Al-treatment of Gyrodactylus salaris infected Atlantic salmon in the hatchery of Statkraft SF, Bjerka, Nordland

Summary

During the winter 2001/2002, the parasite Gyrodactylus salaris was found on Atlantic salmon (Salmo salar) in the hatchery of Statkraft SF in Bjerka, Nordland. The GYROMET-group with the leader Espen Lydersen, NIVA was asked to try and treat the water with aluminium in order to eliminate the parasite. The GYROMET group has had success with this treatment at the laboratory scale. This was the first large-scale treatment. The main challenge with the hatchery water at Bjerka was the high pH and ionic strength compared with most other Norwegian waters. This water quality was the main reason for failure to eliminate the parasite at first. After a long exposure of fish at low Al-concentrations, we decided to increase the Al-dose successively until significant effects were observed on the parasite. Concentrations up to about 1200-1500 g Al L-1 showed significant effects on the parasite, and after 10 days exposure to this Al-dose all Gyrodactylus salaris parasites were eliminated on the fish investigated. Due to the large fish, only parasites were counted on the left breast fin. The optimal Al-dose thus appears dependent on the chemical regime of the water. During periods of rapidly varying water chemistry, such as spring snowmelt, the Al-dose should also vary. We found this very difficult, because we at that time had minor information about Al-doses at different water chemical regimes. Thus we decided to end the Al-treatment, and agreed with the owner of the hatchery to instead try a salt treatment. However the results from the Al-experiments in the hatchery at Bjerka have been very valuable for the GYROMET-group in order to establish a better database about water chemistry and optimal Al-dose for eliminating Gyrodactylus salaris on Atlantic salmon. Today we know a lot more about optimal pH and optimal Al-treatment of Gyrodactylus salaris infected Atlantic salmon, such that the Al-dose can be held as low as possible, and the information from the hatchery at Bjerka has been very important. We have good knowledge about Al-treatment in Gyrodactylus salaris infected rivers with low ionic strength, but still some more information is needed about Al-dose in rivers with higher ionic strength and pH. Such investigations are clearly needed, since several of the Gyrodactylus salaris infected rivers in the Nordland County have high pH and ionic strength and high biodiversity. Al-treatment appears to offer a very good alternative to rotenone.