Timing of habitat shift by Atlantic salmon parr from fluvial to lacustrine habitat: analysis of age distribution, growth, and scale characteristics

Abstract

This study investigated the timing of a habitat shift from fluvial to lacustrine habitat of Atlantic salmon, Salmo salar, parr in a subarctic river in northern Finland and whether the individuals that shifted already had the best growth rate prior to the shift or, alternatively, whether the shift was triggered by competition-induced growth depression. The age distribution of parr indicated that most of the fish that shifted belonged to the age group 2+ and the proportion of 1+ fish was less than 3%. However, the analyses of somatic growth and scale characteristics reflected an earlier shift, at the age of 1+, for the greater part of the lacustrine fish. The growth of lacustrine fish exceeded that of their fluvial counterparts after the first year of life. Based on the growth of the first year, no indications that the lacustrine parr were recruited from the fastest growing part of the population were found. However, the growth patterns during the second year may indicate that fluvial parr with the fastest growth at age 1+ may be those that shift at age 2+. Some evidence of growth depression in fluvial fish was found.