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 zooplan kton collected at 5 stations along the Hardangerfjord (Norway) in spri ng 1992 were investigated. Medusae and siphonophores dominated in abun dance 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), A glantha 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 variet y of food items but the copepods Oithona similis and Temora longicorni s and the cladoceran Evadne nordmanni were the main prey. The daily pr edation rate on copepods estimated for A. digitale was 23.5 to 145.6 c opepods 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 st age was never found in the stomach of the specimens examined, the perc entage consumed increases to 1.6-6.6%. A. digitale potentially ingeste d up to 5.7 and 8.7 % of the O. similis and T. longicornis (adults and copepodites) populations respectively. These low estimations of the p redatory impact might not reduce standing stocks of the copepod popula tions. However, the rates of predation might have been underestimated and the total predatory impact by all cnidarian species could have bee n substantial.