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Rainbowfishes have a number of basic habitat requirements in order to survive from day to day, to breed and to maintain populations over the longer term. A change in any one of these factors reduces the ability of the population to survive in the long term. A reduction in two or more factors will seriously threaten the population. Most rainbowfishes live in the tropical and sub-tropical climates of Australia and New Guinea. They occupy virtually every type of freshwater ecosystem, from wallum swamps to fast flowing rainforest streams; rivers and their tributaries including lagoons, billabongs and the waterholes of arid desert country. Few species of rainbowfishes flourish in bare, barren habitats. A feature of any pristine environment is the huge variety of habitats present. However, there is a pronounced difference between the aquatic habitats of Australia and that of New Guinea.
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| Sepik River, New Guinea |
New Guinea
New Guinea is located just north of Australia and is the world's second largest island with an area of around 836,171 km². The term 'New Guinea' refers to the entire island, consisting of both West Papua and Papua New Guinea. The mainland of New Guinea and its associated archipelagos stretch across a distance of almost 3,000 km between the equator and 12° south on the south-eastern rim of the Pacific Ocean. The climate is basically equatorial and on the coastal seaboard and river basins it is hot, wet and humid. However, New Guinea has a mountainous cordillera which runs along its centre and here ranges rise to 4509 metres at Mount Wilhelm the highest point in Papua New Guinea and to 4884 metres at Pucak Jaya the highest point in West Papua. These highland regions are cooler and less humid but generally equally wet.
The majority of the island is mountainous, except for the south with its extensive and inaccessible swamps and mangrove forests. The mountains, rivers and valleys all act as biological barriers to the movement or migration of plants and animals around the island. Indeed, geologically, the island is extremely complex, comprised of many terrains that have accreted. The biogeography of the island often reflects the independent evolutionary history of these different terrains. The complexity of the province's biogeography contributes to its rich biodiversity. Habitats range from the coastal hot, wet and swampy plains of the south to the permanently snow and ice covered mountains.
The island is divided physically by an extensive mountain range that extends from the Vogelkop Peninsula to the Owen Stanley Ranges in the south-east. This mountain chain forms the backbone of the island and divides the island into a northern and a southern part. The freshwater habitats of New Guinea have been poorly studied. However, the freshwater ichthyofauna can be clearly divided into two biogeographical regions. Freshwater bodies to the south of the central mountain range have an ichthyofauna closely allied with that of northern Australia. Rainbowfishes inhabiting river systems in the north are by and large different species from those in southern water bodies. Because of its mountainous terrain and consequent abundance of isolated freshwater drainage systems, New Guinea represents a particularly rich area for rainbowfishes. More than 80% of the known species of rainbowfishes are found in New Guinea, and no doubt many more will be discovered as a result of future systematic surveys.
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| Palmer River, New Guinea |
New Guinea is drained by 6 major and numerous minor river systems. Three of these, the Mamberamo, the Sepik, and the Markham, flow northward into the Coral Sea. The remainder, the Digoel, the Fly, and the Purari flow south into the Arafura Sea, Gulf of Carpentaria and Torres Strait. Both the northern coastal plain and the interior highlands boast numerous lakes. Three quarters of the country is covered by tropical rain forests and the remainder consists of flat grassland, lowland floodplains and mangrove swamps.
As a result of the high rainfall and rugged topography, most rivers in New Guinea have large flow volumes and high sediment loads, and are generally fast-flowing and turbulent. However, water levels in the main rivers fluctuate dramatically through the year, creating a variety of aquatic habitats including swampy and flooded forest, swampy grasslands, oxbows, and small lakes. Mangroves, brackish swamps, freshwater swamps and alluvial plains account for 7.5% of the total land area of the country. Lowland freshwater wetlands are a mosaic of open water, herbaceous swamp, swamp savannah, and woodland. Most of the wetlands in New Guinea are in pristine condition, but some are under threat from increasing development.
The climate is wet through the year with a relatively dry period between June and August. Precipitation is heavy throughout the island but sharply seasonal in character. Temperatures vary along an altitudinal gradient, the hot (25~30°C) wet tropical climate of the coastal plains giving way to much cooler (12°C at 3000 m) conditions in the highlands. Soft, slightly alkaline water chemistries characterise the larger rivers and most lakes. However, acidic conditions are found in many small creeks flowing through swamps or intact forest along the coast.
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| Fly River (floodplain), New Guinea |
Australia
Australia is a tired landscape, ageing and old before its time thanks to the climate. Australia is the second driest continent on the planet, only the icy Antarctica receives less rain. Australia has a highly variable and unpredictable climate, with long, irregular dry and wet cycles determined by the effects of El Niño and La Niña episodes. Rainfall and runoff vary from month to month and from year to year. Australia's river systems evolved with this variability. Within this variability, longer-term cycles of droughts and floods appear. However, parts of coastal north-eastern Queensland contain well-watered tropical rainforests more akin to that in New Guinea. Rainforest streams are characterised by their clear water, usually high current, lack of abundant aquatic vegetation, and almost complete shade of the water by riparian vegetation. This lack of aquatic vegetation has led to the behaviour of some fish species depositing their spawn in the substrate.
