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October 25, 2001

Fin of the Yellowfin

Distinguishing the difference between big-eye and yellowfin tuna is almost brain surgery.

Q: I need your help in settling a debate with my older and self-proclaimed wiser brother. When we went fishing in Cabo San Lucas, we found great tuna action just a mile or so out of the marina. The fish had yellow finlets and coloration typical of yellowfin tuna. Most of the fish were in the 5- to 10-pound range. A couple of larger fish in the 30- to 40-pound range had the characteristic elongated fins seen on larger yellowfins. However, these long fins weren't very evident on the smaller fish.
The locals called them all yellowfin tuna. My brother, on the other hand, thinks that the smaller fish are actually bigeye tuna. He claims they are often found together and that the lack of elongated fins, even in juveniles, makes them bigeyes. I realize this is tough to settle without a picture, but I would appreciate any information on distinguishing bigeye from yellowfin.
Also, is there any truth to the rumor that bigeye and yellowfin can only be distinguished with certainty by liver biopsies? This was passed on by a local captain. - Jason T. Wilson

A: The best-accepted method of distinguishing a bigeye from a yellowfin is by examining the whole liver, which is different from a liver biopsy (a technique of examining the cells microscopically). When viewed from the bottom, the three lobes of the bigeye tuna's liver are about the same size, and the surface is streaked with lines. The yellowfin has a long lobe at one end of the liver and a short lobe at the other, and the surface of the liver is smooth.
Regarding the long second-dorsal and anal fins, not all yellowfin have the typical, very extended "yellowfin fins." While some adults have very long fins, others' fins are hardly elongated at all. In any case, the fins of small, juvenile yellowfin aren't any longer than the fins of juvenile bigeye. On an average, the yellowfin's fins don't grow noticeably longer until they reach about 60 pounds.
When it comes right down to it, it's hard to tell yellowfin and bigeye apart. Some field samplers say they see a difference in the overall proportions of the fish: The pectoral fin of the yellowfin is slightly longer than that of the bigeye; there's a gold stripe on the flank of a live yellowfin that's not there on a live bigeye; or there's a distinctive difference in the shapes of the notches in the center of the fishes' tails - but none of these criteria is foolproof.