Black jack

Its range

The range is described as circumtropical, but the list of known localities tells a different story from a continuous belt. Western Indian Ocean: Natal and East London in South Africa, Réunion, Mauritius, the Cargados Carajos, the Seychelles. Eastern Atlantic: the Azores, Madeira, St Paul's Rocks, Ascension, Cape Verde, the Gulf of Guinea. Western Atlantic: Bermuda and the northern Gulf of Mexico down to Brazil. Western Pacific: southern Japan to New Caledonia, with a recent report from Tonga. Eastern central Pacific: Mexico, the Revillagigedo Islands included, to Costa Rica.

Almost every name on that list is an island, an archipelago or a rock. It matches what is known of the animal, which is oceanic and insular, tied closely to clear offshore water and rarely found on shallow banks. The range is not a band but a scatter of points — wherever deep clear water meets a steep slope, and nowhere on the shelf in between.

The consequence is a matter of distance. One island population reaches the next only across hundreds of kilometres of open ocean.

Identifying it

The fin counts are no help, and knowing that in advance saves time with a key. Nine dorsal spines, 20 to 22 soft rays, three anal spines, 16 to 19 rays: every one of those values falls inside the range of at least one other jack, and not a single one of them is decisive on its own.

What decides is the overall tone. The fish is dark and uniform, and every language that names it has named the colour. The body is fusiform and laterally compressed, without the opercular spots or the pale fin tips that flag its relatives.

The third character is where the fish was caught, and it is nearly as reliable as anything anatomical. A dark jack brought up from an oceanic drop-off, well away from any continental shelf, at 40 m off an isolated island, has very few alternative identities.

The water it needs

Depth runs from 12 to 354 m, but the working band is 24 to 65. A 40-metre window inside an amplitude of that size is narrow enough to fish directly.

The station is the drop-off, at the outer edge of the reef where the slope tips away into blue water. The fish holds benthopelagically, neither on the bottom nor at the surface, and forms schools occasionally.

What it eats

It feeds on fishes, and it feeds at night.

A fish-eater posted on an oceanic slope intercepts forage that is not the forage of the lagoon. What passes along the edge is small pelagic fish and open-water species, not the residents of the coral flat, and the size and behaviour of those prey is what a lure is being asked to imitate.

How to catch it

The drop-off between 24 and 65 m is the whole brief. Working the flat or the coastal shallows produces nothing, because the species is absent from shallow banks and its presence depends on deep water being immediately at hand.

Feeding is nocturnal, which shifts the best hours away from most reef fishing. The occasional schooling habit stops short of true gregariousness: one fish does not reliably promise the next.

The build is modest for the genus — a common length of 70 cm, maturity from 38, twelve years at the oldest reported. This is a mid-sized jack, and tackle chosen for the Indo-Pacific giants is out of proportion to it.

Conservation

The isolation of the populations is the fact that governs everything else. A species distributed as insular units separated by open ocean is not one stock but a collection of small local ones, and recruitment at one island does not readily replace what was removed at another.

That structure makes any global verdict uninformative. It applies to the sum, while the fishing happens one island at a time, and the reference assessment for this species dates from August 2012 — the oldest of the large jacks.

Commercial exploitation is minor, which limits the pressure for now. What will actually decide the outcome is the artisanal fishery of small islands, where a dark jack off the drop-off can be a meaningful share of the catch.

Other names

Black jack or black trevally in English, jurel negro in Spanish, xaréu-preto in Brazilian Portuguese, schwarze Stachelmakrele in German. For once every language has taken the same character and there is no ambiguity to untangle from one country to the next.

The Latin lugubris means mournful, of mourning. The naturalist named the colour before the fishermen did, and in the same terms.