Most simply, light tuna is tuna that isn’t white. “White tuna” has an official definition: It’s albacore (which tastes mild and feels firm), and it scores high on a color test. Tuna labeled “light” is pretty much everything else.
Light tuna is primarily made up of a species called skipjack, says Gavin Gibbons, a spokesperson for the National Fisheries Institute, but it can include others such as bigeye, yellowfin, and tongol, in “any combination,” says Stephanie Danner, the fisheries research manager at the Monterey Bay Aquarium.
As for the “chunk” part, it means that the tuna in the can will be in smaller pieces that vary in size, as opposed to “solid” tuna, which is in larger, firmer pieces with fewer flakes.
The FDA’s color test is called the Munsell value. According to FDA regulations, tuna labeled “light” cannot be darker than a Munsell value of 5.3. White tuna cannot be darker than a Munsell value of 6.3. The FDA also has official definitions of the terms chunk and solid: They’re measured by the way the pieces fit through a mesh screen, and the percentage of allowable flake. Light and white tuna have similar nutritional profiles, but white has slightly more fat and calories.
There’s one more important distinction, but it has nothing to do with FDA definitions. White tuna has more mercury than light tuna. StarKist notes in its FAQ that “FDA testing has shown that canned light meat tuna has an average of 0.1 parts per million (ppm) and that Albacore (white meat) tuna has an average of 0.35 ppm.” The reason is that albacore are larger, older fish than the types used for light tuna, so they’ve had more time to accumulate methylmercury.
Nevertheless, the lower-in-mercury light tuna is less expensive than the white tuna, probably because people prefer white.