Mobilisation of river transported colloidal aluminium in estuaries and the impact on migrating Atlantic salmon smolts (Salmo salar L.)

The liming of acid rivers in Norway has increased salmon production. In some rivers entering estuarine fjords, however, salmon return rates are low compared to what is expected from the number of fish (smolt) migrating from the rivers spring. Based on in situ Al fractionation of waters and sampling of gills from exposed smolt, results demonstrated that Al-species were mobilised from colloidal river transported material and formed low molecular mass (LMM) Al-species accumulating on gills in the estuaries. In estuarine surface water with 2-10 in salinity the concentration of LMM Al-species and Al accumulation on gills were significantly higher than in the input river water. Since smolt equipped with acoustic tags indicated unormal low swim velocity in part of the fjord with such estuarine waters in 2007 and 2008, the mobilisation of Al in brackish water may have an essential role affecting the migrating and hence return rates of adult salmon.