The below statement came from a cooking site on Yahoo.  It never stated where the 'recent study' came from.

Canned white tuna comes from the large albacore and can be high in mercury content. Chunk light, on the other hand, which comes from smaller fish, skipjack or yellowfin, is best for health-conscious eaters. According to a recent study, canned white tuna samples averaged about 315 percent more mercury than chunk light tuna samples.