Settlement dynamics in the lesser sandeel Ammodytes marinus in the North Sea
Field comparisons of the vertical distribution of pre-settled sandeels, zooplankton and hydrography showed that a significant fraction of A. marinus larvae between 6 and 65mm are found down to a depth of 100m and that the magnitude of vertical movements increases with size of the fish. Vertical distribution is related to feeding with larvae congregating in the water layers with the highest zooplankton abundance during the day. In the absence of thermal stratification of the water column and strong vertical gradients in zooplankton distribution the larvae move down in the water column at dusk and up at dawn. Simulations of the drift trajectory of sandeel larvae suggest that vertical movements did not have a significant influence on the horizontal dispersal pattern of the sandeel larvae during the periods where the investigation was made. Field investigations showed that the horizontal distribution pattern of pre-settled sandeels changed with size of the fish. After hatching sandeel the larvae are transported away from the spawning grounds to deeper water until a length of about 20-30mm. The distribution then becomes more aggregated as larvae larger than 20-30mm start to congregate in highly productive areas close to the habitat of post-settled conspecifics. Active horizontal swimming was indicated as the reason for the observed horizontal distribution of larvae > 20 mm. The 1996 year class of sandeels in the North Sea was a magnitude higher than the 1995 year class (ICES 1999). Larval density measured by the present study was similarly found to be a magnitude higher in 1996 than in 1995, indicating that recruitment strength may be determined in the pre-recruitment stage in this species.