Long-term study of the ecology of wild Atlantic salmon smolts in a small Norwegian river

Abstract

Backcalculated lengths at the end of the first growth season in wild Atlantic salmon Salmo salar differed significantly between parr smolting at age 1, 2 and 3 years over a period of 11 years (i.e. 1983-1993). Mean body lengths of the respective age groups at the end of the first growth period were 11.1, 6.2 and 4.7 cm, respectively. The mean percentage distribution of fish smolting at age 1, 2 and 3 was 14, 78 and 7%, and the mean smolt age was 1-95 years. Mean lengths at smolting of age groups 1, 2 and 3 were 13.6, 15.8 and 17.5 cm, respectively. Females outnumbered males among the downstream migrating smolts with a mean sex ratio (females/males) estimated at 1.61, with a significant female surplus in 7 of the 11 years sampled. Of the smolts sampled, 14% exhibited enlarged gonads indicative of parr maturation, and all were males (37% of the parr males sampled). Mean annual smolt density from 1975 to 1996 was 13.4 individuals 100 m-2 ranging between 0.3-31 smolts 100 m-2. Mean densities (100 m-2) of the smolts aged 1, 2 and 3 years were 1-5, 9.3 and 0.9 fish, respectively. Mean annual biomass for the 22-year period (1975-1996) was estimated at 437 g 100 m-2, with a range of variation from 136 to 683 g 100 m-2. Smolt age 2 made up 81% of the mean annual biomass (355 g 100 m-2) and smolt age 1 and 3, 8% (35 g 100 m-2) and 11% (47 g 100 m-2), respectively.