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Lampris immaculatus Opah (also known colloquially as moonfish , sunfish, kingfish, and Jerusalem haddock) are large, colourful, deep-bodied pelagic lampridiform fish comprising the small family Lampridae (also spelt Lamprididae). There are only two known species in a single genus: Lampris (from the Greek lamprid-, "brilliant" or "clear"). One species is found in tropical to temperate waters of most oceans, while the other is limited to a circumglobal distribution in the Southern Ocean, with the 34th parallel as its northern limit.
Though rarely caught, opah are prized trophies for deep-water anglers as their large size and attractive form lend themselves well to taxidermy. Occasional bycatch from the longline tuna industry is also marketed and prepared as sashimi, as well as being broiled and smoked; opah flesh (despite being stringy and hard to fillet ) has a moderate flavour and is well appreciated in this regard, especially in Hawaii. Opah are garnering increasing interest from restaurateurs as other staple species become unavailable. An average of 35 per cent of an opah's weight is utilised for consumption.
Opah are deeply keeled, compressed and discoid fish with conspicuous coloration: the body is a steely blue grading to rosy on the belly, with white spots covering the flanks. Both the median and paired fins are a bright vermillion, contrasting strongly with the body. The large eyes stand out as well, ringed with golden yellow. The body is covered in minute cycloid scales and its silvery, iridescent guanine coating is easily abraded.
If coloration is ignored, opah closely resemble the unrelated butterfish (family Stromateidae). Both have falcate pectoral fins and forked, emarginate caudal fins. Aside from being significantly larger than butterfish, opah have enlarged, falcate pelvic fins—with ca. 14–17 rays, which distinguish opah from superficially similar carangids —positioned thoracically; adult butterfish lack pelvic fins. The pectorals of opah are also inserted (more or less) horizontally rather than vertically. The anterior portion of an opah's single dorsal fin (with ca. 50–55 rays) is greatly elongated, also in a falcate profile similar to the pelvic fins. The anal fin (ca. 34–41 rays) is about as high and as long as the shorter portion of the dorsal fin, and both fins have corresponding grooves into which they can be depressed.
The snout is pointed and the mouth small, toothless and terminal. The lateral line forms a high arch over the pectoral fins before sweeping down to the caudal peduncle. The largest species, Lampris guttatus , may reach a total length of 2 metres and a weight of 270 kilograms. The lesser-known Lampris immaculatus reaches a recorded total length of just 1.1 metres.
Almost nothing is known of opah biology and ecology. They are presumed to live out their entire lives in the open ocean, at mesopelagic depths of ca. 50–500 metres, with possible forays into the bathypelagic zone. They are apparently solitary but are known to school with tuna and other scombrids. Opah propel themselves via a lift-based labriform mode of swimming; that is, by flapping their pectoral fins. This, together with their forked caudal fins and depressible median fins, indicates that opah—like tuna—maintain themselves at constantly high speeds.
Squid and euphausiids make up the bulk of the opah diet; small fish are also taken. Pop-up archival transmitting (PAT) tagging operations have indicated that (aside from humans) large pelagic sharks, such as Great White Sharks and Mako Sharks, are primary predators of opah.
The planktonic larvae of opah initially resemble those of certain ribbonfish (Trachipteridae), but are distinguished by the former's lack of dorsal and pelvic fin ornamentation. The slender hatchlings later undergo a marked and rapid transformation from a slender to deep-bodied form; this transformation is complete by 10.6 millimetres standard length in Lampris guttatus. Opah are believed to have a low population resilience.
Species and range
- Lampris guttatus (Brünnich , 1788) [Opah] — from the Grand Banks to Argentina in the Western Atlantic; from Norway and Greenland to Senegal and south to Angola (also in the Mediterranean) in the Eastern Atlantic; from the Gulf of Alaska to southern California in the Eastern Pacific; in temperate waters of the Indian Ocean; and rare forays into the Southern Ocean.
- Lampris immaculatus Gilchrist , 1904 [Southern opah] — confined to the Southern Ocean from the 34th parallel south to the Antarctic Polar Front
- "Studying Pelagics: Discovering the long distance migration and deep diving behavior for large pelagics in the central North Pacific with pop-up archival transmitting tags". NOAA. February 2005 version.
- "Opah - Moonfish". Hawaii Seafood Buyer's Guide. February 2005 version. State of Hawaii.
- Fishes: An Introduction to ichthyology. Peter B. Moyle and Joseph J. Cech, Jr; p. 338. Printed in 2004. Prentice-Hall, Inc; Upper Saddle River, NJ. ISBN 0131008471
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