TIME AND INTENSITY OF SETTING OF THE OYSTER, CRASSOSTREA VIRGINICA, IN LONG ISLAND SOUND

Abstract

1. Studies on setting of oysters in Long Island Sound during a 25-year period, 1937-61, have shown that the larvae of advanced stages were never extremely common and were unevenly distributed in the water masses. Their appearance and disappearance did not follow a definite pattern.2. No clear relation was found between the number of certain larval enemies, such as ctenophores, and intensity of setting. Neither could the numbers of larvae be correlated with the numbers of adult oysters.3. Dead or dying larvae were sometimes found in plankton. Disappearance of the larvae possibly may be due to the plankton blooms that produce detrimental external metabolites. A dinoflagellate, Prorocentrum triangulatum, on occasion present in extremely large numbers, may be one of the forms responsible for the absence of larvae during certain periods.4. Setting of oyster larvae was recorded at all depths from the intertidal zone to 100 feet.5. Beginning of setting at all stations at the depth of 30 feet or less normally occurred within the same 24-hour period.6. The average length of setting season of oysters in Long Island Sound was 65 days. The shortest season was 36 days, and the longest was 85 days.7. Setting in different years started between July 9 and August 11; the mean date was about July 20. The earliest end of setting was August 26, and the latest, October 17.8. The setting in Long Island Sound is not continuous or of uniform intensity. Setting ceased almost every year for various periods, sometimes several weeks long.9. Setting exhibited two well-defined waves in most years; each peak had a dearly evident maximum. The time these peaks appeared varied considerably from year to year. The time between the peaks of two waves in different years ranged from 19 to 40 days.10. Neither the first nor second wave of setting was consistently predominant. Within the 25-year period the first wave was the heavier in 15 years, and the second in 10.11. The intensity of setting in the same areas of Long Island Sound differed widely from year to year. Setting was lowest in 1957, when approximately three recently set oysters per 100 shell-surfaces were counted at each station during the entire season; in 1939 and 1940 over 42,000 spat were counted under the same conditions.12. Comparison of the numbers of oyster set at stations of three depths—10-, 20-, and 30-feet—showed that no depth consistently produced the heaviest set. Stations of each of the three depths had the greatest abundance of larvae during one or more years of the period of observations.