Variations in overwintering depth distributions of Calanus finmarchicus in the slope waters of the NW Atlantic continental shelf and the Labrador Sea

Abstract

Vertical depth distributions of Calanus finmarchicus were examined in autumn and winter (2001–2003) for the slope and offshore waters of the Canadian Atlantic continental shelf from the south Labrador Shelf in the north to the southwestern tip of the Scotian Shelf in the south as well as along sections across the Labrador Sea and in Cabot Strait. A total of 324 samples were collected at 53 stations that ranged over ~19 degrees of latitude and ~17 degrees of longitude. Calanus finmarchicus populations in the subsurface layers (depths >100 or 200 m) were dominated (>90%) by CV copepodites off the western Scotian Shelf, in Cabot Strait, the central Newfoundland slope waters and the Labrador Sea. Along the slope between Flemish Pass and the eastern Scotian Shelf CVs made up 60–80% of the population in autumn and 80–86% in winter, with accompanying CIVs being replaced by CVs and adults between autumn and winter. CVs were deepest off the Greenland Shelf (70% below 1 000 m) and shallowest in Cabot Strait (80% in the 100–300 m interval; bottom depth ~450 m). Overwintering depth intervals were generally broad and covered large ranges of temperature and salinity. There were relatively high concentrations (>15 m-3) of late stage (CIV–CVI) C. finmarchicus in the surface layers in the Cabot Strait region in autumn (2003) and in the slope waters off St. Pierre Bank (south Newfoundland Shelf) in winter (2003). These were probably the result of late seasonal production in the Gulf of St. Lawrence and Cabot Strait region, or on the Newfoundland Shelf where vertical ring net tows indicated high numbers (>10 000 m-2) of C. finmarchicus at stations upstream of the St. Pierre Bank sampling area. The vertical distribution data from winter 2003 suggest that transport of overwintering animals around the Tail of the Grand Bank where water depths are >1 000 m and southwestwards along the Scotian shelf break was limited, and that populations along the slope between the NW Grand Bank and the eastern Scotian Shelf had important local sources. There was also evidence of northeasterly transport of overwintering CVs from the western Scotian Shelf to the central Scotian Shelf slope waters between October and December 2003.