The mighty Sepik is the second largest river in Papua New Guinea with a catchment area of approximately 80,000 km². Unlike other rivers in Papua New Guinea it discharges to sea through a single outlet. The river is navigable for about 500 kilometres from its mouth and the main channel is up to 35 metres deep. The Sepik River is one of the great river systems of the world. It has a huge catchment area and landforms that range from lowland swamps and tropical rainforests to frost-covered mountain peaks. Biologically, the region holds some of the most diverse and least described ecosystems on Earth, and is probably the largest uncontaminated freshwater wetland system in the Asia-Pacific region.
The Sepik begins in the Victor Emanuel Range in the central highlands. It leaves the mountains abruptly near Yapsei on the border with West Papua where it becomes a strongly braided channel flowing in a north-westerly, then northerly direction. Turning east it follows the great Central Depression, receiving numerous tributaries draining from the Bewani and Torricelli Mountains in the north and the Central Range in the south before entering the Bismarck Sea through a delta about 1100 km from its source. It flows through the East Sepik and West Sepik provinces. For most of its lower course the river meanders through a wilderness of swamps and lagoons with large floating islands of vegetation.
The Sepik floodplain contains around 1500 oxbow and other lakes. The largest is Chambri Lake that is at most four metres deep with a highly variable seasonal area. A study of 26 lakes in the Sepik floodplain found that there are two types of lakes: those that are connected to the main river and subject to seasonal flooding; and those that are further from the Sepik so that water input is from surrounding swamps with little or no input from the main river. Accordingly, the lakes can be classified into "black water" or "white water" types. Black waters are those stained by humic acids through their association with swamplands, while white waters are those loaded with suspended silt, clay, and detritus originate from the upper reaches of the river. A number of transitional mixtures are recognized and seasonal variations are likely to occur in all water types. Those lakes that have contact with the Sepik receive river-derived water during the wet season and drainage water from the surrounding swamps during the dry season.
Along most of its course the Sepik forms a 5-10 kilometres wide belt of active meanders that has created an active floodplain up to 70 kilometres wide with extensive backwater swamps. North of the Sepik River swamps are markedly rare or absent. Instead variably dissected low-angle fans cover much of the area between river and mountain front. On its way through the lowlands, the Sepik absorbs a considerable number of major tributaries that each command large catchments. The middle and lower Sepik floodplain is at least 30 kilometres wide and is flooded for at least five months of the year. It should be noted that the Ramu River can be seen as a tributary of the Sepik as during the wet season both are connected through the floodplain. There is also a man-made channel between the Ramu River and the Keram River that in turn enters the middle Sepik. This channel has been an important link in both the distribution of fish species and the spread of waterplants.
The water temperature at lower elevations on the Sepik River usually remains within the range 27-29°C. It is thought that average temperatures in the Sepik tributaries stay relatively low at lower altitudes than in most equatorial regions. This is partly due to the unusual topography of the basin in which lowland swamps are surrounded on most sides by relatively steep mountains resulting in cooler water being transported rapidly into the lowlands. Secondly, the degree of riparian shade covering feeder streams and rivers is pronounced in the Sepik in comparison with many other regions which have lost much of their vegetative cover through human activity.
There are at least 282 species of freshwater fish in Papua New Guinea of which approximately 46 per cent are endemic. There are around 75 species of freshwater fish in the Sepik and at least 7 introduced species of unknown status. The ichthyofauna of the Sepik and Ramu rivers are well studied in comparison to most other faunal groups in Papua New Guinea. The lower reaches of the Sepik and Ramu are now interconnected by numerous channels but as the rivers discharge into deep water there is no delta or estuary system. Consequently many types of fish like Lates calcarifer that have pelagic eggs and requirements for brackish habitats are not present in the river system. Piscivores are conspicuously absent except for Oxyeleotris heterodon, and the majority of carnivorous species feed on insects and prawns. Only species like Glossolepis multisquamatus, Giurus margaritacea and Oxyeleotris heterodon migrate on and off the floodplain with the seasonal changes in water levels.
