Eulerian views of layered water currents, vertical distribution of some larval fishes, and inferred advective transport over the continental shelf off North Carolina, USA, in winter

Abstract

ABSTRACT Currents that effect the shoreward transport of the larvae of estuarine‐dependent fishes spawned in winter in Onslow Bay, North Carolina, USA, were driven by winds and pressure gradients, and influenced by the Gulf Stream. Aside from storms, winds over the continental shelf in Onslow Bay blew predominantly alongshore with velocities approaching 14 m s ‐1 during February and March 1986, and January and February 1989. Water masses and currents observed at two current‐meter moorings, one at mid‐shelf and the other on the outer shelf, reflected the onshore (or offshore) advection of interior water in compensation for the offshore (or onshore) advection of wind‐driven surface water. Winds and currents reversed direction approximately every 4 to 6 days. The larvae of Atlantic menhaden, Brevoortia tyrannus , spot, Leiostomus xan‐ thurus, and Atlantic croaker, Micropogonias undulatus , were most abundant in 17–19 o C and 20–21 o C water of the outer shelf and Gulf Stream fronts. There was little indication of diel vertical migration; larval Atlantic menhaden were most abundant in mid‐ and surface water, while spot and Atlantic croaker were most abundant in mid‐ and deep water. Given this distribution, the inferred advective transport of larvae was at times onshore, but at other times it was offshore. Within a spawning season, the prevalence of either reciprocation could determine the number of larvae that reach coastal inlets.