I've heard recently that the Bluefin Tuna is labelled critically endangered by the IUCN. The WWF estimates that the species is in danger of complete collapse. I thought at first that this was used in canned tuna, but I learned that actually, 40% of bluefin is used for sushi in Japan. I looked on the website of the type of tuna I bought, Starkist, but nothing on the label or the website indicated what kind of tuna it was, where it was from, or how it was caught. Whether eating fish is considered sustainable or not really depends on these factors. The Monteray Aquarium Seafood Watch puts out a great guide to what should be avoided, and what types of fish/fishing practices are okay to eat. They also have one about sustainable sushi, for you sushi lovers!
Upon further inspection, I found out that most canned tuna in the US/Canada isn't Bluefin Tuna (phew! I haven't been eating an almost-endangered species for years). It comes from Albacore Tuna, which is more plentiful. However, there are many different ways it can be caught, and some areas are worse than others to fish from. The best choice is to get albacore that has been fished from the US/Canada Pacific by pole-and-line. Since I can't find out about where the Starkist tuna that I had for supper came from, I guess I'll just have to assume that I should avoid it. Long-line tuna caught is the worst (compared to trawl and pole-and-line), not only because it catches the bigger fish which have accumulated more mercury, but because there's also a lot of bycatch. So I think I've made the right choice here: less mercury in my body, and less dead marine mammals, turtles, and other fish and invertebrates.
I know it's important to eat at least a little fish, since it has omega 3 fatty acid, which humans can't produce on their own. Apparently you can also get omega 3 fatty acids from flaxeed oil and algal oil. I eat fish only occasionally, but I'll replace my canned tuna with a more sustainable choice (of a species and fishing method that is easily identified on the package) or by eating more flaxseed oil.
At least until Starkist will tell me what kind of tuna I just ate.