My opinion on fish: I worry that consuming wild-caught fish increases the general demand for fish, and that extra demand is fulfilled by fish farming, which can involve a lot of suffering. As you can see in the graph below, wild fish catches have been stable for many years because we can't sustainably catch more wild fish than we do now.

But this may not apply for all species of wild-caught fish. 

But there are many other complicating factors. For example: 
* Wild animal welfare effects of catching wild fish likely dominate, and they are complex and we are clueless about them, just like we are clueless about everything else. 
* Michael St. Jules pointed out that in some cases reducing the demand for wild-caught fish prevents unsustainable fishing which increases how many fish will be caught in the long-term. So it's unclear what to do if you are worried about suffering during capture which can be long and intense (more on that suffering here).
* If eating wild fish increases fish farming, the effects of that are also complicated. Fish farming is limited by the amount of feed fish that can be caught from the wild. But those are also mostly fished at capacity, or over capacity. Some plan to farm insects in trillions and feed them to farmed fish for a protein source to supplement feed fish. It's possible that those insects will be sentient and will suffer in farms. The bigger demand for fish, the more likely this will happen.

So yeah, it's super complicated.

Yes, it's discussed in some articles by Brian Tomasik that you can see here.

A tuna isn’t like a lion that’s eating large herbivores , It’s going to go after smaller predators that are going after smaller predators… until we get to copepods and such. Except all those animals would die of something without predators, so the relevant question is about impacts on population sizes of all relevant species.

Lets pretend the number of calories available is constant and all consumers are animals. All of those calories will be eaten by some life form, due to small body size/ fast reproduction of algae, ocean trophic pyramids are inverted compared to terrestrial ones, with more biomass at higher trophic levels. And unlike in terrestrial ecosystems with many large herbivores, ocean megafauna are mainly predators. Any calories consumed by sharks/tuna/marine-mammals etc aren’t being put to use making more small scavengers.

Now There are mamy complications with this simple model

-some of those calories being eaten by the large predators might be used by insentient consumers like bacteria in There absence instead of small scavenging animals

-bony fish often have very small offspring, and total-population-biomass/ adult-size might be a really bad proxy for relative population size of marine species

-through diving deep/ defecating near the surface/ trapping nutrients in There bodies they keep nutrients at the surface , which has two big effects

1. fertilizes surface waters and allows for more production of calories by algae

2. means those calories in There waste are available to the surface ecosystems instead of sinking to the deep, where the animals they would feed are long lived and have slow metabolisms due to the cold water and food scarcity.

Either way its more complicated than hunting big fish to save little fish and probably bad?

Good q! To splat some thinking, The act of eating harms no animal so I interpret you to mean economic consumption. Fish killed by humans usually have worse deaths than naturally, but also trophic impacts, I think it's unclear what the effects are and so would favor supporting economically top charities such as fish welfare initiative for better cost effectiveness. Hope these thoughts help!