There is no concrete evidence of this. A lot of sea creatures, including fish, have skin cells that can create polarized light and some seem to have developed communication systems with pulses of polarized light.
Just the other day it was found that some fish can pass the mirror test, didn't you see that?
The prevailing theory is that ratio of brain mass to body mass is more telling than the absolute size of the brain. Fish may fare worse on this metric but I suspect it's just an area where we have very little understanding yet.
Being vegetarian or vegan is not so hard. It is one of many fundamental changes that society should pursue to avert the current ecological crisis and mitigate species loss.
A bit weird nitpicking when you yourself make an insubstantial claim about some "prevailing" theory. I think we can both agree that reducing meat consumption is the right thing to do, it's just sometimes to convince people you need to show some leeway.
Btw I never made any comment about fish's brain size, I said they're generally less intelligent than farm animals, which wikipedia agrees with, fits the brain ratio theory too: https://en.wikipedia.org/wiki/Fish_intelligence#Brain
Thanks for derailing the conversation anyway, at the end, there is no such thing as a fish
I'm all for human survival and well-being, but this is more like convenience: it tastes good, easy to get and cheap. We can at least change to make it not cheap and easy anymore. Values in our economy are artificial anyway. It's not like we have much else to innovate for the next centuries, besides making more internet apps and going to Mars.