Relationships between hydrochemistry and the presence of juvenile brown trout ( Salmo trutta ) in headwater streams recovering from acidification

Salmonid populations are highly sensitive to the impacts of acidification. This has led to the loss of fish populations from heavily impacted areas. There is recent evidence of chemical recovery, but evidence of biological recovery and in particular recovery of fish populations has been rare. This has led fisheries groups to question whether acid recovery is occurring and the likely timescales for tangible improvements. To answer these questions and to provide an evidence base for the management of acidification impacts requires identification of chemical thresholds for sensitive biota. We analysed a robust and powerful chemistry and fish dataset collected from 22 sites in the United Kingdom Acid Water Monitoring Network (AWMN) over a period of 22 years. Specifically we assessed (1) whether there were significant temporal trends in the presence of brown trout across the network, consistent with acid recovery, (2) the relationships between hydrochemical determinands indicative of acidification impacts and the presence of brown trout fry and parr. The effects of chemical determinands were assessed both alone and in combination.