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Australian salmon

Australian salmon

Australian salmon or Australasian salmon, known as kahawai in New Zealand, are medium-sized perciform marine fish of the small family Arripidae (also spelled Arripididae). Four species are recognized, all within the genus Arripis; they are found in the waters off southern Australia (including Tasmania) and New Zealand. Despite the common name, Australian salmon are not related to the salmon (family Salmonidae) of the Northern Hemisphere; the former were named so by early European settlers after their superficial resemblance to the salmoniform fishes.

Relatively long-lived fish, Australian salmon are a favored target of recreational fishers, and both commercial and traditional Māori fisheries. They are also common bycatch of the snapper, mullet, trevally, and mackerel fisheries. These species are all taken in great numbers by way of purse seine nets and trawling. They are also caught by skilled fishermen along the southern coastline of Australia by beach seining. Due to declining numbers and ever-increasing annual catch sizes, the future viability of the Australian salmon stock has been put into question.

Species and range

The ranges of the four species may overlap to some extent, but can be described thus:

It should be noted that some systematists consider A. trutta and A. truttacea as subspecies of the same species.

Physical description

A streamlined, fusiform body and large, powerful forked tail - the upper lobe of which is equal to or less than the length of the head in the Eastern Australian salmon, Arripis trutta - are indications of the fast-paced pelagic lives these fish lead. Upon dissection, the extremely large, dense gills (for maximum oxygen extraction) and large proportion of red aerobic muscle in the 'fillets' is further evidence of a fast-paced pelagic life. The first (spinous, with nine spines) dorsal fin originates behind the pectoral fins, the former being confluent with, but noticeably higher than the much longer soft dorsal fin (with 15 - 19 rays), itself much longer than the anal fin (which has three spines and 9 - 10 soft rays). The pelvic fin is situated in a thoracic position.

Scales and eyes are relatively large - in the Australian herring, Arripis georgianus, the eyes are approximately one fifth the length of the head in diameter. The large mouth is terminal, and the jaws are lined with bands of sharp villiform (brushlike) teeth. The lateral line runs along the upper sides of the body.

The Western Australian salmon, Arripis truttacea, is the largest species at a maximum 96 cm (three feet) standard length (that is, excluding the caudal fin) and 10.5 kg in weight. The Australian herring is the smallest species at maximum 41 cm (16 inches) fork length (that is, from the snout to the middle of the caudal fin's fork) and 800 g. Australian salmon share a passing resemblance to the unrelated yellowtail amberjack, Seriola lalandi, locally known as "kingfish", with which larger salmon are sometimes confused.

All species are strongly countershaded; dorsal colours range from dark blue-green in A. trutta, green in A. georgianus, and steel-blue to grayish- or greenish-black in A. truttacea; the colours fade to a silver-white ventrally. A smattering of yellow, gray, or blackish spots embellishes the dorsal half, the spots arranged vertically or longitudinally in a series of rows. There are marked differences in subadult coloration: for example, on the flanks of juvenile Australian herring are a series of dark golden vertical bars.

Reproduction

Although their reproduction is poorly studied, Australian salmon are all known to be "pelagic spawners"; that is, they spawn in open water during the austral spring, releasing a large number of tiny (1 mm in diameter), smooth spherical eggs made buoyant by lipid droplets. The eggs (and later the larvae), which possess an unsegmented yolk, become part of the zooplankton, drifting at the mercy of the currents until the larvae develop and settle. The young salmon then spend the first 2 - 5 years in sheltered coastal bays, inlets, and estuaries until they become sexually mature and begin to move into more open waters. Relatively long-lived fishes, Australian salmon may attain an age of 26 years in Arripis trutta and 7 - 9 years in other species.

All species are oceanic spawners. Reports of A. trutta being anadromous and spawning in freshwater are not correct; this may be due to confusion with sea run specimens of exotic brown trout, S. trutta, or anadromous populations of native spotted mountain trout, Galaxias truttaceus.

Arripis georgianus are thought - due to females retaining both ripe and unripe eggs - to be "partial spawners"; that is, they may spawn over a long period with no real peaks. In contrast, A. trutta and A. truttacea are thought to be "serial batch spawners", completing their spawning season after a series of small "burst" spawnings.

