The occurrence of Calanus finmarchicus (Gunnerus) and Calanus helgolandicus (Claus) in the western Irish Sea

Abstract

The seasonal abundance of Calanus finmarchicus and Calanus helgolandicus in the North Channel and stratified region of the western Irish Sea is reviewed using data collected between 1992 and 19%. Both species occur in the western Irish Sea, but were more abundant in the stratified region during spring. Increased abundance during May/June was attributed to an increase in copepodite stages. Calanus helgolandicus dominated in both regions, exhibiting spring and autumn peaks in abun- dance in the stratified region. It is argued that the presence of ripe females and naupliar stages in the stratified region is evidence of an in situ breeding population, rather than advection of individuals from population centres outside the Irish Sea. The lack of geographical separation of the two species in the western Irish sea, and reports that both species occur in the Celtic Sea and Malin Shelf, limit the use of either species as indicators of exchange processes between the Irish Sea and neighbouring waters.