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| Lacey Creek, north Queensland |
There are three major landforms on the mainland of Australia; the Great Dividing Range and its associated smaller ranges, the Central Eastern Lowlands (west of the Great Dividing Range) and the Great Western Plateau, which covers about 60% of the continent. These landforms influence the major drainage patterns of the mainland. Within major drainage basins, there are always minor drainage basins. Minor basins sometimes have two or three rivers in a system. Other minor basins have only one river, rising in a mountain range and draining away into lonely desert land. Some only flow after heavy rains, which may be years apart.
The chemistry of Australian surface waters differs from most waters elsewhere, often being dominated by sodium chloride rather than calcium and magnesium bicarbonates. Groundwater is often very old; for example, in the Great Artesian Basin water travels across Queensland, to emerge in central Australia in bores one to two million years after it entered the ground. The generally arid climate means that mainland Australia has relatively few permanent and freshwater lakes. Lakes on the mainland are often shallow, dry and salty. Only on the Central Plateau of Tasmania do a number of larger permanent fresh-water lakes occur.
In Australia, rainbowfishes generally occupy three broad climatic zones. Above the Tropic of Capricorn has a tropical climate characterised by a generally hot, humid summer with strongly seasonal rainfall, and a mild to warm, dry winter. Below the Tropic of Capricorn is sub-tropical with a similarly hot, humid summer and seasonal rainfall, but with some significant rainfall occurring during the mild winter. The interior portion of Australia occupied by rainbowfishes experiences an arid sub-tropical climate. Summers can be extremely hot and dry, with variable rainfall; winters are cool to warm and dry, with irregular light rain.
The majority of rainbowfishes found in Australia are distributed throughout the northern and eastern coastal strips. The presence of substantial rainfall and the range of habitats found in these regions accounts for the relatively greater number of species found in this part of the continent. In southern Australia, their distribution is almost certainly controlled by winter minimum water temperatures. The family's distribution is exceptionally broad, ranging from the tributaries of the Murray-Darling system in Victoria, northwards up the east coast to Cape York, and west to the north-west coast of Western Australia. They also occurs throughout the inland rivers of the arid Lake Eyre drainage in central Australia.
Australia, by virtue of its size, contains a large variety of different freshwater ecosystems. Broadly, the north of the continent has a monsoonal rainfall pattern, while the south generally has a temperate, winter-rainfall pattern. The eastern seaboard and the extreme south-west of the continent are reasonably well-watered, while the arid interior is characterised by rainfall which is extremely variable. The majority of Australia's tropical river systems are characterised by large catchments, with expansive, seasonally-inundated floodplains. From the Fitzroy River near Broome in Western Australia to the Fitzroy River near Rockhampton in Queensland there are more than 60 major rivers and hundreds of smaller streams flowing directly into the sea.
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| Cooper Creek, central Australia (dry season) |
Most rivers, even the larger ones, are ephemeral in most years having highly seasonal and variable flow. Many cease to flow during the dry season and tidal influences can extend some 80~100 km upstream. Some are little more than a chain of elongated waterholes for much of the year. Extended periods of low flows during the dry season not only separate main-channel habitats from off-channel floodplain lagoons, but can also reduce contiguous main-channel habitats to a string of shallow, isolated pools. In the drier areas, these habitat changes can dramatically influence fish community composition by increasing density dependent interactions and causing extreme water quality conditions.
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| Edgbaston Springs |
Artesian springs, such as those found in large numbers on the fringes of the Great Artesian Basin, are of great significance as a focal point for plant and animal life - and as refuges in generally arid regions. The spring water emerges as seepages, as flowing springs, or form pools of standing water. Depending on the rate of water flow, moderately large pools may form over the spring site, some feeding streams or tails several kilometres long. While at least one of the fish (Leiopotherapon unicolor) inhabiting artesian springs, is widespread, many more appear to be restricted to particular groups of springs. The desert goby (Chlamydogobius eremius), which was originally thought to be widespread, is now six species, five of which occur in desert areas, including Chlamydogobius gloveri found only in Dalhousie Springs, and Chlamydogobius squamigenus restricted to Edgbaston Springs, Queensland. Craterocephalus dalhousiensis and Neosilurus gloveri also appear to be narrow endemics, as does Scaturiginichthys vermeilipinnis, which has only been found at Edgbaston Springs.
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| Old Elvire River, Western Australia |
Riverine habitats include open water areas over sandy or muddy bottom substrates. Aquatic vegetation fringes the margins and consists mainly of aquatic beds of floating leaved species and reeds. Mostly the vegetation is in a thin band along the margins, with leaf litter accumulating below. Water flow varies from permanent to seasonal and may dry back to non-flowing deeper holes during dry conditions. Oxygen levels decline during the dry season; pH is mostly neutral and specific conductivity is low. Water flow is relatively slow except for short periods following wet season rainfall. Saltwater enters the lower reaches of coastal riverine habitats and they generally have some tidal movement.
