THE SEASONAL ABUNDANCE OF THE COPEPODITE STAGES OF CALANUS-HELGOLANDICUS AND PSEUDOCALANUS-ELONGATUS OFF PLYMOUTH

The relative abundance of the six copepodite stages (CI-CVI) of Calanus helgolandicus Claus (Copepoda: Calanoida) and Pseudocalanus elongatus Boeck (Copepoda: Calanoida) was recorded at a station in the English Channel throughout 1989. There was a pronounced seasonal variation in the abundance of the early stages. Increased recruitment from the nauplii corresponded in both species to rising primary production early in the year, producing small CI abundance peaks in March. The major period of naupliar development occurred after the April/May spring bloom. During seasonal peaks the CI and CII stages typically formed 40% of the copepodites of these species but were scarce at other times. The body carbon weight of Calanus and Pseudocalanus CIs decreased by 38% and 21% respectively from May to August, indicating an increase in food limitation for the nauplii. Summer was passed predominantly in the CIV and CV stage at low abundances. The over-wintering Pseudocalanus population was formed from these individuals but that of Calanus was derived from nauplii that metamorphosed in late October. Throughout the year the adult sex ratio of both species was biased 5:1 in favour of females. Seasonal variation in copepodite body carbon weight was interpolated from regressions of weight and length. Pseudocalanus attained maximum weight in April, Calanus one month later coincident with the spring bloom, and both declined through the summer. Members of over-wintering populations are 40% lighter than the corresponding spring individuals. The implications of food limitation upon the population dynamics of these species are discussed.