TROPHODYNAMICS OF PLEUROBRACHIA-PILEUS (CTENOPHORA, CYDIPPIDA) AND CTENOPHORE SUMMER OCCURRENCE OFF THE NORWEGIAN NORTH-WEST COAST

Stomach-content analyses and laboratory experiments on Pleurobrachia p ileus (Cydippida) showed an average digestion time of 2.0 h at 12 degr ees C and a high potential predation rate with highest daily ration in terms of prey carbon ingested as percent of predator body carbon for the calanoid copepod Calanus finmarchicus, the biggest prey tested. Pr edation rate increased almost linearly with increased prey abundance o ver the whole range tested (12-1043 l(-1) in start concentration) of m ainly small-sized copepods. Tests of the importance of prey size showe d an individual clearance rate of 6.1 l day(-1) with Calanus prey alon e, which was depressed to 29 % of this when smaller prey was also pres ent in high abundance. This is supposed to be an effect of handling ti me of prey in the feeding process. The laboratory results were used to estimate the impact of this species in Norwegian coastal waters. Abun dance data were collected in summer from 56 stations between 63 degree s and 69 degrees N along a cruise track west of Norway. P. pileus was present in the southern part of the investigated area and was restrict ed to the uppermost 50 m throughout the day. It mainly occurred where its predator, the atentaculate ctenophore Beroe sp., was absent and it s abundance was not correlated with the ambient prey biomass. Estimati on of the predatory impact on the zooplankton stock in the area where P. pileus occurred, using experimentally determined daily ration, clea rance rates, abundance and individual size of ctenophores, and zooplan kton biomass in the field, revealed a rather low impact. Integrated ov er time and depth (down to 100 m) these calculations suggested that up to a few percent of the zooplankton stock could be removed daily by P . pileus predation, but in the surface water, where it was most abunda nt, it could be of considerable importance in controlling the zooplank ton abundance.