Introduction

Low environmental pH and increased concentrations of various metals such as Cu, Mn, and Al interact in determining the toxicity of acidic soft waters to fish. Of the trace metals, Al has been highlighted as causing the greatest concern to date (Rosseland, Eldhurst & Staurnes, 1990), and numerous investigations have reported physiological disturbances in adult or juvenile fish, especially salmonid species, exposed to aluminium in soft acidic waters. Most of these studies have been concerned with the responses of the rainbow trout and brook trout (Neville, 1985; Playle, Goss & Wood, 1989; Witters et al, 1990, 1991, 1992; Wood et al, 1988a,b) while far fewer studies have centred on the brown trout, Salmo trutta, even though this is a major indigenous salmonid in European waters. The responses of brown trout to acidic waters in the absence of aluminium have been investigated in some depth by McWilliams & Potts (McWilliams, 1982, 1983), but the effects of aluminium in acidic waters are less clear though of greater environmental relevance. Some studies have suggested that aluminiumexposed brown trout show osmoregulatory and/or respiratory disturbances similar to those occurring in other salmonids (Muniz & Leivestad, 1980; Rosseland & Skogheim, 1984; Gagen & Sharpe, 1987; Battram, 1988; Dietrich & Schlatter, 1989b; Reader et al, 1991; Sayer et al, 1991) but in most of these studies aluminium concentrations were high and mortalities common. A knowledge of the effects of lower, sub-lethal concentrations of aluminium is particularly important in understanding acclimation and long-term survival as ecosystems recover.