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

Abstract

The abundance, spatial distnbution and d ~e t of the gelatinous zooplankton collected at 5 stations along the Hardangerfjord (Norway) in spnng 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 dlgitale (up to 57.4 ind. m-3), Rathkea octopunctata (up to 14.7 ind. m-3) and Lensia conoldea (up to 38.7 eudoxids n1r3) were the most abundant species. Their stomach contents showed a wide variety of food items but the copepods Oithona similis and Temora longicornls 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-J d.' The percentage of the copepod populat~on consumed dally by A. digitale was 0 2 to 1.5%. Excluding naupl~i, since this stage was never found in the stomach of the specimens examined, the percentage consumed increases to 1.6-6.6%. A. d ~g ~t a l e potentially ingested up to 5.7 and 8.7% of the 0. simllls and 7: longicornis (adults and copepodltes) 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.