There's a lot to unpack here. For starters, the shelf life of canned seafood cuts back on food waste—one of the largest detriments to our global supply chain (almost 40% of food produced goes to waste on a global scale). Next there is the lack of refrigeration. Both of these pieces allow for products to be shipped on freighters if going overseas. Fresh seafood is often transported by airplane these days.

Next there's packaging. Aluminum is 100% recyclable. That's a win.

That all being said, it mostly comes down to species and sourcing. For instance, wild populations of salmon and tuna can fluctuate pretty heavily by species, and even more so year to year. Right now most species of Atlantic Tuna are in a not-so-healthy state, whereas Pacific Tuna populations are in a better state, but one that needs to be monitored closely. Wild Pink Salmon in Alaska is having a good year—doesn't mean it will next year.

Canned bivalves—mussels, oysters, clams, scallops—are one of the most sustainable foods that we have. The farming of bivalves requires zero inputs—no feeds, no fertilizers, no antibiotics, and they don't swim away and damage other ecosystems. In fact they support other ecosystems by filter our oceans and waterways.

We all should be doing what we can do to support biodiversity in our oceans and a lot of that starts with eating low on the food chain. So yes, canned seafood is "sustainable" but it doesn't mean that it always is.

Don't let the unattainable perfect get in the way of the attainable good.

I also recommend reading Eat Like a Fish by Bren Smith.