Hi cakem! Fish mode addict here. It has been a very enjoyable experience seeing the game's recipes in a new light and craving that precious fish per minute statistic. 2.6k is my record :P. I have a minor gripe with the pollution in fish mode however, which I'd like to explain:

My unmodded factorio playthrough has a peak output of 2.8k total pollution per minute with red and green science being unlocked. At that point, 1.6k is from mining drills, 500 from stone furnaces, 500 from boilers, and 200 from assembly machines 1 & 2.

My fish mode playthrough peaked at 250 pollution from boilers and 300 from assembly machine 2s, also while at green science. Even though it was on deathworld settings, 80% of biter evolution came from the time factor, and tiles/trees absorbed all of my pollution without trees dying and without spreading out much, so I got pretty much no biter attacks, aside from a new nest that spawned inside the pollution radius.

Miners & furnaces are a large source of pollution that are not needed in fish mode for the most part. I think that it would be fair to add back in the same amount of average pollution to keep biters equally balanced, or at least close to equally balanced, with vanilla.

Some ways of changing this could be:
- Double/triple power usage of assembling machines

• Add pollution on a per-fish basis to transmutation devices

• Add power consumption and pollution to transmutation devices, either by making it take electricity or scaling up the raw fish fuel value or the pollution value to match.

personally I think adding pollution to do with the transmutation devices or fish production would be best, as my bases are defined more by the fish per minute than anything else. Let me know what you think!