THE OCCURRENCE OF CALANUS-FINMARCHICUS (GUNNERUS) AND CALANUS-HELGOLANDICUS (CLAUS) IN THE WESTERN IRISH SEA

The seasonal abundance of Calanus finmarchicus and Calanus helgolandic us in the North Channel and stratified region of the western Irish Sea is reviewed using data collected between 1992 and 1996. Both species occur in the western Irish Sea, but were more abundant in the stratifi ed region during spring. Increased abundance during May/June was attri buted to an increase in copepodite stages. Calanus helgolandicus domin ated in both regions, exhibiting spring and autumn peaks in abundance in the stratified region. It is argued that the presence of ripe femal es and naupliar stages in the stratified region is evidence of an in s itu breeding population, rather than advection of individuals from pop ulation centres outside the Irish Sea. The lack of geographical separa tion of the two species in the western Irish sea, and reports that bot h species occur in the Celtic Sea and Malin Shelf, limit the use of ei ther species as indicators of exchange processes between the Irish Sea and neighbouring waters.