Fish distribution in a mountain area in south-eastern Norway: human introductions overrule natural immigration

Abstract

The aim of this study is to examine the impact of man’s interventions on the present day distribution of fish in a mountain area in southern Norway, the Atna river system. River Atna originates in the Rondane mountains at altitudes of nearly 1600 m a.s.l., and joins River Glomma at 300 m a.s.l. There are 98 lakes in the watercourse (701–1565 m a.s.l.). Lake Atnsjoen is the largest lake (5.0 km2). Data on the occurrence, origin and status of fish in lakes were obtained by means of interviews, questionnaires and written sources. Occurrence in rivers and streams was surveyed by electrofishing. While the lower reaches of River Glomma contain most of the freshwater fish species found in Norway, the Atna watercourse has a poor fish fauna. Physical conditions, e.g. steep river gradients and several impassable waterfalls have prevented fish from reaching most lakes and river stretches after the deglaciation some 6000 years ago. Five species of fish are regarded as native to the area; brown trout (Salmo trutta), Arctic charr (Salvelinus alpinus), grayling (Thymallus thymallus), Siberian sculpin (Cottus poecilopus) and European minnow (Phoxinus phoxinus). Although only native species are found in the area, the present distribution of these species within the watercourse is to a very large extent a result of human interventions during the past 100–130 years. Brown trout were originally found in the main branch of the river, including Lake Atnsjoen and a few small lakes (n<5) further upstream, but it is now found in 65 lakes. Arctic charr were native only to Lake Atnsjoen, but now inhabit 20 lakes. Grayling remains restricted to River Atna below the waterfall at the outlet of Lake Atnsjoen. The natural distribution of Siberian sculpin is restricted to the main river below Liafossen waterfall (14 km above Lake Atnsjoen). During the 1890s, this species was accidentally introduced to Lake Setningsjoen, and subsequently spread to another three lakes further upstream. The European minnow was native only to the River Atna below Lake Atnsjoen, but was accidentally introduced to seven lakes in the course of this century. There are 24 fishless lakes in the watershed, mainly mountain lakes between 1033 and 1587 m a.s.l.