Abstract
Periphylla periphylla is normally a deep oceanic species, but high densities of this species have persisted for about 20 years and impeded fishing in Lure~orden, western Norway. In October 1991, the biomass was estimated to 6 g AFDW m-2. Mean WW of the individuals was about 410 g, mean DW was 4 % of WW, and AFDW was between 29.2 and 46.0% of DW. Individuals with DW > 5 g had a mean AFDW of 33 %. The population consisted of individuals of rather uniform size, unlike populations in Sognefjorden andii1 the oceans, which have a broad range of size classes and are dominated by very small individuals. P. periphylla is distributed below 60 m during the day, and undergoes a migration to the surface after sunset. Individuals caught alive at the surface in March 1992 were feeding on copepods; P. periphylla is probably the major pelagic predator in the fjord. The pelagic fish community seems to be affected by the presence of P. periphylla. Merlangius merlangus, often reported to be associated with medusae, is the dominant larger pelagic fish in Lurefjorden, contrary to the situation in the nearby fjords. The mesopelagic fishes Maurolicus muelleri and Benthosema glaciale, two of the most important copepod predators in the neighbouring fjords, are not present in Lurefjorden, perhaps as a result of competitive exclusion. Possible mechanisms behind the high density of medusae. in the fjord are discussed, and a review with new information of the occurrence of P. periphylla in Norwegian waters is given.