Ecology

All species are neritic and epipelagic, staying within the upper layers of relatively shallow (from 1 - 80 metres), open and clear coastal waters (although the Western Australian salmon may prefer deeper water). The turbulent surf zone of beaches, rocky reefs, bays, and brackish waters such as estuaries are also frequented, and some species may also enter rivers. Juveniles inhabit estuaries and mangroves, as well as sheltered bays with soft bottoms carpeted with seagrass. Adults undertake seasonal migrations over vast distances, moving into deeper water during winter months.

Australian salmon form immense schools with hundreds to thousands of individuals, as both adults and juveniles. They are carnivorous and feed primarily on small fish such as pilchard; crustaceans such as krill, copepods, and other zooplankton (the latter comprising the bulk of the juvenile diet). The zoobenthos is also sampled to some extent, with primarily shellfish, crabs, and annelid worms eaten. The salmon are very fast swimmers, and are sometimes seen mingling with ostensibly similar species of carangids, such as trevally; this is an example of mutualism.

Together with the carangids, Australian salmon feed en masse by co-operatively bullying baitfish up to the surface; this herding technique is exploited by seabirds which are quickly attracted to, and feed upon, the foaming mass of fish at the surface. This commensal relationship between the salmon and the birds is noted to be especially strong in such species as the White-fronted Tern, Sterna striata, Fluttering Shearwater, Puffinus gavia, and Buller's Shearwater, Puffinus bulleri. The baitfish made available by the salmon's herding behaviour may also be important to the reproductive success of winter-nesting birds; the decline of the salmon stocks has evoked concern for these bird species, some of which - such as the Fairy Tern, Sterna nereis, - are endangered

Aside from seabirds, the salmon are also important in the diets of cetaceans such as Orca and Bottlenose Dolphins; several species of large sharks, for example; great white, dusky, copper, and sand tiger sharks; and eared seals such as the Australian Sea Lion.

Importance to humans

Pungently flavoured, coarse, and slightly oily flesh makes Australian salmon less desirable as a food fish; it is often sold canned or is smoked to improve its flavour, and bleeding the fish out is also said to help. What is not sold for human consumption is used as bait for rock lobster traps and other commercial and recreational fishing. The salmon fetch no more than a few dollars (AU) per kilogram; nonetheless, large numbers are taken via purse seine nets (and to a lesser extent trawling, hauling, gill, and trap nets) annually; the reported 2002 - 2003 commercial New Zealand catch of kahawai was 2,900 tonnes Such reported catches do not include the untold tonnes taken as bycatch from operations targeting more highly valued species. Low-flying planes are used to locate and target sizeable salmon schools, and critics have cited this practice as a means by the industry to artificially inflate catch records (which would give a false impression of abundance). Salmon numbers have declined noticeably however, with large specimens becoming ever rarer; the fish have all but disappeared from some areas. It was not until October 1, 2004 that the New Zealand Ministry of Fisheries included kahawai under its Quota Management System, setting a catch limit of 3,035 tonnes for the season. This was a five percent increase over the previous two years, despite the government's intention of lowering catch limits.

The Māori of New Zealand, to whom the fish are known as kahawai, koopuuhuri and kooukauka, fish for the salmon in subsistence and customary capacities. The fish were (and are) caught with lines of flax fibre and elaborate hooks of bone, wood, shell such as paua , or stone. The salmon are filleted before being hung on racks to dry. Recreational fishers also seek Australian salmon for their renowned mettle when hooked; the salmon are a challenge to land and often jump, occasionally standing on their tails. A significant number are taken for sport. No records of total recreational catches are kept, but the year's estimated catch of Australian herring from Western Australia's Blackwood River estuary beginning May, 1974 was 68,000 individuals

Commercial fishing practices undertaken across Australia and New Zealand have been highly criticised in various recreational fishing magazines as being excessive. Many high profile anglers such as those on Fishing WA have made statements that the commercial fish catch in Western Australia and Australia in general is beyond the scope necessary for human and animal consumption, and that they are far more valuable as a recreational fish species. The ease of catching salmon, which tend to form schools of several tonnes, has meant that recreational fishers are finding fewer of these species in inshore waters during season (the migration patterns of salmon mean they come into warmer waters during the Autumn).

See also

References

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