INVESTIGATION OF OYSTER SPAWNING, ETC., MILFORD, CONN. 



487 



Bay, N. J. In these bodies of water, which are quite similar, they found that oyster 

 larvae are abundant and actively swimming at all stages of the tide and are about 

 evenly distributed from the surface to the bottom during the entire larval period. 

 In order to determine the possible cause for this difference, a comparison was made of 

 the physical conditions in each locality. Various factors, such as temperature, 

 salinity, and hydrogen-ion concentration, were found to vary slightly but gave no 

 indications of being controlling factors. The chief difference appeared in the tidal 

 range and the velocity of the tidal currents in each body of water. In both Great 

 South and Barnegat Bays the mean range of tide is approximately 1 foot and the 

 tidal currents are very weak, while at Milford the mean range is 6.6 feet and the 

 tidal currents attain considerable velocity, as shown previously. Since the oyster 

 larvae were foiind to be most abundant in the harbor during slack-water periods, it 

 is probable that the current velocity is an important controlling factor. To test 

 this, a few experiments were made during the past summer, in which the swimming 

 movements of the larvae were observed in relation to the current. For this purpose, 

 an elliptical wood tank of 1,000 gallons capacity, in which larvae were being reared, 

 was employed, and tests were made as to the presence of the larvae when the water 

 stood or was in circulation. The velocity of the current created in the tank was 0.5 

 foot per second at the surface and 0.3 foot on the bottom. The average results 

 obtained in four experiments are given in the following table: 



Table 17. — Abundance of oyster larvse in a lank when the water is standing or in circulation 



Additional tests were made at various points in the tank during circulation to 

 see if the larvae had collected in eddies or in the center, but none could be found at 

 any place except on the bottom of the tank. The results of these experiments 

 indicate that oyster larvae cease swimming and settle to the bottom in currents 

 having a velocity of approximately 0.5 foot per second. In comparing this with 

 conditions in the harbor, we find that the larvae were most abundant in the slack- 

 water periods, especially at low water, when the current velocity ranged from to 

 0.6 foot per second. When the velocity of the current exceeded this figure, the 

 larvae were absent from the water and were found on the bottom. The correlation 

 between the distribution and abundance of the larvae and the current velocity en- 

 ables us to understand how the natural development and growth of the oyster beds 

 has been possible in many harbors and estuaries where the tidal movement and 

 river drainage would soon carry away free-swimming organisms. 



In many coastal regions we find prolific oyster beds in swift running rivers that 

 empty directly into the ocean and have currents that are many times stronger than 



