Size-dependent survival of juvenile Atlantic salmon Salmo salar and brown trout Salmo trutta from the cold river Beiarelva, northern Norway

Abstract

The growth of Atlantic salmon Salmo salar L. and brown trout Salmo trutta L. in the very cold river Beiarelva is so low that not all individuals reach the critical lengths during their first summer to form scales. Hence, after their first winter, each year-class of Atlantic salmon and brown trout can be divided into two length groups, with the smaller fish lacking the first annulus, and the larger fish having this annulus. For this reason length-dependent survival for the rest of their freshwater life could be examined. In brown trout the same survival was found in small and larger parr of all year-classes investigated. In Atlantic salmon the same was found in all year-classes except one. In one year-class the survival was significantly less in the smaller group than in the larger. In 2+ and older fish of both Atlantic salmon and brown trout some of the small individuals had grown as long as most of the individuals having the first annulus, whereas others were still the smallest ones in the year-class. Hence, the smallest yearlings are not always the smallest fish in later years. The very good food conditions for the parr in river Beiarelva may explain the good survival of small individuals.