That fish farming sustainability article seems highly suspect to say the least, they're complaining about GMOs and such being "unsustainable" which is ridiculous. If anything GMOs are more sustainable in many ways, higher yield for less land, better targeted pesticides, etc.
I'll have to look into it more, there's a few different agencies certifying "sustainably farmed" fish at the moment which seem to be readily available around here at least.
Fish farming is a spooky rabbit hole to look into. All kinds of species, from saltwater to fresh, crustacean to finish, are fed fish that were caught in the wild, fed piles of antibiotics (this part gets really concerning, the implications for water sources connected to the farms are terrible), and all kinds of manual systems for removing polluting farm waste are employed where the pollution simply moves from farm to surrounding ecosystems.
There are better systems that are more sustainable, but my understanding is that most tilapia (for example) you'll find in a freezer at your grocery store is probably produced this way. It's really gross. Farmed shrimp and prawns are another one to stay far away from - they're labelled sustainable, but the farming practices are absurd. I don't trust any sustainability labels, anywhere.
Another one is Atlantic salmon farmed in the Pacific Ocean. That stuff is so gnarly. There's a reason British Columbia is shutting down a lot of these farms (and reviewing the remaining farms in 2022). It's bad for the environment, the product is not healthy, and it's incredibly inefficient. Yet somehow this product was labelled the smart choice for years.
I think you may be incorrect about GMOs actually being more sustainable, as they have a pretty major effect on biodiversity by encouraging higher use of pesticides and insecticides[0]. A great example is the effect they have had on the population of monarch butterflies, mentioned in the linked source.
From that link: "A 2014 meta-analysis covering 147 original studies of farm surveys and field trials, and 15 studies from the researchers conducting the study, concluded that adoption of GM technology had reduced chemical pesticide use by 37%". Scrolling further down shows more citations showing lower use of pesticides, insecticides and herbicides.
Right back at ya' from the same link: "Some doubt still remains on whether the reduced amounts of pesticides used actually invoke a lower negative environmental effect, since there is also a shift in the types of pesticides used, and different pesticides have different environmental effects."
Again, read the monarch butterfly example, where higher rates of Roundup usage has killed milkweed all over, reducing habitat/food for the butterflies.
I'll have to look into it more, there's a few different agencies certifying "sustainably farmed" fish at the moment which seem to be readily available around here at least.