There is a paucity of published information on the population biology of salmonids in Irish upland soft-water streams despite their relative importance as salmonid nursery areas and their susceptibility to acidification. This paper describes the characteristics of brown trout populations in upland streams in County Wicklow and identifies some of the factors affecting their distribution. The impact of plantation forestry on salmonid streams in this area is also evaluated. Sixty sites in the upland reaches of the King's, Liffey and Avonmore rivers and the feeder-streams of the Vartry Reservoir were studied in July/October between 1990 and 1992 (Vartry 1990 only). Summer trout densities ranged from 0.009 to 2.93 fish/m(2) but were generally well below 0.6 fish/m(2). At an elevation above c. 300m numbers fell below 0.2 fish/m(2) at most sites, Fish were recorded at altitudes as high as 460m but were generally not found in stretches of stream above c. 400m, especially where gradients were steep. Trout have, however, been observed in Wicklow streams above the 500m contour. Alkalinity, calcium and pH have been shown to be the key factors affecting the distribution of trout. Adverse changes in water quality, such as depressed pH and elevated levels of rode inorganic aluminium, may be associated with high levels of forestry in poorly buffered streams, and can in rum deplete trout populations.
