Southern bluefin tuna

Its range

The water it lives in and the water it breeds in have nothing in common, and that is the species in one line.

The first is a continuous southern belt crossing the Atlantic, the Indian Ocean and the Pacific between 30°S and 50°S, reaching almost to 60°S. Temperate and cold seas, five to twenty degrees.

The second is a wedge of tropical ocean off the west coast of Australia, as far up as 10°S. Large fish run north to it, spawn, and drop back down.

A population spread across three oceans therefore assembles on one ground to reproduce. Everything else follows from that: there is no reserve spawning area, and a failed year has nowhere else to have gone right.

Identifying it

The median caudal keel is yellow in an adult. That is the character, and it exists because everything else about the fish repeats the Atlantic bluefin closely enough to be of no use.

Everything they have in common is useless for telling them apart. Same heavy build with the greatest depth under the first dorsal, same pale ladder of colourless bars and dot rows down the lower flank, same first dorsal tending to yellow or blue, same dull finlets ringed in black. 39 vertebrae and a swim bladder, as in its northern counterpart.

The dimensions sit a little under the Atlantic fish without being modest: 160 cm for an ordinary specimen, and twenty years of life.

In practice, geography decides before anatomy does. A large bluefin taken between 30°S and 50°S can only be this one.

Breeding

The central question has no answer. Nobody knows whether mature fish spawn every year, every few years, or once in a lifetime, and that gap sits underneath a species that has been fished hard for decades.

What is known concerns place and temperature. Spawning fish and larvae are found in surface water of 20 to 30 degrees, far above the band the species occupies for the whole of the rest of its existence. Maturity comes at about 122 cm, within a range of 120 to 130.

The water it needs

Once mature, the animal leads an oceanic, pelagic life. The recorded depth range reaches 2,743 m, which is deep even by the standards of the genus.

It joins schools with other scombrids, albacore among them, in the temperate water of the south.

What it eats

An opportunist in the strict sense. Stomach contents put a variety of fishes together with crustaceans, cephalopods, salps and whatever else the water offered.

The salps deserve a line of their own. These gelatinous tunicates return next to nothing in energy, and a two-metre predator eating them is taking what the Southern Ocean has put in front of it rather than choosing. The diet describes the patchy productivity of the water as much as the tastes of the animal.

How to catch it

Two constraints intersect. The first is thermal — five to twenty degrees describes southern seas that few recreational fleets work at all. The second is seasonal: the spawning migration periodically empties the temperate belt of its large fish, which are then off northwestern Australia.

A specialised fishery for sashimi-grade fish has been developed by New Zealand fishermen, alongside the canning trade that absorbs most of the catch.

Conservation

This is the tuna whose curve never turned. Where the Atlantic bluefin eventually got a management regime that produced a measurable rebuild, the southern fish is still on the slope that decades of exploitation cut into it: the published projection is that at current rates the adult population falls below 500 mature individuals within a century.

Three traits make recovery slow. One spawning ground, where a bad year cannot be made good anywhere else. An unknown spawning periodicity, which prevents any count of adults from being converted into a production of eggs. And twenty years of life against maturity at 122 cm, which spreads the effect of anything decided now across decades.

Farming offers no way round it. The industry rests on cage-fattening juveniles caught alive offshore, not on breeding in tanks, so the tonnage announced comes out of the wild stock — and it takes precisely the age class that has never spawned.

Demand does not soften. The Japanese sashimi market pays very highly for this flesh, and the unit price of a single fish can justify the effort even as yields fall. That is the mechanism which separates luxury fisheries from the rest, where scarcity eventually stops the boats.

Other names

Southern bluefin tuna in English, minami-maguro in Japanese — the tuna of the south — and it is on that market that its value is set.

The epithet honours Frederick McCoy, who directed the museum in Melbourne at the time of the description. The English name places the fish, the scientific one places the naturalist, and only the first is any help in identifying what is on the deck.