Gyrodactilus is present in all scottish waters at the moment, however the disaster strain Gyrodactilus Salaris is a baltic species that is not here yet!. It is 1mm long and born pregnant, so once it is here it will stay until every single one is killed. It has no effect on adult fish. It kills salmon parr by basically an allergic reaction, they cannot cope with the parasite and die.
The mechanism of infestation would be like this. An angler returns to Scotland with wet gear. He fishes with the gear and a GS parasite gets from the felt on the sole of his waders into the river. It is pregnant and infects a number of minnows, trout and parr, the infestation spreads and no one notices. Adult salmon return in huge numbers and spawn, we all think it has been a superb season. The following year the same happens. The next year the grilse run is very poor and the next year it does not come. The following year the 2,3,4 SW salmon do not return. Then we notice there are no parr in the river!
The trout, minnows, eels and the like are not effected but are effective hosts, and both these species and GS flourish. It can only be removed if every single fish is killed!
But GS is a weak parasite, and easily killed. It was described to me like this, anything that would kill a gold fish will kill it. So heating, freezing, drying and salt all kill it.
Interestingly, during research into it it was found it could not live in tanks with aluminium frames. It is very intolerant of aluminium salts. Research continues into this and hopefully a less drastic solution is possible. The Dee and Don are checked very regularly for GS by monitoring parr.
GS has been around forever and is easily killed. It cannot survive on ducks feet or any other natural mechanisim. It was spread in europe by fish farms and stocking but could easily be moved by anglers and canoeists. In Aberdeen Orvis, Somers and Aberdeen Fieldsports can all disinfect tackle quickly and effectively.