Distribution of <i>Calanus</i> spp. on and around the Nova Scotia Shelf in April: evidence for an offshore source of <i>Calanus finmarchicus</i> to the central and western regions

Abstract

Observations of hydrography and Calanus spp. abundance were made at stations on lines across the central and eastern Scotian Shelf (SS) in April 1995 and 1997 and on additional lines on the western SS and across Cabot Strait (CS) and the Laurentian Channel (LC) in 1997. Calanus finmarchicus was more abundant on the central and western SS than in the east, and Calanus hyperboreus was concentrated in the east and in areas influenced by the Nova Scotia Current. Calanus spp. on the eastern SS probably originated mainly from populations overwintering in the Gulf of St. Lawrence. High abundances of C. finmarchicus occurred at or beyond the shelf break when warm slope water was present, suggesting that they derived from a population overwintering in the deep water south of the SS. In both years, slope water was present in the near-surface layers at shelf stations of the Halifax Line (HL), but in 1997 the degree of intrusion was greater. Calanus finmarchicus abundances were also substantially higher at shelf stations of the HL in 1997, implying contribution from the offshore population to the populations in central and western SS regions. Consistent with the suggested difference in their origins, C. finmarchicus on the western SS belonged primarily to the new year's generation, whereas those on the eastern SS belonged mainly to that of the previous year.