© 2003 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Vertical distribution of pre-settled sandeel (Ammodytes marinus) in the North Sea in relation to size and environmental variables
a Danish Institute for Fisheries Research, Charlottenlund Castle DK-2920 Charlottenlund, Denmark
b FRS, Marine Laboratory Victoria Road, Aberdeen AB119DB, Scotland, UK
*Correspondence to H. Jensen; tel: +45 33963370; fax: +45 33963333. e-mail: hj{at}dfu.min.dk.
Vertical distribution patterns of larval and juvenile sandeels were investigated at four locations in the North Sea. Sandeels between 6 and 65 mm were found to depths of 80 m, with vertical distributions dependent on both length and environmental factors. At one location with a stratified water column, the highest densities were found during the day in midwater where food concentration was also highest. In areas without marked vertical hydrographic gradients, larvae were relatively more abundant in surface waters during the day. At all locations, larvae of all sizes were generally more homogeneously distributed in the water column during night than during day. The extent of vertical migration, as measured by the standard deviation of the mean depth, increased generally with length. Gear avoidance was evident for larvae 
20 mm. Catch efficiency generally depended on both length class and surface light intensity. A simulated drift pattern of larvae, based on ADCP current measurements from two locations, predicts that the horizontal drift trajectory would only be affected slightly by the vertical positioning of the larvae in the water column during the time of sampling. The implication of vertical migrations for dispersal of larvae away from the spawning grounds is discussed.
Keywords: Ammodytes marinus, fish larvae, frontal zones, gear avoidance, hydrography, sandeel, vertical distribution
Received 4 November 2002; accepted 20 June 2003.