Avoidance of toxic mixing zones by Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) in the limed River Audna, southern Norway.

The mixing zone that occurs where an acidic tributary enters a limed river can be highly toxic to fish. Toxic mixing zones are characterised by a rapid increase in pH and thereby polymerisation of aluminium from the acid tributary. The toxicity is most likely to be caused by Al precipitation onto the fish gills. The effects of toxic mixing zones on the fish in its natural environment, will be dependent on the ability of the fish to avoid these areas. In the present study the habitat use of Atlantic salmon and brown trout was investigated in a limed river with tributaries of different water chemistry, and thereby mixing zones of different toxicity. Electrofishing demonstrated that the mixing zones, with one exception were empty of fish when the tributary had a pH below 4.8 with labile Al-concentration above 200 microg litre(-1) and a Ca-concentration below 3 mg litre(-1). For the mixing zones with fish, the tributaries had a significantly higher pH and Ca-concentration and lower concentration of labile Al than mixing zones with no fish. Averagefish density downstream of the mixing zone was not significantly correlated to the water chemistry of the inlet tributary. The present results indicate that Atlantic salmon and brown trout can avoid the most toxic mixing zones.