Densities of Atlantic salmon (Salmo salar L.) fry in relation to physical habitat, spawning redd distribution and parr abundance in a small subarctic river

Abstract

Two small-scale surveys were conducted in a small river to examine the effects of physical habitat characteristics, spatial spawning redd distribution and abundance of older juveniles on the Atlantic salmon (Salmo salar L.) fry (age 0+) distribution 1) shortly after the emergence and 2) after the assumed main dispersal period of fry. The effect of physical habitat variables (water depth, water velocity and substratum size), spatial spawning redd distribution and densities of older juveniles were examined using multiple linear regressions. Identical models were fitted with generalized habitat suitability indices to test their applicability to predict the variation in fry densities. Clear patterns of habitat use were found in relation to individual physical habitat variables and suitability indices, but there was a large variation in densities, with this variation often being largest within optimum habitats and the capability of regression models to explain this variation was relatively poor, ranging from 22 to 34 %. It is suggested that physical habitat variables may rather act as limiting edges than determinants of habitat selection of fry at within-reach scales. Fry densities were not significantly related to densities of older juveniles. Spatial spawning redd distribution was positively correlated with fry density approximately 2 months after the emergence. This result supports previous findings that the spatial distribution of spawning areas is an important factor that affects habitat selection of Atlantic salmon fry.