Overlapping size ranges of Calanus spp. off the Canadian Arctic and Atlantic Coasts: impact on species' abundances

In the North Atlantic, Calanus finmarchicus, C. glacialis and C. hyperboreus are identified based on subtle morphological traits, which is fastidious, or based on their allegedly non-overlapping prosome length ranges. We reappraised the prosome length-based diagnosis for the copepodite stage V stage by coupling prosome length and molecular identifications (mtDNA, 16S gene) for 1159 individuals collected over 2 years from 15 stations off the Canadian coast from the Arctic to the Atlantic. We observed spatial but no intra-annual variation in species' average prosome length. At sympatric coastal sites, prosome length overlap was frequent between C. finmarchicus and C. glacialis and restricted to the Estuary/Gulf of St. Lawrence between C. glacialis and C. hyperboreus. We used discriminant analyses to redefine prosome length criteria to minimize errors in species identification. Species abundances were corrected and this affected mostly C. glacialis. In the St. Lawrence Estuary and on the Labrador shelf, abundance of C. glacialis was underestimated by 19 and 35%, respectively, with important interannual variations since 2000. This increase in abundance could enhance estimation of the role of C. glacialis in the food web and potentially alter our view of the long-term changes along the eastern Canadian coast.