The easiest solution: Don't eat fish. Or our oceans may never recover.
[1] https://ourworldindata.org/grapher/ghg-per-protein-poore
I'd argue that if we're looking for a full top-to-bottom sustainable food system, animals will play a role. But we need to be cognizant of the whole system, not playing whack-a-mole with issues.
According to your source, there are 15 sources of protein that emit less greenhouse gases (GHGs) per 100g of protein than farmed fish, including poultry and eggs, and 16 sources that emit more (including items that are not known for their protein content like coffee, apples, and dark chocolate). Being highly charitable, farmed fish is squarely in the middle.
Additionally, farmed fish emits twice the GHGs of tofu, and almost 22 times that of nuts. So just comparing placements on the list paints a misleading picture.
As for "not willing to go full vegetarian": you may as well say "not willing to stop eating fish", because they are equally unserious limitations when discussing these topics. "Not being willing" is only a slightly more mature version of a child saying "I don't want to".
If you're quibbling about wording, all I meant was: farmed fish and chicken are among the most sustainable meat sources.
I'm not making a statement that people should eat meat, but many people do eat meat, so it's worth comparing which meat sources are better than others. I think it would be great if more people knew that beef produces 10x the greenhouse gases than chicken/fish do.
If you'll forgive me borrowing your logic: "I'm not saying that people should eat beef, but many people do eat beef, so it's worth comparing which beef sources are better than others."
Plant-based diets are a very good answer to the problems caused by animal agriculture. If someone takes issue with that answer, I'd need a better reason than their personal pleasure to take them seriously in the conversation.
I think this harm-reduction approach might make more sense from a governmental policy perspective, but is otherwise silly for us to take as individuals because we have such comparatively little influence over each other's choices. I wouldn't waste that small influence encouraging someone to make a slightly less bad choice.
The comparison of food to transportation is a bad one. Nutrients are nutrients, and everything else is personal pleasure. In other words, you can easily hit your same macros by replacing animal products with plant products without even having to change grocery stores. You cannot easily transport a mattress on a bicycle instead of a car.
Calling harm reduction "silly" because tofu exists just shifts the target. We can hold two thoughts at once: (1) plant-heavy diets are best, and (2) for the vast majority who aren’t going vegan tomorrow, steering from beef to chicken/fish dramatically reduces damage right now. Dismissing that because it’s not maximal purity guarantees we leave real cuts on the table.
So is there any hypothetical harm reduction that you believe is too small to be worth your time to encourage?
Good in theory, horrible in practice.
[1] FDA “Approved Drugs for Use in Aquaculture” — https://www.fda.gov/media/80297/download
[2] Jensen et al., Nutrients 2020 — https://doi.org/10.3390/nu12123665
Also what I wrote about shrimp from any 3rd world country is valid - I've seen such place this summer in Indonesia, and from what I've heard whole south east Asia is exactly like that, or worse. Getting shrimp from some western democracy with strong consumer protection rights ain't possible in many parts of Europe, not sure about other places