eastward to at least the Lakekamu River.
Although the invasive fish species already present in New Guinea appear to be undergoing population expansions, thereby posing a grave threat to native species (Allen 1991), the specific impacts of such invasives on aquatic organisms endemic to New Guinea have for the most part not been determined. Similarly, little work has been undertaken regarding the identity or spread of other invasive freshwater animal species, particularly invertebrates.
The following section provides additional detail on many of the most significant invasive fishes documented from the New Guinea region, and their varying degrees of ecological impact as known to date.
Carp
Carp (Cyprinus carpio) are common in a few areas such as the upper Baliem River in Papua, Lake Kopiago, and the Lower and Middle Sepik and Ramu river systems of PNG (Allen 1991). Like many invasive fish species, carp modify their environment to conditions for which they are better suited to survive in than native fish species. World-wide, carp are regarded as a pest fish because of their tendency to uproot and destroy aquatic vegetation which results in increased turbidity and deterioration of habitats (Fuller et al. 1999). Carp have also been found to not only impact native fish species directly through egg predation, but also negatively impact waterfowl by increasing turbidity causing a reduction in food availability needed by both birds and native fish (Fuller et al. 1999).
Tilapia
Tilapia (Oreochromis or Sarotherodon spp.) are perhaps one of the most adaptable and widespread species of fish in existence, and have been stocked throughout the world. These highly invasive fish are now abundant in the Timika region of Papua (Allen et al. 2000), and in the lower Ramu and middle and lower Sepik rivers of Papua New Guinea, and have become the most important food fish in the Sepik area (Allen 1991). Tilapia have ecological impacts similar to carp in that they uproot aquatic plants, are known to feed on wetland taro, and reduce food supplies for native bird species (Englund and Eldredge 2001). In contrast to carp, tilapia are even more invasive in tropical areas because their ability to withstand saline and brackish-water environments (Englund and Eldredge 2001) allows them to spread along a coastline.
Snakeheads
Native to areas of Indonesia west of Weber’s Line, snakeheads (Channa striata) currently are found on Waigeo Island off western Papua (Allen et al. 2000); in streams near Bintuni on the Vogelkop Peninsula (Allen 1991); in the Timika region (Allen et al. 2000); and in the vicinity of Merauke (Hitchcock 2002). Because migrants prefer eating this fish, it is commonly first found near their settlements (Allen et al. 2000) and would be expected to be spread throughout Papua by migrant communities. Species in the genus Channa are voracious and highly effective predators, and further establishment in New Guinea would have highly detrimental impacts to all freshwater biota. Snakeheads have been implicated in the extinction of at least four species of fish in Madagascar, displacing the formerly common native Cichlid genus Paratilapia from the central plateau and Lake Aloatra (Courtney et al. 2004; Courtenay and Williams 2004). In New Guinea, snake-heads appear to have caused a general reduction of native fish numbers and diversity in areas where they are present (Allen et al. 2000).
Trout
Although Brown Trout (Salmo trutta) were introduced in 1949 to the highland regions of Papua New Guinea, and by 1952 had become established in this area (Werry 1998), there is no record of similar introduction into the upland streams of Indonesian New Guinea. In Papua New Guinea, the effect of trout on the native ichthyofauna appears to be minimal because they have survived in high elevation (> 2,000 m) areas lacking native fishes (Allen 1991), although they have been documented to prey upon two species of endemic waterbugs, Nesocricos mion and Tanycricos acumentum, the latter of which also occurs in the central ranges of Papua (Polhemus and Polhemus 1985, 1986). The impacts of this predation are unknown, however, and these insect species still remain common enough after the introduction of trout to be used as bait by highland tribesman (Polhemus and Polhemus 1985).
Livebearers (Poeciliids)
At least three poeciliid species have been recorded from New Guinea: Mosquito-fish (Gambusia affinis), Guppies (Poecilia reticulata), and Green Swordtails (Xiphophorus helleri). Although Mosquitofish and the other two species of poeciliids were introduced into New Guinea to control mosquitoes, their impact in this regard has been minimal, and they instead crowd out effective native predators of mosquitoes such as rainbowfishes (Allen 1991). Given their continuing popularity as supposed biocontrol agents, however, such fishes will likely continue to be introduced to Papua, particularly in streams near larger towns. The situation in Papua New Guinea provides a cautionary example: guppies are now common in the Goldie River and in streams around Port Moresby, and are often the only fish present. In 2004, surveys funded by Conservation International also found guppies in the lower Gumini River system of Milne Bay Province; these guppies were the only invasive fish species found in this otherwise faunally pristine stream. Green Swordtails, although unknown from the main body of Papua province, are present in at least one upland stream on Biak Island, and are highly invasive, having the ability to penetrate streams far inland (Englund and Eldredge 2001).
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