Jan Helge Fosså

Mass occurrence of Periphylla periphylla (Scyphozoa, Coronatae) in a Norwegian fjord

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, ...

The holopelagic life cycle of the deep-sea medusa Periphylla periphylla (Scyphozoa, Coronatae)

Abstract The life cycle of Periphylla periphylla (Peron & Lesueur, 1809) is described. it is the only known direct development of a coronate scyphozoa, showing a holopelagic life cycle without any sessile stage. It also lacks an ephyra stage that normally precedes the medusa stage and it is the first scyphozoan life cycle with sexual propagation where the planula stage is missing. The morphology and histology in the early development is described and a description of eight well recognised developmental stages, covering the period from egg to young medusa, is presented. A brief comparison of the population structure in 1992 and 1997 in Lurefjorden, Norway shows that there has been a dramatic change. The population in 1992 showed a normal size distribution with a median individual wet weight of 540 g, eggs and stages did not occur in the plankton and there was no significant recruitment. Eggs and larvae were first observed in 1993 and since then they have been present in all seasons. The population therefor...

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

Abstract 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...

Near-bottom vertical zonation during daytime of deep-living hyperbenthic mysids (Crustacea: Mysidacea)

Abstract Animals were collected in a Norwegian fjord from 220 m depth with an epibenthic sledge sampling in three levels above the bottom (0–30, 30–80, and 80–130 cm). A height index, Hy, was calculated for 9 sampling dates between February 1980 and January 1981. Different species differed in height above the bottom. There was a general trend that juveniles remained closer to the bottom than older age stages. Statistically significant difference in Hy between sexes was found only for one species. Five species differed significantly in Hy between dates. Hy, calculated from one 305 m locality and from other data sets, showed that species may have different Hy’s at different localities whereas interspecific rank in height above the bottom is similar. Preferred swimming height, or swimming activity, above the bottom may be important when interpreting population fluctuations on a locality with bottom currents. No single environmental or biological factor seems to regulate the daytime height above the bottom.

Diet overlap between two subsequent year-classes of juvenile coastal cod (Gadus morhua L.) and wild and reared cod

Abstract Juvenile coastal cod (Gadus morhua) inhabit the sublittoral part of west Norwegian fjords. This paper analyses whether different co-occurring year-classes share food resources, during a period when food seemed scarce. Stomach contents were compared of the 1987 year-class and wild and reared cod of the 1988 year-class. To a large degree the three predator groups preyed upon the same prey groups, and also upon the same size-classes of Amphipoda, Portunidae and Teleostei. The similarities were higher between wild and reared cod of the same year-class, than between subsequent year-classes. Some differences in prey choice were found as wild and reared cod of the 1988 year-class ate more Copepoda, Amphipoda, Paguridae and less Portunidae and Teleostei than their one year older conspecifics. The stomachs of the 1987 year-class also included larger individuals of Portunidae and Teleostei than the one year younger cod. Possible competition between the year-classes and its implication in cod enhancement in...

Aquarium observations on vertical zonation and bottom relationships of some deep-living hyperbenthic mysids (Crustacea: Mysidacea)

Abstract Animals for aquarium observations were collected at depths of 220–240 m west of Bergen, Norway, using an epibenthic sledge. A special constructed cod-end protected the animals from mechanical damage and exposure to light. The animals were observed in simulated habitat light conditions by means of an infra-red viewing system. The vertical distribution above the sediment in light and darkness was observed for Boreomysis megalops, Erythrops serrata, Hemimysis abyssicola and Mysideis insigrtis and confirmed that light conditions had great impact on the behaviour. Behavioural notes on six other mysids are given and deep-living fjord mysids are classified into five groups with different association to the bottom, from pelagic species found near the bottom to burrowing species. The zonations in the aquarium were in good agreement with zonation data from nature. Aquarium studies are, however, superior for revealing the microdistribution immediately above the sediment.

Changes in behaviour and eye-morphology of BoreomysisMegalops G.O. Sars (Crustacea : Mysidacea) following exposure to short periods of artificial and natural daylight

Abstract Boreomysis megalops G.O. Sars were collected from 240 m depth using a protective cod-end, which shielded the eyes from light exposure. Animals were divided into groups which were exposed to darkness (DE) or to natural or artificial daylight (LE) for periods of 1–15 min or 4 h. Some animals were thereafter kept in aquaria under simulated habitat light conditions (light/dark cycles). The animals were observed in infra-red (IR) light. DE animals showed a vertical zonation behaviour in the laboratory similar to that obtained from sledge data: i.e. the animals stayed close to the bottom during daytime (light) and spread out vertically at night (darkness). LE individuals showed putative pathological changes both in zonation behaviour in the laboratory and in eye-morphology. The photoreceptor membranes of the six main retinula cells were damaged. The seventh cell was unaffected by light exposures. A correlation was found between the extent of damage to the eye and the degree of modification of vertical zonation behaviour. The groups exposed to the least light showed the smallest changes in zonation behaviour and eye-morphology. The light induced changes were not reversed even after 4 days in the aquarium under simulated habitat light conditions. In conclusion, deep living animals with well-developed and very light sensitive eyes should be protected from daylight during sampling and handling. IR light should be used for observation purposes.

A pneumatic, bottom-activated, opening and closing device for epibenthic sledges

Abstract The sledge door opens upon contact with the sea floor and the pneumatic system keeps the door in a fully opened position as long as the sledge has proper contact with the bottom. When the bottom-contact is weak, or the sledge is off the bottom, the door is kept closed. Fluttering of the door when the sledge has weak bottom contact is thus avoided. The system is suggested for use at moderate depths down to about 500 m.

A drop-net method for sampling Gobiusculus flavescens (Fabricius) and other fishes on hard and mixed bottoms with algal cover

Abstract The construction, the sampling procedure and some typical results are described. The device consists of a floating frame (7 × 7 m) with a net suspended on the inside. It is transported by a boat and deployed by means of a pneumatic system and a heavy chain which pulls the net rapidly to the bottom. The net is pursed on the inside of the frame by hand and lifted by a hydraulic winch onboard the boat. The fish fauna caught by the drop-net is compared with the fauna caught in a beach seine, and results on the abundance of Gobiusculus flavescens are presented.