Ocean Watch

Monday, May 5, 1997

Hot tuna: Northern and
Southern bluefins

Ted Farm, a longtime reader from Ewa Beach, wrote recently, asking a question about tuna. It's a hard one. Ted wants to know why Atlantic bluefin tunas grow five to six times larger than Pacific bluefin tunas.

To answer this question, I first had to learn about bluefins in general. These famous giant tunas come in two species, but not Atlantic and Pacific, as I first thought. Rather, there are Northern bluefins and Southern bluefins, separated by Earth's north and south hemispheres.

The scientific name for the bluefins of the south is Thunnus maccoyii. These fish spawn in one large area off the west coast of Australia. From there they swim throughout the Indian, South Pacific and South Atlantic oceans.

The record weight for a Southern bluefin is 348 pounds. It was caught in New Zealand waters in 1981.

OK, now we head north. The bluefins up here, both in the Atlantic and Pacific, are all the same species: Thunnus thynnus. (Scientific names are important here because common names vary throughout regions.) These tunas of the north have three spawning grounds: The Atlantic fish spawn in the Caribbean or the Mediterranean, the Pacific fish near Japan.

The biggest Northern bluefin on record weighed a whopping 1,496 pounds. It was caught in 1979 off Nova Scotia.

This makes the biggest Northern species about four times larger than the biggest Southern. But how do Northern bluefins of the Pacific compare to those of the Atlantic? They're smaller, but not much smaller.

According to researcher William Bayliff of the Inter-American Tropical Tuna Commission in La Jolla, Calif., Pacific bluefin tunas have been caught off both Japan and Southern California in the range of 1,000 pounds, give or take 50 or so pounds.

That, of course, isn't the average catch size. One Hawaii fish book says most Pacific specimens are in the 250-pound range. Last week, however, the United Fishing Agency (which runs the Honolulu fish auction) sold a 517-pound Pacific bluefin caught by a long-liner. "It was one of the larger bluefins we see here," a spokesman said.

Most Atlantic bluefins don't approach their 1,500-pound record either. Catches usually weigh in the several-hundred-pound range.

Still, these numbers show that Ted Farm is right: The Pacific bluefins don't appear to grow as big as the Atlantic stock, though they're the same species. Why?

"It's not uncommon for members of the same species to be different sizes in different regions," said Bayliff. "King salmon are enormous in British Colombia and smaller in Alaska.

"Another example that comes to mind are people. Pygmies are smaller than Europeans, yet we're certainly all the same species."

This genetic variation among races occurs during the process of evolution because somehow the change is to the group's advantage.

So, how does being somewhat smaller than the rest of the species benefit Pacific bluefins? No one knows.

During my search for an answer, I learned a lot of other interesting facts about bluefin tunas.

Only about 20 years ago, Atlantic bluefins, also called horse mackerels, were worth about a nickel a pound and fed to cats. During fishing tournaments, these giant fish were hauled up the scales as trophies, then thrown out either at sea or in the town dump.

That's quite a contrast to the late '80s when other tuna stocks got low. Now, raw bluefin meat can sell for up to $50 a pound, making a 1,000-pound fish worth $50,000.

The results of this bonanza were inevitable. Today, most researchers agree that the stocks of Southern and Northern Atlantic bluefins are seriously depleted. However, according to Bayliff, the Northern Pacific stock appears to be holding its own.

I wonder how long it will last.