Tuna is one of the most ubiquitous seafoods. It can be eaten from a can or as high-end sashimi and in many forms in between. But some species are over-fished and some fishing methods are unsustainable. How do you know which type of tuna you’re eating?Some tuna is certified as sustainably caught by groups such as the Marine Stewardship Council (MSC) that set standards for sustainable fishing. But these certifications are only good if they are credible.