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Kakadu is approximately 257 km from Darwin along the sealed Arnhem Highway. Located at the northern fringe of the Northern Territory, Kakadu is the largest national park in Australia. It comprises almost 13760 km² of spectacular wildlife habitat. At the heart of Kakadu is the South Alligator River, so named by a 19th century explorer, who mistook the native crocodiles for alligators.
The Kakadu area, while not limited to wetlands, encompasses a mosaic of contiguous wetlands comprising the catchments of three large river systems, the East, West and South Alligator rivers and nearly all the Wildman River system. With the inclusion of seasonal creeks, the lower reaches of the East Alligator River, the lower South Alligator floodplain and the Magela Creek floodplain. These wetlands included seasonal creeks, extensive freshwater floodplains, permanent lagoons, paperbark swamps and semipermanent billabongs. The large Mary and Katherine river systems also have part of their catchments in Kakadu.
The rivers are tidal in their lower reaches and are associated with extensive tidal flats formed from river borne mud. Up to 1465 km² of floodplain may be inundated during the wet season. The climate is monsoonal with a hot wet season from November to March accounting for, on average, 90% of the annual rainfall which ranges from approximately 1300-1565 mm. In May the floodplains begin to dry out with the water contracting into lagoons and billabongs. Mean monthly temperatures range from minima in the low twenties to maxima in the mid-thirties. The coolest months are in the dry season.
From the headwaters to the mouths of the creek systems, the habitat conditions presents a continuous gradient of conditions; cooler, clearer waters over rocky and sandy substrates moving downstream to hotter, more turbid waters over muddy and clayey substrates with hydrophytes. Distinctive fish communities are present in the upper catchments and lower reach floodplain zones. These zones contained major refuge habitats for fish.
The fish fauna of Kakadu is significant, being richer than in similarly sized drainage systems in tropical areas of the world and elsewhere in Australia. Fifty-nine fish species, 12 of them essentially marine, have been recorded from freshwater locations in Kakadu. This represents about one-third of all fish species found in Australian freshwater habitats. Five species, the exquisite rainbowfish (Melanotaenia exquisita), the Magela Hardyhead (Craterocephalus marianae), the sharp-nosed grunter (Syncomistes butleri), Midgley's grunter (Pingalla midgleyi) and a number of undescribed species have been recorded only from the northern part of the Northern Territory, and Kakadu contains a significant portion of the total of these species
Some species, such as the primitive archerfish (Toxotes lorentzi), the sooty grunter (Hephaestus fuliginosus), and the toothless catfish (Anodontiglanis dahli), live mainly in clear water near the escarpments. In the billabongs and creeks, some of the more common fish are barramundi, freshwater long-toms (Strongylura spp.), salmon-tailed catfish (Neosilurus spp.), chequered rainbowfish (Melanotaenia splendida inornata), and the Saratoga (Scleropages spp.).
Smaller juveniles of the larger fish species are found in lowland backflow billabongs and floodplain billabongs. These habitats are particularly important as nursery areas. Larger species tended to have their spawning limited to a single season, primarily either the early-wet or mid-wet seasons. Smaller fish species such as rainbowfishes have continuous spawning with a few peaks occurring during the year. The primary spawning peaks of such species typically occurred in the mid-wet season.
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| Melanotaenia solata? - Günther Schmida |
However, most fish species breed around the onset of the wet season. This is the time when initial flooding hugely increases the area and diversity of aquatic habitats available as well as initiating major increases in plankton and other foods. The most important spawning habitats are the lowland backflow billabongs. Feeding activity increased most dramatically between the late-dry season and the early-wet season. By the mid-wet season feeding activity has peaked, and then decreased slightly by the late-wet - early-dry season. The highest feeding activity occurs in lowland backflow billabongs followed by floodplain billabongs. Upstream the highest feeding activity occurs in the main channel water bodies followed by lowland billabongs.
Rainbowfish species found in the Kakadu region include Melanotaenia exquisita, Melanotaenia australis, Melanotaenia splendida inornata, Melanotaenia trifasciata, Pseudomugil gertrudae, Pseudomugil tenellus, and possibly Pseudomugil inconspicuus and Pseudomugil cyanodorsalis.
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| Waterfall Creek Exquisita - Neil Armstrong |
Another rainbowfish species known in the hobby as "Waterfall Creek Exquisita" comes from above the Gunlom Falls (Waterfall Creek), a tributary of the South Alligator River. Consensus is that this fish may be a new species and not a variety of Melanotaenia exquisita. The males show a charcoal black chequer-board pattern over the body with pink edging on the dorsal and anal fins. When spawning the nape band is bright orange. Also its possible that Melanotaenia australis found in the Kakadu region may be an earlier described species, Melanotaenia solata (Northern Rainbowfish)
Seven species of freshwater turtle can be found in Kakadu, and the pig-nosed turtle (Carettochelys insculpta) is particularly significant scientifically. This turtle is the sole surviving member of a family that has undergone some 40 million years of independent evolution and adaptation.
Over 200 amphibious and aquatic species have been recorded from the Kakadu region. Floating aquatic plants dominate areas covered by water for six to nine months a year. A variety of waterlilies, such as Nymphoides crenata and Nymphoides indica, are commonly found in these areas. Utricularia vulva is found along sandy creek banks. Nymphaea violacea is commonly seen along the margins of billabongs. Its violet-tipped white flowers appear between January and July. Nelumbo nucifera grows in lowland wetlands. Its leaves are very large and stand erect above the water. Large, fragrant deep-pink flowers appear between March and November. In more permanently wet areas, remarkable 'floating mats' of grasses (Hymenachne acutigluma and Pseudoraphis spinescens) can be found. These rafts are often sufficiently stable to walk on and are a favoured crocodile-nesting habitat.
Many other aquatic plants flourish in the Kakadu wetlands. Examples are Azolla pinnata, Baumea articulata, Baumea rubiginosa, Blyxa aubertii var. aubertii, Blyxa aubertii var. echinosperma, Blyxa octandra, Eleocharis (c. 4 species), Eriocaulon (c. 14 species), Fimbristylis spp., Lemna tenera, Limnophila chinensis, Limnophila brownii, Ludwigia octovalvis, Myriophyllum (c. 5 species), Najas (c. 3 species), Nymphaea (c. 4 species), Nymphoides (c. 5 species), Potamogeton javanicus, Pogostemon stellatus, Utricularia (c. 22 species), Vallisneria caulescens and Vallisneria annua.
© Copyright Adrian R. Tappin
Created July, 2001
Updated July, 2005
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