Bigeye and yellowfin tuna are over exploited in all oceans - suffering serious trouble in the Western and Central Pacific Ocean. In 1999, Greenpeace recorded an 80% decline in Mediterranean bluefin. Skipjack and yellow fin are most liekly to make up your canned tuna. Along with 100,000,000 sharks a year, too.
Advancements in technology are only exasperating the problem. Floating factories now hauling in as much as whole countries could have done a few years ago. Purse-seining methods reeling in all sorts of marine animals in huge, devestating quantity.
Fish Aggregating Devices (FADs) catch a higher proportion of juvenile tuna, including the overfished and vulnerable bigeye tuna. And they also increase the bycatch of vulnerable species including sharks and turtles.
Fish are generally unsustainable so be open to a plant-powered diet.
But if you are buying fish, check fish species, catch method, and location for sustainability.