Invasiveness of brook charr (Salvelinus fontinalis) in small boreal headwater streams

Invasion of the introduced North American species brook charr (Salvelinus fontinalis) is believed to threaten the native aquatic fauna in Swedish streams and lakes. Although introductions have been made since 1892 and the species is widely spread in the country, little is known about invasion dynamics and patterns. The ability of brook charr to disperse upstream through small boreal streams was investigated with electro-fishing. The study includes 18 headwater streams with documented time and place of stocking. Vertical water falls up to 1,3m and complex falls up to 1,15m had been ascended. Slopes with gradients up to 22%, measured with clinometer in the field and 31% measured in ArcGIS did not stop the upstream invasion. The obstacles that turned out to be barriers were: areas with subsurface stream flow through boulders (3,5-100m), a four-meter long plug of debris/sediment and a 65m long steep (22,6%) flat rock. Very small streams with mean annual discharge as low as 2,5l/s have been invaded. The invasion rate, in streams without barriers to invasion, ranged from 19,7 – 71,2 m/year, with a mean of 48 m/year. Increase in mean annual discharge (27-171l/s) and total stream gradient (0,564,30%) significantly increased the invasion rate. If charr had been introduced in a lake or a stream did not affect the invasion rate. Neither did the presence of brown trout in the streams or the number of repeated stockings