The Role of Hydrology, Water Quality and Temperature in Determining Juvenile Atlantic Salmon Population Status

Catches of adult Atlantic salmon, Salmo salar, have declined dramatically across much of their global distribution, particularly at the southern edge of the range. Identifying the environmental factors responsible for these population trends provides a framework for future conservation strategies and will underpin management for multiple end-users. Using a multi-decadal dataset, changes in Atlantic salmon populations across rivers in England and Wales were examined in relation to variation in putative environmental drivers. To provide a mechanistic link between environmental conditions and population size, metrics were developed that described the hydrology, water quality and temperature of these rivers during key periods of the Atlantic salmon’s freshwater life history. Hydrological variations were expressed to capture multiple aspects of the natural flow regime, with the link between life history stages and the frequency, duration and rate of change of high and low flows investigated. Understanding how these factors influence Atlantic salmon population status will allow targeted effort in terms of legislation, monitoring, and habitat rehabilitation.