Diel Movements of Juvenile Illex illecebrosus and Other Cephalopods in the Shelf Water-Slope Water Frontal Zone off the Scotian Shelf in Spring

Vertical water structure at two diel stations in the Shelf Water-Slope Water frontal zone seaward of the Scotian Shelf could be classified into five layers on the basis of some rather specific hydrographic characteristics. The near-surface layer (0-30 m) consisted of Shelf Water of low temperature and salinity, the second layer (30-100 m) represented the transition from Shelf Water to Slope Water, the third layer (100-200 m) consisted of Slope Water with maximum temperature and salinity, the fourth layer (200-300 m) extended from the lower boundary of Slope Water to the depth of minimum oxygen content, and the fifth layer (300-500 m) extended downward to the upper boundary of North Atlantic Central Water. Vertical distribution and diel migrations of juvenile Illex illecebrosus were closely associated with these layers and differed for the three modal size-groups (3, 5 and 8 cm mantle length) of squid that were caught in a small-meshed midwater trawl. They preferred the second and fourth layers of increasing and decreasing temperatures respectively and tended to avoid the maximum temperatures of the third layer. The vertical distribution of other squid species, euphausiids and planktonic fishes seemed to be closely related to the hydrography of the defined layers.