Influences of mean advection and simple behavior on the distribution of cod and haddock early life stages on Georges Bank

Abstract

ABSTRACT Results of a modeling study designed to explore the influences of physical advection and certain biological mechanisms on the distribution of cod (Gadus morhua) and haddock (Melarwgrammus aeglefinus) early life stages on Georges Bank are described. Using a late‐winter/early‐spring 3‐D circulation field driven by the M 2 tidal current, mean wind stress and Scotian Shelf inflow, we examine the distribution of cod and haddock larvae spawned on the Northeast Peak of the Bank. The sensitivity to a March‐April baroclinic field is also explored. Results indicate that larvae remaining in the surface Ekman layer are generally advected off‐bank. However, downwelling associated with Ekman layer convergence near the shelf break provides a mechanism for larvae to exit from the off‐bank surface drift. Larvae below the surface layer are transported south‐westward along the southern flank of Georges Bank and are retained on the Bank if their position immediately upstream of the Great South Channel is shoalward of (roughly) the 70 m isobath. Within the Great South Channel region and between the 50 and 70 m isobaths, retention can depend on the phase of the tide. Spawning shoalward of the 50 m isobath on the Northeast Peak greatly increases the chances of retention. These results apply to passive larvae and to those with specified vertical distributions and migration based on observations. Directional on‐bank swimming at rates of 0.5 to 1 body length per second would substantially enhance shoalward displacement, resulting in larval distributions during the first 2 months that are consistent with field observations.