Gyrodactylus salaris is a parasite that lives on the skin and fins of salmon. It measures approximately 0.5 mm and cannot be detected with the naked eye. G. salaris is also capable of living and reproducing on the skin of other species of fish. The parasite occurs naturally in rivers that drain into the Baltic Sea. G. salaris is common among salmon born in the Torne, but it causes no visible harm or mortality to the fish.

G. salaris spread to Norway as a result of fish stocking in the 1970s. Several salmon rivers were stocked with infected fish. The parasite has resulted in high salmon mortality in Norway’s salmon rivers and depleted the salmon populations of more than 40 rivers. The reason for this is believed to be that the salmon in the rivers that drain into the Atlantic do not have immunity against the excessive reproduction of G. salaris.

Due to this lack of immunity, it is extremely important to prevent the spread of G. salaris to the Tana and the Neiden. For this reason, all kinds of fish stocking or transferring of live or dead fish from other water bodies is strictly prohibited. Boats and canoes brought to the Tana and the Neiden from other water bodies as well as all fishing equipment and accessories, such as reels, rods, lures, nets, boots, waders, and gutting tools, must be completely dry or disinfected before use. G. salaris is able to live for several days without a host and can therefore spread simply through water or wet fishing equipment or accessories. All fishing equipment must be disinfected before use in the sections of the rivers that run in Norway.

The Natural Resources Institute Finland collects fry samples from the Tana and the Neiden every year to test for the parasite. All the samples analysed so far have been clean.

Picture on top of the page: Panu Orell