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 OC a nd a high potential predation rate with highest daily ration in terms of prey carbon ingested as percent of predator body carbon for the cal anoid copepod Calanus finmarchicus, the biggest prey tested. predation rate increased almost linearly with increased prey abundance over the whole range tested (12-1033 l(-1) in start concentration) of mainly s mall-sized copepods. Tests of the importance of prey size showed an in dividual clearance rate of 6.11 day(-1) with Calanus prey alone, which was depressed to 29 % of this when smaller prey was also present in h igh abundance. This is supposed to be an effect of handling time of pr ey in the feeding process. The laboratory results were used to estimat e the impact of this species in Norwegian coastal waters. Abundance da ta were collected in summer from 56 stations between 63 degrees 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 restricted to th e uppermost 50 m throughout the day. It mainly occurred where its pred ator, the atentaculate ctenophore Beroe sp., was absent and its abunda nce was not correlated with the ambient prey biomass. Estimation of th e predatory impact on the zooplankton stock in the area where P. pileu s occurred, using experimentally determined daily ration, clearance ra tes, abundance and individual size of ctenophores, and zooplankton bio mass in the field, revealed a rather low impact. Integrated over time and depth (down to 100 m) these calculations suggested that up to a fe w percent of the zooplankton stock could be removed daily by P. pileus predation, but in the surface water, where it was most abundant, it c ould be of considerable importance in controlling the zooplankton abun dance.