The Sepik-Ramu river basin and its ichthyofauna can be divided into two major ecological zones. The first is in the lower altitudes where the system is dominated by extensive floodplains and associated lakes. At higher altitudes (200-4500 metres) the freshwater environment is dominated by a higher gradient, tributary streams and rivers. Greatest fish diversity is found in the lower reaches of the river; above 200 metres diversity declines dramatically.
In families and genera, the Sepik ichthyofauna is similar to that of the Fly River in southern Papua New Guinea. The main differences are in several genera that are absent from the Sepik. All of the native species are either diadromous (47%), and temporary visitors from sea, or freshwater species that belong to families that have marine origins. The only exception is the rainbowfish (Melanotaeniidae) which mostly occur in fresh waters. The Sepik, like most of the other river systems in New Guinea, is characterised by a number of endemic species and amongst these there is a predominance of rainbowfishes. The Sepik fish fauna also shares great similarities with that of the Mamberamo River of West Papua.
Fish Species List
Allomogurnda nesolepis - Yellowbelly gudgeon
Ambassis buruensis - Buru glass perchlet
Ambassis interruptus - Long-spined glassfish
Anguilla bicolor - Indian Short-finned eel
Anguilla marmorata - Giant long-finned eel
Brustiarius nox (Arius nox) - Comb-gilled catfish
Brustiarius solidus (Arius solidus) - Hard-Palate catfish
Brachyamblyopus urolepis - Scaleless worm goby
Bunaka gyrinoides (Oxyeleotris gyrinoides) - Greenback Gudgeon
Butis amboinenis - Ambon gudgeon
Caranx sexfasciatus - Bigeye trevally
Carcharhinus leucas - Bull shark
Chanos chanos - Milkfish
Chilatherina bulolo - Bulolo rainbowfish
Chilatherina campsi - Highland rainbowfish
Chilatherina crassispinosa - Silver rainbowfish
Chilatherina fasciata - Barred rainbowfish
Ctenotrypauchen microcephalus - Comb goby
Eleotris aquadulcis - Freshwater gudgeon
Eleotris margaritacea - Gudgeon
Eleotris melanosoma - Ebony gudgeon
Giurus margaritaceus - Snakehead gudgeon
Glossamia gjellerupi - Gjellerup's mouth almighty
Glossamia wichmanni - Wichmann's mouth almighty
Glossogobius bulmeri - Bulmer's goby
Glossogobius coatesi - Coates' goby
Glossogobius giurus - Flathead goby
Glossogobius koragensis - Sepik goby
Glossogobius torrentis - White water goby
Glossogobius sp.14 - Goby
Glossolepis maculosus - Spotted rainbowfish
Glossolepis multisquamatus - Sepik rainbowfish
Glossolepis ramuensis - Ramu rainbowfish
Hephaestus transmontanus - Sepik grunter
Hypseleotris guntheri - Günther's gudgeon
Kuhlia marginata - Dark-margined flagtail
Kuhlia rupestris - Jungle perch
Leiognathus equulus - Common ponyfish
Liza alata - Diamond mullet
Liza macrolepis - Largescale mullet
Liza melinoptera - Cream mullet
Liza tade - Flathead mullet
Lutjanus goldiei - Papuan black bass
Megalops cyprinoides - Oxeye herring, Tarpon
Melanotaenia affinis - New Guinea rainbowfish
Mesopristes argenteus - Silver grunter
Microphis brachyurus - Short-tailed pipefish
Microphis spinachioides - Spinach pipefish
Mogurnda aurifodinae - Northern mogurnda
Mugilogobius fusculus - One-thread goby
Neosilurus gjellerupi coatesi
Neosilurus gjellerupi gjellerupi
Neosilurus idenburgi - Idenburg tandan
Neosilurus novaeguineae - New Guinea tandan
Neosilurus sp.
