There are alot more spots than the three at the river area. But again this is easiest with two accounts, or a really bored friend.
I set one account up with a necro swords in vamp form wielding a demon slayer 100% cold kryss. I put him in the demon spawn area just above the river so all the hydras and serpents aggro him. He makes short work of any demons that target him. The demons spawn all over that area and eventually they all spawn off screen from my necro.
Then I run my fisher on a second account up and down the river fishing. The noxious gas only activates if you are standing right on the edge of the river. So stand a few tiles back and cast on the edge. As long as your fisherman isnt closer to the serpents and hydras than the necro, they will stay focused on the necro and leave your fisherman alone.