VARIABILITY IN DRY-WEIGHT AND VERTICAL DISTRIBUTIONS OF DECAPOD LARVAE IN THE IRISH-SEA AND NORTH-SEA DURING THE SPRING

Decapod larvae and early post-larvae were extracted from detailed vert ically stratified samples taken during research cruises in the Irish S ea from April to early June 1988 and in the North Sea in June 1989. Th e total dry weight of decapods was 4 to 106 mg m(-2) which represented 0.5 to 8.6% of the total dry weight of zooplankton. The vertical dist ributions of abundant species were examined in relation to physical an d biological variables. Nephrops norvegicus and Pagurus bernhardus wer e found nearer to the surface during the spring bloom in April than af ter the bloom in May. In the near-surface waters of the North Sea in J une, larvae of Liocarcinus spp. were more abundant than those of other decapods. Diel migrations varied, apparently due to differences in th e physical structure of the water column and the distribution of poten tial food. Conventional migrations were modified or absent in stratifi ed areas when near-surface concentrations of chlorophyll were present and in well-mixed water. Few significant ontogenetic variations were f ound. The most striking case was of Liocarcinus spp. larvae during the day in stratified water in the North Sea, where the early zoeal stage s (at a mean depth of < 6 m), the fifth zoeae (15.5 m mean) and the me galopas (26.7 m mean) occupied different positions in the water column .