Tidal-front entrainment and retention of fish larvae on the southern flank of Georges Bank

From a grid of stations across the southern flank of Georges Bank in May 1997, larval haddock and cod were observed to be concentrated across the tidal front in a band 10–20 km wide. Several cross-bank CTD and acoustic Doppler current profiler (ADCP) sections indicated a narrow (<10 km) but significant tidal-front jet with residual currents moving southwest along the 60-m isobath at 10–20 cm/s in the upper water column (<35 m), consistent with a satellite-tracked drifter that logged a mean residual velocity of 13 cm/s for over a week. The tidal-front jet moved laterally with the strong M2 tide, but generally occurred in the region of maximum cross-bank density gradient. Evidence is provided for a two-cell secondary circulation pattern, with upwelling flow on the mixed side of the front and a surface convergence and downwelling on the stratified side. A surface convergence zone was observed with drifters and also predicted from a modeled flow field within 10 km of the jet on the stratified side, which generally coincided with the highest concentration of larvae. Model simulations, initialized with observed density structure from a CTD survey, suggest that larvae located in the convergence zone could be advected onto the shoals, under the jet in the near-bottom (>35 m), cross-isobath residual flow (0.6–1.2 cm/s).