Influence of Residual Circulation and Vertical Distribution on the Abundance and Horizontal Transport of Larval Atlantic Herring (Clupea harengus) in a Maine Estuary

Abstract

Horizontal movement of fall-spawned Atlantic herring (Clupea harengus) larvae in the upper Sheepscot River estuary, Maine (October–April) during three consecutive years), was a function of residual tidal flow at depth, vertical distribution of larvae during flood and ebb tides, and amount of river runoff. Larvae were vertically stratified 57% of the time and made significant shifts in position between tides in fall. They were distributed deeper than expected on both flood and ebb tides in October, either deeper or shallower than expected on flood tides during the remainder of the larval season, shallower on ebb tides during November–February, and either deeper or shallower on ebb tides in March and April. Larvae appeared to move either landward or seaward during fall and winter at the deepest station and seaward during spring. Seaward transport was associated with high river runoff and landward transport with low runoff. Results suggest that larvae are more likely to enter the estuary during fall and winter in years of low runoff and to be flushed out of the estuary when runoff is high. Severe mortality of larvae that remain in estuaries during winter may adversely affect recruitment to the juvenile stage in spring.