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| South Alligator River (floodplain), Northern Territory |
Many rivers in Australia are floodplain rivers, and during the wet season they break their banks to cover large areas of flat country. The floodplain surrounding lowland rivers contains a mosaic of habitats. All are replenished, regularly or irregularly, by flooding. Such waterbodies include intermittent lakes, billabongs (lagoons) and various types of flood runners (deep channels that only have water in them during high floods), as well as backwaters, anabranches and creeks. Equally importantly, river floodplains also contain swamps, marshes and other intermittently wetted areas, all of which play crucial roles in conserving river health. Indeed, the whole of a river floodplain can be considered a single, but extremely diverse, wetland. Floodplains are important areas for rainbowfishes moving upstream or downstream and for feeding and spawning. The floods covering floodplains in most areas are short lived. Flow in the river channel may persist for a longer period, but the period of extensive flooding rarely exceeds two to three weeks. However, this may occur several times in the course of a good wet season, but is dependent upon cyclones, rain depressions and monsoonal development. While perhaps not important as habitat for some species, floodplain and temporary wetland habitats may be important to rainbowfishes through the entrainment of terrestrial organic matter into organic food webs thereby increasing the abundance of food for larval and juvenile fish.
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| McIvor River, north Queensland |
Tributary streams are mostly slow-flowing and seasonal in nature. They can be clear or turbid with fringing water plants such as waterlilies, emergent grasses, and sedges. Rainforest streams are characterised by their clear water, usually high current, sparse aquatic plants, and almost complete shade of the water by riparian forest.
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| King River (billabong), Western Australia |
Lagoon (billabong) habitats differ significantly from riverine habitats. Billabongs are pools or lagoons left behind in a river or in a branch of a river when the water flow ceases. Billabongs are often formed when floodwaters recede, replenished only when the stream floods again. Flowing water in the river channels provide few niches for rainbowfishes to live in, while lagoons have a diverse range of areas for breeding and feeding. Substrates are mud or silt, and there is an abundance of water plants growing to the surface around the margins. Sometimes they may have water plants growing in the deeper water in the middle. Lagoons are habitats for decomposition of organic matter from terrestrial sources, often having a thick layer of leaf litter around the margins. In the wet season they often turn green, due to influx of nutrients in runoff water. A number of the smaller species of rainbowfishes and blue-eyes appear to be dependent upon these specialised habitats for their survival. Iriatherina werneri and Pseudomugil gertrudae are almost exclusively found in vegetated lagoons.
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| Burster Creek (swamp), north Queensland |
Swamps can be broadly defined as areas featuring permanent or temporary shallow, open water. This includes virtually any land, which is regularly or intermittently inundated. Swamps near river mouths are mostly slightly saline. Upstream swamps tend to be shallow and support mainly emergent water plants. There may be standing water in these swamps for most of the year. The ground storey may contain insectivorous plants (Byblis and Utricularia spp.), ferns, grasses, and a variety of sedges.
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| Fraser Island habitat, Queensland |
Large stretches of dune field and coastal heathland (wallum) swamps; lakes and streams are found dotted along the eastern Australian coast. These habitats are generally acidic, have low conductivity (dissolved ions), but vary in their pH levels, dissolved organic matter, ionic composition, and colour. Factors contributing to these variations are age, formation, layers of low permeability and peats, proximity to the sea, surrounding vegetation, and the extent to which leaf litter accumulates and decays in the water. The creeks and swamps contain dissolved organic acids (humic acids) which give the waters their dark brown colour and low pH (3.9 to 6.8). Rainbowfishes often found in these habitats include Iriatherina werneri, Melanotaenia maccullochi, Cairnsichthys rhombosomoides, Pseudomugil gertrudae, Pseudomugil mellis and Rhadinocentrus ornatus.
The sandmass water bodies are usually well oxygenated but highly oligotrophic (low nutrient levels due to the surrounding infertile sands) and of low biological productivity. The dominance of humic acids among this organic material and the relatively low pH are not conducive to bacterial degradation, so particulate and dissolved humic compounds are metabolised very slowly. The brown (tannin) colour of the water severely limits penetration of light, which, together with low concentrations of inorganic ions, restricts photosynthetic activity in aquatic plants. Limited photosynthesis and slow bacterial degradation results in low zooplankton and phytoplankton development.
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| Mangrove habitat, north Queensland |
Extensive areas of intertidal mangrove forests occur at the lower reaches of coastal topical rivers in both Australia and New Guinea. These mangrove forests are comparable in diversity to those of Southeast Asia, which are acclaimed as being among the richest mangrove areas in the world. During the wet season, freshwater flowing into these habitats dilutes the waters to nearly fresh. Water thus varies from saline through brackish to fresh. Rainbowfishes are seldom, if ever, found in these habitats. However, they are the preferred environments for a number of blue-eye species. Pseudomugil cyanodorsalis, Pseudomugil inconspicuus, Pseudomugil majusculus, and Pseudomugil signifer are regularly found in brackish water habitats.
© Copyright Adrian R. Tappin
Updated December, 2008
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