It's good to be fat, at least in the fish world. So nobody's cheering about the skinny bluefin tuna in Maine.
Creative science found skinny bluefin tuna by relying on the meticulous work of tuna grader Robert Campbell. Since bluefin can fetch a scary price when prime, upwards of $50 per pound, much attention is giving to separating the prime from the not-quite-prime.
Fortunately for fishery science, what makes bluefin tuna good to eat also makes them good for making baby tuna. It's all about the fat content, like pretty much any good desert and most good food. Is it fatty? Then it's good.
There is no obvious reason for skinny bluefin tuna, there seems to be plenty of herring to eat. Maybe it's the influence of mainstream media and unhealthy body images.