Introduction of the European minnow into a subalpine lake: habitat use and long-term changes in population dynamics

Minnows Phoxinus phoxinus, studied 30 years after the first record of the species in the subalpine Lake Ovre Heimdalsvatn, Norway, greater than or equal to55 min L-T, were estimated to have densities of c. 4.7 kg ha(-1) (120 000 fish) in June 1999 and 2.1 kg ha(-1) (63 000 fish) in June 2000. The population was characterized by low individual growth, delayed age of maturity and lived longer when compared to values reported in a previous study in the early phase of its establishment, and other values reported in the literature. Most minnows reached sexual maturity at 4-5 years and >55 mm L-T. Although the estimated annual survival of minnows >55 min was low (S=0-2), ages up to 13 years were recorded. Despite a moderate increase in the population size during the last 20 years, the present reduction in individual growth, followed by delayed age of maturity, suggested the existence of density-dependent effects on the population dynamics of the minnows. The minnows were restricted to the littoral zone and near bottom areas. A vertical or horizontal expansion in habitat use was probably prevented by the presence of piscivorous brown trout Salmo trutta. (C) 2002 The Fisheries Society of the British Isles. Published by Elsevier Science Ltd. All rights reserved.