Is a fish based diet most sustainable?

Question

It is in the news (paper, writeup) that a study has just come out that some seafood, particularly small wild pelagics such as mackerel, herring and pilchard (eg. Japanese pilchard = 0.498 kg CO2e per kg), have a far smaller green house gas emissions than meat (eg. beef = 56 kg CO2e per kg), and comparing the figures to other datasources lower even than vegan protein sources, eg. soya at 6.44 kg CO2e per kg.

Does this actually mean that a small wild pelagic fish based diet is sustainable, or do other factors (such as the recent work on plastic polution) mean that the environmental harm is actually greater than more traditional "green" foods?

Nutrient density scores are based on the 21 nutrients common to all species (full bars) and, where possible, 23 nutrients (grey lines) (for nutrients see Methods). GHG emissions of individual seafood species are representative of the dominant production method for each (or weighted if multiple major production method is employed globally). Solid bars indicate species from fisheries, and striped bars species from aquaculture. Comparisons to land-based animal proteins are based on nutritional content of averaged meat cuts for beef and pork, and fillets for chicken. GHG emissions of beef are beyond the scale at 56 kg CO2e per kg edible product.

Answer 1

Greenhouse gases aren't the only consideration.

A problem is the sustainability of fishing. Are the fished species in danger of extinction? Do the fish or fishing have extinction-causing effects on other animals (for example by fishing nets catching or human-introduced fish consuming unwanted fish species, or fishing nets causing drowning of animals like Saimaa ringed seal in Finland).

Also for farmed fish, you should really consider the area where they are farmed. Fish farmed in water tanks on land isn't a problem because they won't leave excess nutrients in water. Fish farmed in large oceans like in Norway isn't a problem too -- the Norwegians are attempting to eutrophicate the Atlantic ocean, but the Atlantic ocean is so large they won't succeed in that. So Norwegian farmed fish should be reasonably good to consume. But fish farmed in Baltic Sea, a very small sea, is in an entirely another category because the sea is so small that eutrophication is a real problem.

As an example, the country where I live, Finland, has numerous lakes with lots of fish. There are more lakes and more fish than needed to feed the entire country. Most of that fish is left uncaught. So consuming more Finnish fish is obviously a very climate-friendly and otherwise environment-friendly diet, but then again if you live so far away from Finland that consuming Finnish fish has huge transportation climate footprint, it might not make sense. And local fish may not be as sustainable depending on the area where you live.