Børre Kind Dervo

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.

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.

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.

The old trout Salmo trutta in the pond

The last of 12 brown trout Salmo trutta stocked in a small pond in south-east Norway in 1979 were caught in July 2011. These fish represent a curiosity regarding longevity and growth pattern, and ageing from otoliths was consistent with the known life history of this specimen. Despite almost total stagnation in growth since the age of 16 years, the testes were fully developed.