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.Significant positive temporal trends in fish presence were observed at two of the most acidified sites on the network indicating that limited recovery is occurring where satisfactory chemical conditions have now been attained. Fry were substantially more sensitive to water quality than parr. L-Al was the best single chemical determinand for predicting the presence of fry. L-Al therefore represented the most sensitive overall indicator of acid impacts.It is suggested that (1) chemical thresholds should be derived from chemistry-fry response relationships, (2) standards should be precautionary reflecting the uncertainty in the fry response relationships, (3) environmental standards be developed for L-Al in acidified systems, (4) monitoring of L-Al becomes standard practice in acidified areas and (5) the different methods of calculating ANC are not used interchangeably given their substantially different fish response relationships.