Jostein Skurdal

Crayfish plague and management strategies in Norway.

Abstract This paper describes the crayfish plague situation in Norway as at October 1991 and describes the spread within and between watersheds. Four watersheds are so far affected: the Glomma, the Store Le, the Halden and the Veksa/Vrangselva. The last was affected during 1971–1974 whereas in the others the disease occurred in the period 1987–1989. In the Veksa/Vrangselva watercourse a dense crayfish population has been re-established due to restocking. In addition, the paper describes the management strategies implemented by the authorities in order to reduce the risk of spreading the disease. The attempts to re-establish extinct and protect threatened populations are also described.

Niche overlap between young brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) in a subalpine Norwegian river

The density, diet and habitat use of brown trout (Salmo trutta) and Siberian sculpin (Cottus poecilopus) were studied in the subalpine River Atna in southeastern Norway in the autumn during a six year period (1986–1991). There was an inverse relationship between the density of brown trout and Siberian sculpin. Diet overlap, as indicated by the Schoener index, was high between the two species, ranging between 0.48 and 0.86. Chironomid larvae and other aquatic insects were the most common food items for both species. Brown trout also consumed substantive amounts of surface insects. Siberian sculpin typically occupied sites with finer substrates and greater water depths than brown trout, even though there was considerable overlap in habitat use between the two species. Because the two species shared similar habitats, we suggest that the potential for species interactions exists, particularly at sites where density of sculpin is high.

Vertical distribution and substrate preference of brown trout in a littoral zone

SynopsisWe studied vertical distribution, substrate preference and food choice of brown trout, Salmo trutta, from benthic gillnet catches at four littoral sampling locations in a Norwegian hydroelectric reservoir. The sampling locations had different bottom substrates; at one location the bottom substrate consisted of sand, while at the other three, substrates consisted of stones ranging 2–5 cm, 10–30 cm and 30–150 cm in diameter, respectively. Small-sized (< 160 cm) and intermediate-size (164–269 mm) brown trout were mainly caught close to the bottom (0–0.5 m above). Small-sized brown trout were caught in the highest frequency at the location with substrate consisting of 10–30 cm large stones. Intermediate-sized brown trout were also caught in highest frequency at this location, but were also caught in a high frequency at the location with sandy substrate. In contrast, the catches of large-sized (≥ 270 mm) brown trout did not vary with distance from the bottom or with substrate coarseness. The most important food items for the brown trout were aquatic insects, surface insects, Eurycercus lamellatus and crustacean zooplankton, mainly Daphnia longispina, Bythotrephes longimanus, and Holopedium gibberum. In accordance with the differences in vertical distribution, benthic food was more important to small than to large brown trout. We argue that small brown trout stayed close to the bottom to reduce aggressive behaviour from larger specimens, and that small brown trout were therefore more dependent on benthic food items. We also argue that the observed differences in substrate preference between the size groups of brown trout is explained by variation in access to shelter, visual isolation between individuals and benthic feeding conditions between locations.

Age and Size at Sexual Maturity of Heavily Exploited Arctic Char and Brown Trout in Lake Atnsjø, Southeastern Norway

Abstract In Lake Atnsjo, Southeastern Norway, Arctic char Salvelinus alpinus and brown trout Salmo trutta both attained sexual maturity mainly at 6 years of age and older, usually at a body length larger than 26 cm. Both species have been heavily exploited at least since the turn of the century. The annual survival rate was estimated at 0.38 for Arctic char (age-groups 6–9) and 0.49 for brown trout (age-groups 4–9). Mean female age at maturity was higher than the optimal age at maturity predicted if female reproductive success only depended on fecundity. We argue that reproductive advantages of larger parental body size, due to high competitive ability during spawning, favored late female maturation and also limited the selective effect by the fishery for earlier maturation.

Habitat use among size groups of monomorphic whitefishCoregonus lavaretus

The whitefish (Coregonus lavaretus) in Lake Tyrifjorden, southeastern Norway, is monomorphic with regard to gill raker number: (x=32), and exhibits a bimodal length distribution. The small whitefish (<27 cm) were distributed mainly in the littoral zone and the deeper layers of the pelagic zone. The larger whitefish (>27 cm) were mainly distributed in the upper layers of the pelagic zone. Both small and large whitefish fed on zooplankton in the pelagic zone. In the littoral zone the diet of the two size groups was significantly different; the small whitefish fed mainly on zooplankton whereas the large whitefish fed mainly on zoobenthos.

Effects of Temperature and Precipitation on Breeding Migrations of Amphibian Species in Southeastern Norway

To reveal the effects of climate, a generalized linear mixed model was used to explore the variation in onset of spawning migration for the two newt species T. cristatus and L. vulgaris in southern Norway. Amphibians are highly influenced by the physical environment, such as temperature and rainfall. The first migrating newts were observed subsequently to the three first consecutive days with mean temperature close to or above 4°C. Further, migration of L. vulgaris was facilitated at lower temperatures compared to T. cristatus, but the migration was dependent on higher precipitation levels. Northern populations of T. cristatus and L. vulgaris may already benefit from a warmer climate due to increased recruitment and juvenile survival. However, an offset in the migration phenology due to climate change might further alter the recruitment and survival rates with either positive or negative outcome. Thus, variations in migration phenology for newts due to climate change may have implications for management and protection status in many systems. In a general context, we should increase emphasis on protecting newts and support increased populations and distribution.

Increased proportion of mature females of the noble crayfish (Astacus astacus L.) in culture conditions

Abstract Crayfish were collected from Lake Steinsfjorden, Norway, one month before maturation, and transferred to culture conditions. Of the females in culture, 92.8% matured, whereas only 36.3% of females in the lake in the same year (1986) matured. During the preceding 5 years the proportion of mature females in the lake had varied between 6.5 and 62.1%. In 1986 females maturing in the lake ranged in length from 75 to 94 mm whilst those maturing in culture ranged from 85 to 109 mm. However, neither this difference in length distribution nor differences in food and temperature regimes could account for the difference in maturation frequency. It is suggested that stress may act as a maturation trigger in culture.

Radiocesium decay in populations of brown trout and Arctic char in the alpine Atna area, south-eastern Norway

Some of the highest recordings of radioactive fallout from the Chernobyl accident in Norway were found in the south-eastern Oppland and Hedmark counties. Cesium content and decay rates were followed in populations of brown trout and Arctic char in Lake Atnsjøen over the years 1986–1995. These results were compared with samples for adjacent lakes to test for between-lake variabilities within the same region. The data on brown trout was compared with samples from a wider region, where more than 1800 individual samples of brown trout were collected from nearly 100 localities. Back-calculated initial activity per 1 January 1987 showed a strong regional and within-lake variability for these localities, ranging from 437 to 18000 Bq kg−1 (average: 2416 Bq kg−1), while trout and char from Atnsjøen had initial activities of 1259 and 1122 Bq kg−1, respectively. Most of the other populations from the Atna region also had initial activities below the average (around 2000 Bq kg−1). Ecological half-life for both brown trout and Artic char in lake Atnsjøen was close to 1.7 years (corresponding to a decay rate of 0.4 year), which was consistently lower than the 2.5 years average for the entire lake data set. Decay constants from linear regressions of total decay over time ranged from 0.15 to 0.28. For trout populations, a positive correlation was found between initial load and decay constants. Although trout and char clearly differ in their diet, no consistent differences were recorded in initial activities or decay rates for these two species in the Atna area.