Galaxias maculates
Written by William Gumbley (Lincoln University)
July 2005
Edited by Michael Hudson, Lincoln University, May 2006
Taxonomy
- Scientific name: Galaxias maculates (Jenyns)
- Synonyms: Galaxias attenuatus
- Popular Common names: inanga, whitebait.
- Other common names: cowfish, milkfish, minnow.
- Kingdom: Animalia
- Phylum: Chordata
- Class: Actinopterygii (ray/finned fishes)
- Order: Osmeriformes (smelts)
- Family: Galaxiidae
- New Zealand Status: native
- Comment: whitebait is the catch made up of five species of Galaxias and the juvenile smelt, Retropinna retropinna.
- Taxonomy sources: Bailey (1973), McDowall (1990), Richardson & Taylor (2002).
Identification
Species Description
Galaxias, the name given to species of the Galaxiidae family, is attributed to their profusely spotted coloration which resembled the Milky Way galaxy (McDowall, 2000). Each of the species of Galaxias has distinctive features and coloration. Inanga (Galaxis maculates) has a greenish translucent band along the middle of the sides which has resulted in a certain variety of greenstone being named after it by the Maori (McDowall, 1990). The eyes, gill covers and belly are bright and silvery in coloration but the whitebait (juveniles) are translucent. The body is small, sleek and slender, with a small head and large eyes (McDowall, 2000).
Similar species
There are 10 species of Galaxias, 5 of which make up the whitebait catch; although within the Galaxiidae family there are also some species of mudfish. The Galaxias divergens (dwarf galaxias) and the Galaxias gracilis (dwarf inanga) are two species that cause confusion with the inanga as the whitebait look very similar.
Geographic Distribution
Full natural and naturalized range
Galaxias maculatus can be found naturally in New Zealand, western and south-eastern Australia, Tasmania, Lord Howe Island, Chile, Argentina, Tierra del Fuego and the Falkland Islands, which make it one of the most widely distributed freshwater species in the world (McDowall, 1990).
New Zealand
The distribution range of inanga covers a great deal of coastal New Zealand in lowland and coastal rivers, streams, lakes and swamps (McDowall, 1990). Inanga are usually present at altitudes less than 20 meters and less than 10 kilometers inland, although they have been found at altitudes as high as 160 meters and distances of 160 kilometers inland (Richardson & Taylor, 2002). Inanga do not have the ability to climb waterfalls, unlike some galaxiids, which often restricts the distance of which they can travel inland (McDowall, 1990). Being diadromous means they migrate between freshwater and saltwater (Harding et al. 2004), but there are some populations that are confined to small, land-locked lakes in Northland (McDowall, 1990). Figure 2. New Zealand distribution of inanga (Galaxias maculatus) (image source: McDowall, 1990)
Canterbury
In relation to the rest of the country, inanga are reasonably abundant in Canterbury. They can be found all around the coastal areas of Banks Peninsula, including Akaroa and also Lake Ellesmere. Inanga make their way up most of the Canterbury Rivers from the coast, including the Rangitata, Rakaia and Waimakariri rivers (McDowall, 1990).
Natural History in Canterbury
Habitats
Once they have migrated inland from the sea, the habitats of inanga are within flowing waters such as rivers and streams and also coastal lagoons, swamps, and lakes (Richardson & Taylor, 2002). The rivers or streams need to be relatively slow flowing because this is a habitat condition needed in order for the inanga to feed.
Description of preferred habitats
Once inanga migrate into fresh water they grow to maturity in low gradient and low altitude waterways which include pools, slow runs and backwaters (Richardson & Taylor, 2002). Inanga prefer habitats that have overhanging vegetation or macrophyte beds as cover and require moist riparian vegetation for spawning (Richardson & Taylor, 2002). Inanga are intermediate in their tolerance to water quality compared to other fish as their preferred water temperature is about 20 degrees C, pH of about 9.5 and they are relatively tolerant to moderately high levels of suspended sediment (Richardson & Taylor, 2002).
