 contained no shells or other objects that could obstruct the currents. The finding 

 of oyster larvae on the bottom at certain stages of the tide shows that they are not 

 passive planktonic forms, and therefore are not subject to wide dispersal by the tides 

 and currents. By remaining on the bottom during the greater part of the larval 

 period, and by limiting their swimming activities to the tidal periods, when horizontal 

 movement of the water is least, the oyster larvae are able to remain and set on and 

 near to the spawning bed that produced them. 



One of the important questions that has presented itself in the study of the oyster 

 is. Where does the spawn or larvse from a bed of oysters finally attach or set? The 

 greater production of oyster larvae and spat in Milford Harbor foUowing its rehabili- 

 tation has shown definitely, during the past three years, that oyster larvse are not 

 distributed far from the spawning bed by the currents, the predominating drift of 

 which is out of the harbor. The final distribution of the larvae can be determined 

 easily by studying the relationship of setting areas to spawning bed. It has been 

 found that the majority of the larvse set within a radius of 300 yards from the center 

 of the spawning bed and that the greatest number of spat per square inch or per shell 

 is to be found on the bed within a radius of 100 yards. As indicated by setting, the 

 larvse are distributed both above and below the spawning bed and attach in greater 

 numbers on the areas that are below or in the direction of the Sound. Though the 

 intensity of setting varies from year to year, the distribution of the larvae was found 

 always to have this same relation each year to the spawning bed. 



The occurrence and distribution of the oyster larvae in Milford Harbor was 

 entirely diflerent from that found in Great South Bay, Long Island, by Churchill 

 and Outsell (1920 and 1921; unpublished reports) and by Nelson (1922) in Barnegat 



