Diet of the gelatinous zooplankton in Hardangerfjord (Norway) and potential predatory impact by Aglantha digitale (Trachymedusae)

The abundance, spatial distribution and diet of the gelatinous zooplankton collected at 5 stations along the Hardangerfjord (Norway) in spring 1992 were investigated. Medusae and siphonophores dominated in abundance and were concentrated in the upper 50 m where a strong halocline (28.2 to 34.5 psu) was present. Obelia spp. (up to 158.1 ind, m 3), Aglantha digitale (up to 57.4 ind. m(-3)), Rathkea octopunctata (up to 14.7 ind. m(-3)) and Lensia conoidea (up to 38.7 eudoxids m(-3)) were the most abundant species. Their stomach contents showed a wide variety of food items but the copepods Oithona similis and Temora longicornis and the cladoceran Evadne nordmanni were the main prey. The daily predation rate on copepods estimated for A. digitale was 23.5 to 145.6 copepods m(-3) d(-1) The percentage of the copepod population consumed daily by A. digitale was 0.2 to 1.5%. Excluding nauplii, since this stage was never found in the stomach of the specimens examined, the percentage consumed increases to 1.6-6.6%. A. digitale potentially ingested up to 5.7 and 8.7 % of the O. similis and T. longicornis (adults and copepodites) populations respectively. These low estimations of the predatory impact might not reduce standing stocks of the copepod populations. However, the rates of predation might have been underestimated and the total predatory impact by all cnidarian species could have been substantial.