Jan Ivar Koksvik

Food and feeding habits of Triturus vulgaris (L.) and T. cristatus (LAURENTI) (Amphibia) in two bog tarns in central Norway

The diets oftwo sympatric species of newts, Triturus vulgaris(L.) and T. cristatus(LAURENTI), from two localities in central Norway, have been compared. Adult T. vulgaris ate mainly those Cladocera which can be classified as planktonic, and to a lesser degree those which are predominantly benthic. The food of T. cristatus was predominantly benthic invertebrates and Cladocera associated with the bottom. T. vulgaris larvae ate planktonic - and predominantly benthic Cladocera, and to a certain degree also larger benthic invertebrates, while T. cristatus larvae (July-August) had taken almost exclusively Cladocera, 75% of which can be classified as planktonic. Large T. cristatus larvae (in September) had eaten relatively more benthic prey. These data would fit the hypothesis that the modes of feeding of the species are different: Adult T. vulgaris swim about in the water much of the time, while T. cristatus stay on the bottom. Among the young, T. vulgaris larvae are mainly associated with the bottom, while T. cristatus larvae are definitely more nektonic during most of the summer; large T. cristatus larvae, however, when nearing the time for metamorphosis, become benthic.

Does reduced feeding prior to release improve the marine migration of hatchery brown trout Salmo trutta smolts

The aim of this study was to test the hypothesis that hatchery brown trout Salmo trutta smolts, with 50% reduced or no feeding over the last 5 months before release, were more likely to migrate to the sea than individuals with standard feeding ratios. The juvenile fish were divided into three groups 176 days before release: (A) with no feeding, (B) with 50% and (C) with 100% feeding. To study their seaward migration, 40 fish from each feeding group were tagged with acoustic transmitters and tracked by automatic listening stations in the River Nidelva, Trondheim, Norway, its estuary and in the nearest marine environment. At the time of release, mean condition factor was significantly lower in group A and the fish from groups A and B had higher levels of Na+, K+-ATPase. Significantly more fish from group A migrated to the sea, but the rate of downstream progression from release to the estuary did not differ between the three groups. In conclusion, the S. trutta smolts with no access to food in the last 176 day before release were more likely to migrate to the sea. Fish from all three feeding groups, however, appeared to smoltify and had the same rate of downstream progression to the estuary. This indicates that differences in migratory behaviour between individuals from the three feeding groups begin from the time when the fish reach saline waters. It is suggested that feeding in hatcheries has to be greatly reduced (by 50% or more) over several months to have a pronounced effect on the migratory behaviour in S. trutta.

Seasonal occurrence and diel locomotor activity in littoral Cladocera in a mesohumic lake in Norway

In situ studies were carried out using activity traps during the course of one year. Of 35 species recorded, only one, Alonella nana, was found to be perennial. The majority of species hatched in May/June, at temperatures of 4–15 °C and disappeared in October/November at temperatures of 1–6 °C. Males were found in 23 species, and always at the end of the active period. No species were found to have more than one period of gamogenesis. Day activity, which was most intense in the afternoon, seemed to be the common pattern. The activity pattern in some species seemed to be influenced by the presence of males.

Prey availability and juvenile Atlantic salmon feeding during winter in a regulated subarctic river subject to loss of ice cover

Increased mortality of juvenile Atlantic salmon (Salmo salar L.), related to lowered levels of stored energy following the loss of ice cover during winter, has been observed after hydropower development in the subarctic River Alta, northern Norway. Drift samples were compared to examine if drift densities, and thus drift prey availabilities for juvenile salmon, were lower in the ice-free than the ice-covered area. In addition, juvenile salmon stomach contents were compared to benthos and drift in the ice-free area to examine salmon winter feeding habitat. Zooplankton, originating from the reservoir, dominated drift at the ice-free site but had lower densities at the downstream ice-covered site. Excluding zooplankton, Chironomidae comprised most of the remaining drift at both the ice-free and ice-covered site, followed by Ephemeroptera, Plecoptera and Simuliidae. No Trichoptera were found in the drift samples. There was no consistent diel periodicity in drift. Benthos was dominated by Chironomidae, followed by Ephemeroptera, Plecoptera and Trichoptera. Other invertebrates occurred in low numbers. Juvenile salmon demonstrated size-selective feeding and fed mainly on Ephemeroptera, followed by Trichoptera and Plecoptera. No zooplankton and few Chironomidae were found in the stomach samples. Stomach content was more similar to benthos than to drift, indicating a larger extent of benthic than drift feeding. No evidence was found for the hypothesis that lack of ice cover reduced the invertebrate drift or caused diel periodicity in the drift. Differences in drift between areas with and without ice could not account for the observed differences in mortality of juvenile salmon during the winter in these areas.

Riverine and near coastal migration performance of hatchery brown trout Salmo trutta

To study migration performance and return rates of hatchery brown trout Salmo trutta smolts the first 5 months after release, 50 fish in each year (fork length, LF , 158-288 mm) were in two subsequent years tagged with acoustic transmitters and recorded by automatic listening stations in the River Nidelva (central Norway), its estuary and in the marine environment. More than half of the smolts became anadromous migrants (52% in 2011 and 70% in 2012). The fish spent longer time in the estuary than in the marine environment and the results suggest that migratory behaviour of S. trutta smolts is not only restricted to be resident or anadrome-lacustrine, but that there is also an intermediary strategy of estuarine feeding. There were no differences in LF or mass between groups of smolts with different migration patterns. Return rates from the sea within the first 5 months after release were in both years 16%. Median progression rate in the river was 0·090 LF s(-1) but decreased significantly as the smolts entered the estuary (0·015 LF s(-1) ). The long residential time in the estuary may increase the risk of negative effects of anthropogenic activities in estuaries, such as harbours and industrial development, and special attention should be given to evaluate effects of such activities.

Some observations on seasonal variation of radio-Cesium contamination in trout (Salmo trutta L.) and arctic char (Salvelinus alpinus (L.)) in a Norwegian lake after the Chernobyl fall-out

This study of radioactive contamination in freshwater fish shows a clear dependence to seasonal feeding habits. The mechanism of radioactivity storage in the fish body seems to be selected to concern mainly radio-Cesium (strontium content not explored). The biological half-life of this element is estimated to be in the order of one to one and a half month. The fall-out particles must have a faster rate of sedimentation than earlier expected.

Effects of experimental nutrient enrichment on the macroinvertebrates in a stream

Litjvasselva is a tributary of the River Vefsna in northern Norway, l 00 krn south of the Arctic Circle. It is a small stream with a tora! length of 14 krn. In the upper part i t is 2-3 m wide, in the lower parts u p to l O m. Mean summer water flow in rhe middle part was estimated at 0.5 m/s. The ice-free period normally lasts from late May to the end of October. There are no human setdements or direct pollution from human activities in the catchment area, which mainly consists of deciduous and coniferous forests and marshland.