The opah, or moonfish, belongs to a group of oceanic, mid-water fishes – the Lampriformes – which includes the huge snake-like oarfish and the weird looking bandfish. Like the opah, these also have silvery bodies with brilliant orange/rose coloured fins. The reason for this general colour pattern in fishes of such differing body forms which live in mid ocean depths, is not known, although it probably has something to do with orange and red objects appearing jet black at depth.

DISTINGUISHING FEATURES

Opahs are large fish, characterised by an almost round, laterally compressed body, and greatly elongated pelvic fins (the ones under the body). The striking colours are hard to describe, but here goes. As mentioned, all of the fins, including the tail are a bright orange to red colour, often described as vermilion. The back is dark green, gradually fading down the flanks to silvery gold. The body is covered with large silver spots, the whole effect sometimes being compared with a peacock’s feather. Simply stunning.

DISTRIBUTION AND MOVEMENTS

The common opah is distributed through the warmer waters of all oceans, including the Mediterranean. In Australia, it is a reasonably common bycatch of longliners fishing between southern Queensland and Bermagui, NSW. It occupies depths from the surface to about 350m, and while most are caught between 100 and 250m, it is also taken by drift nets set at the surface at night and occasionally by game fishers using surface baits. A second species, the southern opah (Lampris immaculatus), lives in much colder water and has a circumglobal distribution in the southern hemisphere between latitudes 40° and 60°S.

Very little is known about movements of opah. Like some other open ocean fishes, they are sometimes found washed up on beaches after storms, but otherwise, are not seen near land. Several opah have been tagged with satellite tags off Hawaii, and these demonstrated considerable movement but with no set pattern. It was also shown that opah dive to as deep as 400m during the day but like many other open ocean fishes, ascend to near the surface at night.

SIZE AND GROWTH

Growth rates are not well understood, but are probably rapid since large adults are relatively common throughout the population. A recent Hawaiian study estimated an age range of one to six years for fish between 20kg and 70kg. The maximum size quoted in the scientific literature is a whopping 275kg, however, this is very unlikely since no opah over 90kg has been recorded in the Hawaiian longline fishery, despite more than 30,000 having been measured.

SPORTING AND EATING QUALITIES

While the opah is almost exclusively a commercial species, there are scattered records of its capture by anglers. A 58kg specimen was caught in 1973 by an angler off Penzance, England, while the all tackle record was a much more recent capture weighing 74kg caught off California in 1998.

The opah swims strongly by flapping its pectoral fins like the wings of a penguin. To achieve this, the muscle supplying these fins is extremely well developed, and it is this muscle which is considered a great delicacy. In fact, the Polynesians hold opah in great reverence, their local name literally meaning ‘fish of the gods’.

Opah fillets are usually marketed under the name ‘moonfish’. If you see it for sale, buy some and try it. I like to cook it very simply by quickly searing fillets in a pan with a little peanut and sesame oil. Turn down the heat, add some dry white wine, lime juice and fresh dill. Simmer until the liquid is reduced and the fish is just cooked through. Serve with rice and a splash of soy for a truly special treat.

By Dr Julian Pepperell, as featured in Trailerboat issue 223