Relationships between afforestation, water chemistry and fish stocks in an upland catchment in south west Scotland

Abstract

The soils and waters in the Galloway hills of south-west Scotland have undergone considerable acidification during the present century. The aims of this study were to examine the extent to which afforestation can influence water chemistry, and the occurrence of fish stocks within an individual catchment in this area. Studies were carried out at 45 sites in the Bladnoch catchment during 1994. River water samples were collected at all sites and determinations made of pH, total organic carbon, calcium and monomeric aluminium concentrations. In order to quantify the size and abundance of fish populations, electrofishing was carried out at 21 of the sites. Estimates of afforestation were made by a combination of field surveys and the use of 1∶:10 000 scale ordnance survey maps. The presence of coniferous trees had a substantial affect on water chemistry. In areas where the whole catchment was afforested, the water pH was 0.7 units lower than in unafforested areas (p<0.001). Afforestation was also correlated with low concentrations of calcium, and increases in the concentrations of monomeric aluminium and total organic carbon. There was a clear relationship between fish populations and water chemistry. Numbers of trout were positively correlated with pH (r2=0.69; p<0.001), and with concentrations of calcium (r2=0.55; p<0.001). The study has shown that salmonid fish populations are sensitive to acidity and the associated changes in river water chemistry; these problems were exacerbated by afforestation.