THE DISTRIBUTION AND REPRODUCTION OF SAGITTA ELEGANS ON GEORGES BANK IN RELATION TO THE HYDROGRAPHICAL CONDITIONS

Abstract

1. The quantitative distribution, size, and stages of maturity of Sagitta elegans in the waters of Georges Bank have been determined from plankton hauls made on a network of stations occupied during 11 cruises from September 1939, to June 1941. 2. The area of relatively homogeneous water overlying the central portion of Georges Bank was found to change in extent from cruise to cruise, but to be sharply delineated from the surrounding stratified water masses, and has been designated as the "Mixed Area." 3. The abundance of S. elegans varied in individual hauls from a maximum of 165 specimens per cubic meter to zero, but averaged more than 10/m.3 at stations within the Mixed Area. The deeper hauls were numerically richer than the shallow hauls at more than half the stations. The existence of a diurnal vertical migration was revealed in certain cases, but varied greatly from cruise to cruise. 4. In horizontal distribution the greatest concentrations of S. elegans were found within the central homogeneous water mass of the Bank and a close agreement was disclosed between the contours of abundance and the boundary line of the Mixed Area. A center of abundance for this species existed on the Bank throughout the year. 5. The modal length of the mature sagittae increased from 16 mm. in September, when the water temperature was the highest, to 23 mm. in March, when the temperature was at the minimum for the year. Some specimens as long as 30 mm. were encountered. 6. The chief period of reproduction for S. elegans on Georges Bank centered in April in 1940 and in May in 1941. Following these periods very large numbers of small, immature individuals appeared. Evidence is presented that a distinct, second generation was produced during the late summer or autumn. 7. S. serratodentata and S. enflata, which were taken chiefly in the September cruise, occurred entirely outside the margin of the Mixed Area. Comparison is made with a similar case of reciprocal distribution found for two species of copepods: Calanus finmarchicus, which was excluded from the Mixed Area, and Pseudocalanus minutus, which was chiefly abundant within the Area. 8. Our observations support the suggestion of Redfield and Beale that S. elegans tends to accumulate on Georges Bank because of the relative absence of dislocating currents. However, reasons are advanced for believing that other characteristics of the Mixed Area water should be scrutinized as being particularly favorable for the growth and reproduction of S. elegans, and particularly unfavorable for the existence of other species of plankton. 9. The persistence of the population of S. elegans on Georges Bank throughout the year is a valuable index of the degree of permanence of the homogeneous bank water. The species recommends itself as an indicator for tracing movements of the Mixed Area water in future studies.