Reproductive cycles of dominant calanoid copepods in the North Sea

The seasonal cycles of reproduction of the dominant copepods Acartia clausi, Temora longicornis, Centropages hamatus, C. typicus and Pseudocalanus spp. were studied at Helgoland Roads, southern North Sea. Egg production rate, clutch size, number of spawning females and prosome length were compared with temperature and phytoplankton concentration. Females of Temora, Acartia and Pseudocalanus were present all year round. Temora and Pseudocalanus bred continuously throughout the year, although egg production rates and the proportion of spawning females were low in winter. Acartia did not spawn between the end of September and the end of January. Female Centropages hamatus were absent from the water column from November to May, and C. typicus from February to August. As the latter three species spawn year round in warmer regions, low temperature inhibition of reproduction is suggested for Helgoland, where winter temperatures are below 0°C. Maximum egg production was observed in all species, except the late arriving C. typicus, in April/May, when females were largest. Significant correlations were only obtained between clutch size or egg production rate and prosome length, which in turn was correlated with temperature. It is therefore concluded that for Helgoland water temperature controlled egg production by its effect on prosome length, and that food conditions were not limiting.