Mass occurrence of the physonect siphonophore Apolemia uvaria (LESUEUR) in Norwegian waters

At the end of November 1997 salmon farmers in Hordaland County on the coast of western Norway reported mass occurrence of an unknown animal that caused severe lesions and death of fishes. The organism was identified as the physonect siphonophore Apolemia uvaria. A survey of reported sightings (Fig. 1) shows that the species ranged from Gullmarsfjorden on the Swedish west coast to Finnmark in northern Norway. The first known observation from Scandinavia was from Kosterfjorden in western Sweden in early November. After news about death of farmed salmon was first made public by radio, television and newspapers in the last part of November, additional reports arrived almost concomitantly from sites all along the coast (see reports in Havbruksrevyen, internet: http:// www.intrafish.no). Based on problems reported by salmon farmers it seemed that the abundance of siphonophores along the mid-Norway coast decreased after mid December (Karl Tangen, pers. commn). In mid December we made a field survey from SvinøY along a 300 km transect to the northwest (see Fig. I), where CTD casts and zooplankton sampling (l-m2 MOCNESS and a WP2 net) were performed. These collections showed that A. uvaria was present in coastal water as well as in open ocean water and that it occurred as deep as 300 m, mainly in Atlantic water, indicating an origin from the Atlantic (Fig. 2). Only one of four samples from the transect parallel to the coastline south of SvinøY contained A. uvaria. Between mid December and the beginning of February 1998 Apolemia also occurred within the inner parts of fjords in Rogaland County and southern Hordaland County, and was also present in open waters, although no further problems for salmon farmers were reported after December 1997. © 1998 Taylor & Francis Group, LLC.