Mesohabitat Evaluation Reveals Variable Abundances and Habitat Choice in Juvenile Atlantic Salmon Across Diverse Habitats in the Main Stem of a Large Sub-Arctic River

Abstract

Large main stem rivers typically provide a variety of mesohabitat types, but their abundance, distribution and role in providing habitat for juvenile salmonids have rarely been analysed. The quantity of meso-scale habitats and their juvenile Atlantic salmon abundance was surveyed in the large sub-arctic River Teno in northernmost Fennoscandia. Different habitat types were mapped using the Norwegian Mesohabitat Classification Method (NMCM). Juvenile salmon abundance in different mesohabitats, on various substrates, depths and lateral positions across the channel was estimated by electrofishing boat surveys. Most of the Teno main stem (67%) was dominated by fine substrate and > 50 cm s⁻¹ flow velocity. Typical juvenile salmon-rearing habitats, such as rapids, riffles and glides, comprised 32% of the riverbed. Data from NMCM and electrofishing surveys were used in generalised linear models to describe the relationship between habitat variables and juvenile salmon (YOY and parr) abundance. Juvenile abundance varied a lot across and within mesohabitats; in general, abundance was higher in areas with depth ≤ 70 cm than in deeper areas, and in middle sections of the river compared to those near shorelines. Juvenile salmon were documented also in sandy areas, although in low abundances, and in relatively high abundances in areas deeper than typically considered important for juvenile salmon. These areas should be considered in assessing habitat potential for juvenile salmon production. NMCM proved to be a fast, cost-effective method for surveying large areas for habitat assessment.