Taimen are probably the biggest of any trout/salmon (listed as Hucho Taimen, not sure where that classifies it) of the world. They can grow to over 200lbs and 7ft long. Interestingly, they never venture out of fresh water. As for fishing them, its a slow game. Because of their size there might be only 1 per mile of water. So when you do find one you might just get a couple casts before they spook and you're off to find another giant miles away.

http://www.combat-fishing.com/taimen.html

http://www.ultimaterivers.com/Taimen.htm