Nibea soldado - Soldier croaker
Ophiocara porocephala - Spangled gudgeon
Ophisternon bengalense - Bengal eel
Oxyeleotris fimbriata - Fimbriate gudgeon
Oxyeleotris heterodon - Sentani gudgeon
Parambassis confines - Sepik glass perchlet
Potamosilurus coatesi (Arius coatesi) - Coates' catfish
Potamosilurus velutinus (Arius velutinus) - Papillate catfish
Pristis microdon - Largetooth sawfish
Pseudosciaena soldado - Silver perch
Redigobius bikolanus - Speckled goby
Scatophagus argus - Spotted scat
Sciades utarus (Arius utarus) - Northern Rivers catfish
Stenogobius laterisquamatus - Goby
Taenioides anguillaris - Eel worm goby
Toxotes chatareus - Largescale archerfish
Zappa confluentus - New Guinea slender mudskipper
Zenarchopterus kampeni - Sepik River garfish
Introduced
Barbonymus gonionotus - Javanese carp
Cyprinus carpio - Common carp
Gambusia holbrooki - Mosquitofish
Oreochromis mossambica - Tilapia
Osphronemus goramy - Giant goramy
Piaractus brachypomus - Red-bellied pacu
Tilapia rendalli - Redbreast tilapia
A major fish stocking project (the Sepik River Fish Stock Enhancement Project) commenced in 1987 and concluded in 1993. The project addressed the feasibility of introducing further species of fish into the Sepik and Ramu River basin to boost the fishery. The scientific advisory group to the project unanimously endorsed the recommendation to introduce further species of fish into the Sepik-Ramu basin. This project had a limited duration in which to actually obtain and import all of the fish recommended. In fact, only one species, Tilapia rendalli was introduced. Tilapia rendalli introductions were successful and the species has formed a significant population in the middle Sepik. It is interesting to note that in 1989, a consultant's report to the FAO stated that Tilapia rendalli had the potential to create severe ecological disturbances in the Sepik on account of being a voracious macrophyte feeder. It was stated that if substantial amounts of macrophytes were systematically removed there may be a subsequent effect on the rest of the food chain through the removal of epiphytic algae, habitats for invertebrates (food for endemic species of fish) and an ultimate reduction in primary productivity.
Shortly after the Sepik River Fish Stock Enhancement Project ended, finance was provided by the FAO for a further fish release program. Called FISHAID (Fisheries Improvement through Stocking High Altitudes for Inland Fisheries Development), the new project took over the stock enhancement recommendations arising from the previous initiatives. The chief task of the FISHAID project was to import and quarantine approved species of fish and release them at suitable locations in the Sepik-Ramu basin in order to improve "the naturally poor fish stocks in that region" (Coates, 1997). By the project's end in 1997, five new species had been introduced to the Sepik River, as well as further introductions of Tilapia rendalli. The population of Tilapia rendalli is still increasing.
The FISHAID project stocked edible coldwater fish species imported from Nepal and India into higher coldwater streams and warmwater species imported from Malaysia and South America into waters at lower elevations. The coldwater species were Neolissochilus hexagonolepis (Copper mahseer), Schizothorax richardsonii (Snow trout) and Tor putitora (Golden mahseer). The warmwater species imported for stocking in the flood plains of the Sepik and Ramu River Basin were Barbonymus gonionotus (Java barb), Piaractus brachypomus (syn. Colossoma bidens) and Prochilodus lineatus (Streaked prochilod).
At this stage most of the species released have not had time to breed and disperse. It is known that Tilapia rendalli and Barbonymus gonionotus have established small populations although it will take longer for them to build up to their final population levels and distribution. However, a fish biologist who was involved in some stream monitoring in the upper reaches of the Ramu River in 2005 reported that native species had declined from about six to only one or two while the exotics were the dominant biomass.
