Monitoring fish stocks in relation to acidification in Norwegian watersheds

The Norwegian Monitoring Programme for Long-Range Transported Air Pollutants started in 1980. The biological part of this programme includes besides invertebrate studies in streams, (i) fish community status in lakes by means of interviews, test-fishing in lakes by using standard gill-net series, recruitment studies of brown trout in inland streams, and juvenile stock assess and monitoring of fish kills in salmon rivers. Damaged fish stocks are recognized within a land area of 51,500 km² in southern Norway and 30 km² in northern Norway. At least 6,000 lake-dwelling fish stocks have either been lost or are at various stages of reduction. Brown trout (Salmo trutta) is the most widespread and abundant species of fish in Norwegian watersheds, and is also most severe affected by acidification. More recently, there are some indications of an increase in the abundance of brown trout in some areas. However, analysis of age structure in lakes, and fry densities in streams in such areas revealed large annual variations in recruitment rate, which indicates unstable water chemical conditions. Atlantic salmon (Salmo salar) is virtually extinct in 25 rivers in southernmost, southwestern and western Norway.