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The shortjaw kokopu Galaxias postvectis, first identified by F.E. Clark in 1899, belongs to the Galaxiidae family of the Osmeriformes order.  The species is endemic to New Zealand, but is not found in the Chatham Islands or Stewart Island.

Shortjaw kokopu are threatened with listings of 'endangered' in the 2008 IUCN Red List of Threatened Species and 'nationally vulnerable' in the 2013 New Zealand Threat Classification System.

A few dispersed populations occur in Northland.  Other North Island locations include the Bay of Plenty, southwest Waikato, Taranaki, the Wanganui River catchment, and Wairarapa and Wellington coasts.

In the South Island, shortjaw kokopu have the highest concentration on the Westland coast, and occur in northwest Nelson, and the Marlborough Sounds.

Shortjaw kokopu appear to be the rarest of the five galaxiid species in whitebait runs, which enter river and stream estuaries in the spring.

Little is known about this secretive, nocturnal fish because it was rarely found until new populations were recently discovered.  With better detection methods and more surveys, it may turn out to be less uncommon than previously thought.

Shortjaw kokopu mostly live at low to moderate elevations, however they climb upstream, can penetrate up to 200 km inland, and reach an elevation of 500 metres in many catchments.

There is a distinct preference for faster moving, small tributary streams with boulders, and complete vegetation cover of native forest, particularly podocarp/broadleaf forest, often podocarp/broadleaf/beech forest, and rarely beech forest.  In fact it may be restricted to streams with native forest cover.

During the day it hides in small pools under logs and overhanging banks, or around rocks, with the overhead canopy of native riparian plants.  An effective method of finding them is proving to be by spotlight at night.

It gets its' name from the lower jaw being shorter, tucked under the upper jaw, a feature shared with koaro.  The shortjaw has somber, blotched spots of brown and grey colouring with faint bands, and a black patch behind the gills.

Shortjaws are smaller than giant kokopu and a bit larger than banded kokopu, growing to a length of 15-20 cm, and occasionally reaching 35 cm.

In typical Galaxias fashion, a large rounded single dorsal and anal fins are set back close to the large square caudal fin.  And as with all galaxiids it does not have scales, and instead a leathery, thick skin covered with mucus.

Shortjaw kokopu are nocturnal feeders, mainly of terrestrial invertebrates such as spiders, ants, caterpillars and moths that fall onto the water surface, and caddisfly larvae from boulders.

Shortjaw kokopu have declined in locations where introduced trout populations have become established.  Trout have changed their habits, and learnt to follow the whitebait run and become a predator of young kokopu.


An adult shortjaw kokopu Galaxias postvectis that has lost the definitive banding of the juvenile stage.
Photo Stephen Moore, Crown Copyright © Department of Conservation.


Whitebait and the migration cycle of diadromous galaxiids

New Zealand's freshwater fish fauna is sparse with 41 native species in 9 families, but unique with the predominance of 24 species of Galaxias and Neochanna genera in the Galaxiidae family.

More than half of the freshwater fish fauna is diadromous, moving between fresh and salt water during their life cycle.

The five whitebait galaxiids, shortjaw kokopu, giant kokopu, banded kokopu, inanga and koaro are amphidromous, a particular form of diadromy relating to breeding in freshwater.

Adult galaxiids make their way downstream to rivermouths in the autumn to spawn. Thousands of eggs are laid amongst vegetation on river and stream banks flooded by a spring (very high) tide. Males fertilize the eggs with large amounts of sperm (milt) that make the water milky.

When the tide recedes the eggs sit out of water for two weeks or more remaining moist in the vegetation until the next spring tide, when they hatch and larvae are washed out to sea.

After spending about four or five months of winter in the ocean, large shoals of juvenile fish (whitebait), up to 5 cm long and 3 to 4 mm wide, swim into river and stream estuaries on the incoming tide in spring.  They make their way upstream to develop into adult freshwater fish.

Shortjaw kokopu are probably a small part of the whitebait run, but this is difficult to determine as juveniles cannot be distinguished from koaro, banded kokopu and giant kokopu.

The tiny, almost transparent New Zealand whitebait, caught in a net or screen, are primarily the migratory juveniles of inanga, koaro, and banded kokopu.  Inanga make up the bulk of the whitebait catch in New Zealand, but are not present in sufficient quantities in Australia and South America.

The regulated whitebait season is from mid-August until the end of November throughout most of New Zealand except Westland.  Any whitebait catch outside this period is prohibited.  There are restrictions on the size of fishing gear, but there is no catch limit.

Many New Zealanders become familiar with whitebait in fritters, a traditional culinary delight, without realising that they would have grown into galaxiid fish, if they had made it through the lines of whitebaiters in river estuaries.

In 1902, McKenzie argued that whitebait were a distinct species from the Galaxias species, and not the young of another fish.  This claim continued until 1928 when Hope kept whitebait in captivity, and watched them grow into inanga.

At different times and in some parts of the country, whitebait runs include juvenile and adult Retropinna (smelt), and juvenile Gobiomorphus (bullies), and Anguilla (eel) species. The whitebait of extinct grayling Prototroctes oxyrhynchus used to be in the runs.

The ancient Gondwana ancestry of galaxiid fish .....

Only two of New Zealand's 19 Galaxias species are not endemic.  Inanga G. maculatus is widespread throughout the southern oceans in Tasmania, southeastern and southwestern Australia, South America, and some Pacific Islands.  Koaro G. brevipinnis is found in south eastern Australia and Tasmania.

This is the first indication of a Gondwanan ancestry of galaxiids. The question remains, whether the distribution of galaxiids around the Southern Pacific ocean was by dispersal following the breakup of the Gondwana continent, and separation of the Zealandia continent 80 million years ago; or by vicariance within Gondwana through Antarctica before separation of the continents.

Dispersal is the migration of an ancestral population across a pre-formed barrier, such as an ocean.

Vicariance is the separation of a population by a barrier, which isolates a new population, and in many cases is the initiation of speciation and the evolution of new species.

The linked continents of South America, Antarctica, Australia and New Zealand were an overland dispersal route 130 to 35 million years ago.

Evidence of a vicariant origin of the New Zealand freshwater fauna is provided by a relation-ship between Galaxiid fish and tiny parasitic larva of the freshwater mussel, genus Hyridella, called a glochidium.

The larva attach to the fish's gills to obtain nutrients and get a free ride.  The minute larval would be washed away in the current without an attachment to the fish.

By coincidence, the mussel family Hyriidae occurs in New Zealand, South America and eastern Australia in the same habitat with galaxiid fishes.

The dispersal of larva to coincide with the galaxiid fish, and evolution of the same relation-ship on different continents, would have been an ecological phenomenon that is hard to imagine.  It is easier to accept vicariant separation of populations, when the larva and fish were associated before Gondwana broke up.

An ancient ancestry, Gondwana origin, and vicariant origin of New Zealand freshwater fauna further revolves around the only two species of freshwater crayfish Paranephrops planifrons and P. zealandicus.  Crayfish relatives in the Parastacidae family in Australia (yabby), and South America are thought to be of Gondwanan origin.

Another coincidence is a relation-ship between flatworms of the genus Temnocephala that attach to the claws of koura.  They only live on a host.

Temnocephala have the same relationship with freshwater crayfish in Australia, and other relatives attach themselves to various hosts, including snails, crayfish and turtles in Brazil, Chile, Argentina, Uruguay, Indonesia and New Guinea.

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