Vertical distribution and seasonal numerical abundance of the Calanidae in oceanic waters to the south-west of the British Isles

Abstract

The vertical distribution and seasonal abundance of the copepodite and adult stages of Calanus finmarchicus, C. helgolandicus, C. tenuicornis, Neocalanus gracilis, Nannocalanus minor and Calanoides carinatus from a series of Longhurst Hardy Plankton Recorder hauls taken in the oceanic waters, off the continental shelf, to the south-west of the British Isles are described. The sampling area was selected because the geographical distributions of the major Calanidae copepods of the north-east Atlantic Ocean are shown to overlap in this region. It marks the southern boundary of the distribution of C. finmarchicus, the central area of C. helgolandicus and the approximate northern limit of distribution of C. tenuicornis, N. gracilis, N. minor and C. carinatus. These four southern species occasionally penetrate further north (60° N) in the open ocean but do not breed at these northern latitudes. In autumn and winter, when C. finmarchicus and C. helgolandicus were overwintering below 400 m primarily as Stage V copepodites, N. gracilis occurred in the upper 200 m of the water column in a breeding condition; all copepodite stages were present. This copepod reproduced throughout the year in this regions while C. tenuicornis was observed to breed primarily in spring and summer. The geographical and vertical distributions of the Calanidae are related to the observed seasonal temperatures of the North Atlantic and the breeding strategies of species arc compared.