Presumably the optimal size for survival when humans aren't applying pressure via fishing is bigger. Perhaps "ruining" is hyperbolic, but this is making the fish less fit for their environment.

...less fit for their environment excluding human fishing pressure.

It is making them less fit for their environment? I can see how it is making them less fit for the environment they used to be in before they were subject to large scale human fishing, but that's not their environment now.

If that’s the case it should return if we stopped fishing.

Not always the case. Any number of things can happen that could prevent that from happening, like extinction, as an extreme example.

No. A certain gene expression can just be gone forever. Nothing guarantees the desired expression will evolve again.

Sure it does, there is still variation in the sizing of them. If the previous size was the ideal one, and assuming that has otherwise not changed, the larger cod will be more fit and breed more.

Their environment is one where humans prey on them, especially on the large ones. It's a selection pressure like any other and becoming smaller on average makes them more fit for that environment.

if you're interested in maintenance of the fishery, size reduction is a primary indicator of failure. so yes

> Is being bigger Platonically better than being smaller?

Maybe, most animals evolve towards larger sizes. Except on smallish islands, where they tend to become smaller.

> Maybe, most animals evolve towards larger sizes.

This is not even a little bit true.

> Maybe, most animals evolve towards larger sizes.

Animals tend towards whatever size lets them maximise survival. This is based on available niches and other selective pressures.

> Except on smallish islands, where they tend to become smaller.

Island syndrome leads to both gigantism and dwarfism.

> Animals tend towards whatever size lets them maximise survival.

Evolution is a filter that removes unfit, it does not select the best fit. So a lot of mutations that slightly decreases fitness for the environment stay as they do not make an organism too unfit. This results in a gen pool diversity in a population that helps to survive if the environment changes as previously somewhat harmful mutations can be essential for survival in the new situation.

It's better if you want to eat them lol

Small fish generally taste better in my experience. Small of course implies younger, so we let the biger ones go as well as small ones - there is a too small to eat point.

In the sense that the gene pool is being unintentionally manipulated by humans, when the original goal was to try and leave it undisturbed to an extent.

Yes. It is.