Phenology/Breeding, Lifecycle, Dispersal
Inanga are catadromous, meaning that they migrate from freshwater to saltwater to spawn (Harding et al., 2004). Their one year diadromous lifecycle is typical of the whitebait species where the juvenile inanga migrate upstream in spring, then over the summer inanga grow to maturity in freshwater habitats. In autumn, mature inanga migrate back down stream to spawn among riparian vegetation that is flooded by spring tides (Richardson & Taylor, 2002).
During spawning the male inanga releases milky white milt from their testes and can be seen in the water at spawning time. This fertilizes the eggs which take in water and stick to the riverside vegetation. The eggs then hatch two weeks later when the next cycle of spring tides occur, and the larvae are washed into the river estuary and flushed out into the sea as the tide falls (McDowall, 1990). After spawning the adults migrate downstream, but not into the sea, where it is believed that nearly all the adults die (McDowall, 1990).
The winter months spent out at sea, feeding on marine plankton, are a crucial growing stage for inanga (McDowall, 1990). The growth made during the 6 months spent at sea is so great that there was doubt whether this amount of growth could occur in such a short period, and that it could very well be more like 1 or 2 years and 6 months. This was proven otherwise by studies made on the growth rings on the otoliths (inner ear drum) of the migratory whitebait (McDowall, 1990). As spring begins, especially if there have been floods, the inanga whitebait begin to move inshore as they come across fresh water flowing in from the rivers to the coastal seas. This triggers the mass migration where even just a small flow of fresh water from a creek can attract the whitebait (McDowall, 1990), revealing that they can disperse amongst a wide range of freshwater habitats from the sea. Inanga mature over the summer following migration where they dwell in pools associated with the rivers (McDowall, 1990). Once they have reached maturity they will make their way back downstream to the estuaries where they spawn and die, thus completing the lifecycle (McDowall, 1990).
Diet
When at sea the inanga live at or near the surface, feeding on small crustaceans such as young shrimps and crabs that live within the plankton (McDowall, 1990). After migration, in their fresh water habitat inanga display very cosmopolitan feeding habits as they will feed on a great variety of aquatic organisms and terrestrial animals that have been blown into or onto the surface of the water (McDowall, 1990). Their particular diet may vary with different habitats. In sandy streams there is an abundance of larvae to feed on but in rocky streams, insects such as mayfly (Ephemeroptera) and caddis (Trichoptera) larvae are the staple diet (McDowall, 1990).
Behavior
Inanga are often found in ones and twos amongst vegetation cover in streams and rivers (McDowall, 1990). Their behavior changes dramatically during spawning and it is said that schools of inanga can be seen swimming and wriggling around near the waters edge where the eggs are deposited and fertilized by the male’s milt. It is possible to hear splattering sounds and even for the water to appear as though it is boiling from the movement of the fish (McDowall, 1990).
Predators
At sea, despite their transparency inanga are preyed upon by marine fish such as hapuka (Polyprion oxygeneios), red cod (Pseudophycis bachus) and kahawai (Arripis trutta) as whitebait have been found in the stomachs of these fish (McDowall, 1990). As inanga migrate back inland they are intensely preyed upon. It has been found that sea birds chase them into the river mouths and estuaries where the whiteface heron (Ardea novaehollandiae novaehollandiae)and kingfisher (Halcyon sancta vagans) feed upon them (McDowall, 1990). At the river mouths, fish such as kahawai, brown trout (Salmo trutta), eels (Anguilla spp.) and black flounder (Rhombosolea retiaria) feed upon them and this predation continues as the inanga move further up into their fresh water habitats (McDowall, 1990). Slugs appear to consume small numbers of whitebait eggs, as they feed on their own eggs which look very similar to those of the whitebait and are often found amongst them (Madgewick et al., 1992).
