Is a fish diet more ethical than plant based diet?

I don't see the big issue as being animals lost to farming. I understand eating plants requires more volume on the people side of eating, but eating animals require more deaths on the animal feed side. Even when you argue animals can graze, they require constant rotations of growth. If you argue for free range, pasture grazing, you need to equate that side with a comparable produce farming, which could as easily be verticle growth, home gardens, or greenhouses.
Actually, it isn't farming that impacts animals as much as the decimation of their homes and food sources. Know what is a huge impact? Animal farming as well as factories that produce products for new technology

Humans are the only animals that have sought to live outside of the natural world in a world of their own creation
I remember when I first went vegan with the thought that everyone should turn vegan. I then realized it was going against nature that is the main cause for people to become vegan.
Sure, there are places where eating only plants could be the default, but those who choose to live in the woods, build there houses, plant their gardens, and live amongst the animals. I know many who have grown up hunting and fishing, and would defend their love of animals and nature as deeper than most vegans I'd ever known-because they know their lives. they know their families, their foods, where they drink, where they sleep, the old and the young. they hunt them with intent, in the way animals hunt. they have spent more time ensuring the animal families prosperity far more than they do hunting them for food

I have no need to hunt--no one in my city in the US does. We are fat, exercise intentionally, and a vegan diet is by far the best for our health and environment.

While I don't have too much of an opinion on eating eggs, I don't believe the eggs that replace beans or grains are saving any more lives than if you declined