ECOLOGY OF LARVAL HERRING IN RELATION TO THE OCEANOGRAPHY OF THE GULF OF MAINE

The dynamics of larval herring in the north-eastern Gulf of Maine are reviewed with reference to the important physical and biological oceanographic processes of the region. Particular attention is given to the apparently conflicting hypotheses of larval drift from the tidally well-mixed spawning areas to nursery areas, and larval retention in spawning areas for a period of several months. Both processes have been reported for the eastern Maine-Grand Manan spawning area of the Gulf of Maine, but the relative importance of each to larval survival through the winter and recruitment to the juvenile stage is not clear. Both transport and retention are interpreted in light of oceanographic processes that might impart variability, especially between years, in the proportion of larvae transported away versus that retained. Results of recent survey cruises in the Gulf of Maine show both hypotheses to have merit. It is suggested that (i) interannual differences in slope water intrusions into the Gulf of Maine as they affect the residual circulation, (ii) the lunar periodicity in the intensity of tidal mixing in relation to hatching times, and (iii) the potential for variable spawning locations relative to the tidal fronts, may affect the distributions of herring larvae immediately after hatching in the fall, and may control the proportion of larvae that are advected away with the residual currents versus that retained in the vicinity of spawning. It is also suggested that those processes that affect larval distributions and survival in the fall are important in determining the overwintering distributions of larvae in the Gulf of Maine, where the subtle influences of variable food regimes and water temperatures could potentially exact large interannual differences in winter survival and recruitment ot the juvenile stage.