Competitors/Mutualisms
There is no apparent strong competition between inanga and other species, possibly due to the decrease in numbers from human impact on their habitat. An apparent mutualism is the way in which the inanga in its whitebait form migrates with the other four species of Galaxias and the juvenile smelt where in a school formation, individuals would be at a lesser risk of predation.
How to find inanga
It is highly unlikely that you could come across inanga during winter because at this time most of the adults are dead and the young are out at sea. To see inanga in their juvenile form would be in early spring, when the whitebait are migrating back into the fresh water. This is the time when many New Zealanders fish for the whitebait at river mouths near or at high tide (Richardson & Taylor, 2002), which would be an ideal time to view them. Canterbury Rivers such as the Rangitata, Rakaia and Waimakariri are popular whitebaiting rivers.
In summer, the maturing inanga can be found in shoals, in a wide range of freshwater habitats such as the pools, backwaters and lagoons of rivers (McDowall, 1990). At the autumn spawning time, it is possible to find mature inanga in shoals also and during spawning they are much livelier and more likely to be found further down river in the shallow areas of estuaries, amongst the riverside vegetation (McDowall, 1990).
Abundance and Conservation Status
- New Zealand: abundant
- Canterbury: abundant
Inanga are widespread and abundant, although they have suffered from extensive habitat deterioration (McDowall, 2000). One of the greatest threats to inanga is the trampling of livestock on their habitat as inanga thrive in streams flowing through pasture land (Richardson & Taylor, 2002). Another threat to inanga is the human impact on their habitat. This is through urban and rural development in the way that inanga are being prevented from traveling upstream by floodgates, culverts, weirs and dams (Richardson & Taylor, 2002). The most obvious threat to inanga is through the fishing of whitebait in spring, which has led to many restrictions being put on the way they are fished.
Inanga are not known to pose a threat on any particular species, although it was wrongly thought that they may have been responsible for the disappearance of trout from New Zealand streams in the early days (McDowall, 1990).
Significance for people
Modern Uses
The annual upstream migration of whitebait is an important recreational and commercial fishery in New Zealand as many people head to the river mouths in spring to catch the cultural and gastronomic icon that is whitebait (Richardson & Taylor, 2002). This is where inanga is important because it makes up more than 95% of the whitebait catch and due to its size and coloration it is the most commercially attractive of the Galaxias species (McDowall, 1990).
Traditional Maori significance and uses
Maori valued inanga as a food because both the mature adults and juvenile whitebait are rich and oily (McDowall, 2000).
Historical uses
There have been reports of cartloads of whitebait being caught in the 19th century but this makes it quite clear that the abundance of inanga and whitebait has declined dramatically since then (McDowall, 1990), underlining the need for this species to be more closely watched to conserve its ecological status in New Zealand.
References
Bailey, K., Leutscher, A., Pope, J., & Roberts, D. (1973). Nature (p 910). Glasgow, Great Britain: William Collins Sons and Company Limited.
Harding, J., Mosley, P., Pearson, C., & Sorrell, B. (2004). Freshwaters of New Zealand (pp 17.2–17.3). Christchurch, New Zealand: NEW ZEALAND HYDROLOGICAL SOCIETY INC. NEW ZEALAND LIMNOLOGICAL SOCIETY INC.
Madgewick, H.H., Mitchell, C.P., Strickland, R.R., & Van Boven, R.J. (1992). The Use of Larval Fish as an Aid to Identifying Whitebait Spawning Grounds, and the Role of Slugs as Predators on Whitebait Eggs. New Zealand Freshwater Fisheries Miscellaneous Report, 127, p 13.
McDowall, R.M. (1990). New Zealand Freshwater Fishes: a Natural History and Guide (pp 87–127). Auckland, New Zealand: Heinemann Reed.
Mcdowall, R.M. (2000). New Zealand Freshwater Fishes (pp 83–85). Auckland, New Zealand: Reed Books.
Richardson, J. & Taylor, M.J. (2002). A Guide to Restoring Inanga Habitat. NIWA Science and Technology Series, 50, pp 5–25.
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