Henning, thanks for bringing back some happy memories for me. I worked and fished on the Vosso for about six years in the '80s, and saw it in good times and bad.
When things were good it was truly a remarkable river. Early in the season the salmon used to average about 28lbs; a figure that left just about all other famous big fish rivers trailing in its wake. Even towards the end of the season in August, when we would see many more 2SW fish, and proportionally fewer 3- and 4SW, we still averaged about 22lbs. Grilse were quite uncommon; sometimes fish over 40lbs outnumbered those under 10lbs caught in a season!
As a brief note on the fishing, it was not an easy river to fish with a fly, being very fast and deep, but over the time that my employers had the tenancy there, we more or less reintroduced the fly as an effective method; it having been largely neglected during the previous 20 years or so when Odd Haraldsen had the fishing. It was Oglesby who fished and wrote about the river, as a friend of Haraldsen, and although he claimed to prefer fly to other methods he only caught a handful of fish with it in all the years he visited. But I think he didn't try very hard; certainly our results proved that fish would take a fly in most of the pools on fly if you were prepared to give it a go.
The sudden collapse in fish numbers is still, so far as I know, unexplained. The NSA site touches on some possible theories, but nobody seems to know why the population crashed from one season to another. In (I think) 1987 the Bolstad beat had its record season since the 1960's. The following year we caught only just over one tenth of that number, and things never recovered. The problem of fish farm escapees was becoming increasingly manifest at the same time, and to anyone who was aware of the river's uniquely late developing strain of fish, this immediately spelt bad news. The average weight of the fish we caught was so high purely because the fish matured much later and so stayed at sea for longer. Diluting this unique strain could only lead to the loss of the very thing that made the river and its salmon unique.
The research mentioned on the NSA website was conducted at another place I know a bit, the Salmon Research Trust in Co Mayo, Ireland. It demonstrates graphically the danger to wild fish stocks of interbreeding with farmed salmon. This is an aspect of salmon farming that has perhaps not fully been picked up in the way that issues of pollution, disease and parasites have been publicised. But it is no less serious; in fact arguably it is worse because once widespread interbreeding has taken place there is absolutely no way to get back to the genetically distinct strain of salmon that each river should support. In fact, I believe it may have implications for any fishery that relies on stocking to supplement wild fish numbers, unless the stocked fish are triploid or prevented from spawning by some other means. For some more information about this important piece of research, here are links to the BBC's report, and also to the Royal Society's Journal.
http://www.journals.royalsoc.ac.uk/app/home/contribution.asp?wasp=461e9458df6541bfa0172e7345ca 9c7d&referrer=parent&backto=issue,4,16;journal,31, 185;linkingpublicationresults,1:102024,1