The Prism is probably one of the top 3 worst places to fish due to lack of accessible spots and spawn. Easiest method for doing this that I have found is to take in another char ahead of your fisher who can survive long enough in the daemon room to lead the spawn in the river room down by the hole that drops into vortex room and then ticket out. Then bring your fisher in since the upper half of the river should now be clear. There are 3 spots you can fish there at the top of the river. If you start just by the falls and creep down slowly you can fish 3 holes without the radiating ice damage and you should be left well enough alone by spawn. The occasional serpent is not a big deal it is the hydras that are the problem. And even led off they may randomly respawn periodically like any spawn and be on your head at the top of the river at which point you ticket out. Option 2, bring an area peacer to keep things calm while you fish. Hope this helps and good luck. The Lantern fish lives in Prism of light as well. Happy Fishing.
Lord Aphid/ The Albatross LS