Seasonal and interannual variation in abundance of Calanus finmarchicus (Gunnerus) and Calanus helgolandicus (Claus) in inshore waters (west coast of the Isle of Man) in the central Irish Sea

Abstract

The seasonal and interannual variability in abundance of Calanus finmarchicus and Calanus helgolandicus in the Irish Sea was examined on the west side of the Isle of Man (central Irish Sea) between 1995 and 2001. There was considerable interannual variability in abundance of both species that was greater than that seen in adjacent seas. In general, maximum abundance occurred between May and July each year with C. finmarchicus tending to occur earlier in the year than C. helgolandicus. There was a significant difference in interannual maximum abundances in the area (14-18 stations treated as replicates) with most years having a low abundance and one year a relatively high abundance. The maximum abundance of C. finmarchicus (1996) coincided with a low (negative) index value of the North Atlantic Oscillation (NAO), and with a high abundance of young gadoids (cod, haddock and whiting) in the water column and a large settlement of plaice. A high abundance of helgolandicus coincided with a relatively high (positive) index value of the NAO in 2000. The magnitude of interannual and seasonal variability in abundance was in close agreement with data from an inshore area (Port Erin Bay, Isle of Man) between 1907 and 1920. As with many Calanus populations, the adults in both species were predominantly female. As shown previously, Calanus is not one of the numerically dominant copepod species in the Irish Sea. However, the presence of young stages of Calanus within the central Irish Sea suggests that it must support a breeding population.