Tuna is the world's most popular fish, and by that I mean as a food product, not it's social standing (although who knows, maybe they're well thought-of in the oceans too?) Jokes aside, it's not hard to see why tuna is the most popular fish on the market. The prevalence of canned tuna in western countries means the low fat, high omega-3 protein source is readily available in convenient single serve packets. It's not uncommon to see people piling cans upon cans into their trolleys at the supermarket.
But being the world's most popular fish has it downsides. Sometimes, you're so popular that there's hardly any of you left. Like yellowfin and bigeye tuna, two species used for canning, that are now overfished and classified as 'at risk'. Or, you're so popular, that when people try to catch you, they take in everything else nearby in their haste. The majority of tuna caught in the world is done with 'Fish Aggregating Devices' and large purse seine nets (like a large drawstring bag), which produces high levels of bycatch (unwanted marine life - sharks, turtles, etc).
The good news is it's very easy to make good choice at the supermarket, and it's only getting easier. Look for 'Pole & Line' caught, and choose skipjack or albacore tuna species.
When I first stared looking in to sustainable tuna options, there was only one brand offering a pole & line caught option in Australian supermarkets - Safcol. Now each of the major supermarkets (Coles, Woolworths and Aldi) all have pole & line caught options as part of their own brand offerings, and the other major tuna brands are not too far behind, with all committing to source 100% pole and line in the next few years.
To see how your favourite brand ranks, take a look at the Greenpeace Canned Tuna Guide.