Retention of Ichthyoplankton in the Georges Bank Region During the Autumn-Winter Seasons, 1971-77

Abstract

The distribution, abundance and length-frequency data for all species of ichthyoplankton have been examined for 27 Coordinated Larval Herring Surveys covering the Georges Bank-Nantucket Shoals region during the period for September 1971 to February 1977. Two major faunal zones were delimited by the numerical classification technique of Divisive Information Analysis: a northern zone centered on Nantucket Shoals and Georges Bank and composed of taxa native to the shelf region, and a southern faunal zone dominated by subtropical and mesopelagic taxa which was found to be associated with broadscale intrusions of Slope Water onto the southern flank and around the perimeter of Georges Bank, lending support to the inferred clockwise mean residual flow. The persistence of this clockwise circulation pattern on the shoaler part of Georges Bank and the maintenance of a sharp frontal boundary along the southern flank of the bank by the Slope Water tend to retain fish larvae within the Georges Bank region. Ichthyoplankton distributional anomalies, such as geographical displacement following prolonged periods of wind stress or passage of warm-core eddies south of the shelf, were clearly highlighted by this technique.