The vegetation of New Guinea is more closely allied to the flora of western Asia than to that of the Australian continent. For this reason it has been termed "Malesian", part Asian and part Melanesian. Freshwater plants recorded in the Sepik River and floodplain includes:
Acornus calamus
Acrostichum aureum
Aeschynomene indica
Ampelopteris prolifera
Azolla pinnata
Ceratophyllum demersum
Ceratopteris thalictroides
Cyclosorus interruptus
Cyperus platystylis
Echinochloa proestans
Eleocharis dulcis
Eleocharis retroflexa
Equisetum debile
Hanguana malayana
Hydrilla verticillata
Hydrocharis dubia
Hydrostemma motleyi
Hymenachne acutigluma
Ischaemum polystachyum
Isoëtes habbemensis
Lasia spinosa
Leersia hexandra
Lemna perpusilla
Lemna trisulca
Limnophila aromatica
Limnophila indica
Ludwigia adscendens
Ludwigia octovalvis
Machaerina rubiginosa
Monochoria hastata
Monochoria vaginalis
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Nelumbo nucifera
Nephrolepis biserrata
Nymphaea dictyophlebia
Nymphaea pubescens
Nymphoides exiliflora
Nymphoides indica
Oryza rufipogon
Panicum auritum
Panicum paludosum
Phragmites karka
Pistia stratiotes
Pogostemon stellatus var. roxburgianus
Pogostemon stellatus var. stellatus
Polygonum attenuatum
Polygonum minus
Scirpus grossus
Scirpus mucronatus ssp. mucronatus
Scirpus mucronatus ssp. clemensii
Sesbania javanica
Spirodela polyrhiza
Stenochlaena milnei
Stenochlaena palustris
Utricularia aurea
Utricularia exoleta
Introduced:
Alternanthera philoxeroides
Eichhornia crassipes
Salvinia molesta
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Source
Allen, G. R. and D. Coates (1990). An ichthyological survey of the Sepik River, Papua New Guinea. Records of the Western Australia Museum Supplement 34: 31-116.
Allen, G. R. (1991). Field Guide to the Freshwater Fishes of New Guinea. Madang: Christensen Research Institute.
Allen, G. R. (2003). Allomogurnda, A New Genus of Gudgeon (Eleotridae) from Fresh waters of New Guinea, with Descriptions of Seven New Species. Fishes of Sahul 17 (3-4): 978-97.
Coates, D. and P.A.M. Van Zwieten (1992). Biology of the freshwater halfbeak Zenarchopterus kampeni (Teleostei: Hemiramphidae) from the Sepik and Ramu River basin, northern Papua New Guinea. Ichthyological Exploration of Freshwaters 3: 25-36.
Coates, D. (1993). Fish ecology and management of the Sepik-Ramu, New Guinea, a large contemporary tropical river basin. Environmental Biology of Fishes 38: 345-368.
Coates, D. (1993). Recommendations regarding fish species suitable for stocking: Recommendation #9: The stocking of the "Pacu", Colossoma bidens Spix (Characidae), and responses from the advisory group regarding its suitability in-line with the Code of Practice Regarding Fish Species Transfers. Field Document No. 17, FI: PNG/85/001. FAO, Rome. 20p.
Coates, D. (1997). Fish stocking activities undertaken by the Sepik River Fish Stock Enhancement Project (1987-1993) and the FISHAID Project (1993-1997). Field Document No.5, FI:P.N.G./93/007. FAO, Rome. 41p.
Kolkolo, U. M. (1996). Quarantine practices used in Papua New Guinea for introductions and transfers of live fish. In Health Management in Asian Aquaculture. Proceedings of the Regional Expert Consultation on Aquaculture Health Management in Asia and the Pacific. R. P. Subasinghe, J. R. Arthur & M. Shariff (eds.), p. 140-142. FAO Fisheries Technical Paper No. 360, Rome, FAO. 142 p.
Marceniuk, A.P & Menezes, N.A. (2007). Systematics of the family Ariidae (Ostariophysi, Siluriformes), with a redefinition of the genera. Zootaxa 1416: 1-126.
Richards, S. J. and S. Suryadi (editors) (2002). A Biodiversity Assessment of Yongsu - Cyclops Mountains and the Southern Mamberamo Basin, Papua, Indonesia. RAP Bulletin of Biological Assessment 25. Conservation International, Washington, DC, USA.
Shearman, P. (1999). The Sepik River: A Natural History. WWF World Wide Fund for Nature, South Pacific Program, PMB, GPO, Suva, Fiji, 54 pp. and 7 Appendices.
© Copyright Adrian R. Tappin
Created April, 2004
Updated April, 2007
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