Ecology of Indonesian Papua Part Two. Andrew J. Marshall
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of Food Fish Harvesting on Native Fishes
There are virtually no data on the harvest of native fishes for human consumption or the possible impact of this activity on native fishes in general. Compared to the considerable harvest of marine fishes, the take of freshwater fishes seems relatively insignificant. Nevertheless, people living along the major river systems depend on freshwater fishes for a significant portion of their diet. Most of the larger villages have regular fish markets, which appear to be dominated by forktail catfishes, large gudgeons (Eleotris and Oxyeleotris), and various introduced fish, especially carp and tilapia. Forktail catfishes (family Ariidae) are represented in New Guinea fresh waters by 21 species and are probably the most important food fish in this habitat. Although they are heavily targeted by gill netting and traditional fishing methods their numbers do not appear to be declining, at least in major Papuan river systems such as the Digul and Mamberamo.
A variety of fishing methods are employed including hook and line from canoes, home-made traps, and various nets ranging from simple one-person hoop nets to large seines and gill nets. Streams, some of considerable size, are sometimes diverted and the former channel containing isolated pools with dense fish concentrations are then netted or speared. Some villages also employ Derris (Fabaceae) root to poison ponds, stagnant pools or slow flowing sections of creeks. In addition, local fishers are usually adept at catching by hand gudgeons and other fishes that hide in crevices.
Traditional fishing methods appear to have insignificant impact on the native fish fauna. After all, they have been used for centuries and continue to be sustainable. The problem lies with more modern techniques, especially when outboard motors have been introduced in combination with gill nets. It is our opinion that gill nets should be banned from areas of special biological significance in Papua, such as Lake Sentani and Lake Yamur. Gill netting has certainly played a major role in the demise of the Freshwater Shark (Carcharinus leucas) in Lake Yamur and the Giant Sawfish (Pristis microdon) in Lake Sentani.
INVASIVE SPECIES
In relation to its overall size, the New Guinea region exhibits a remarkably low incidence of invasive freshwater species. This fortuitous situation appears to result from the fact that the region is lightly inhabited, has not experienced extensive colonization and settlement by foreign peoples (although this situation is changing in Indonesian New Guinea with a continuing influx of western Indonesian settlers that were initiated through now-defunct government-sponsored transmigration programs), and is still not well integrated into the global economy. The result is that freshwater ecosystems in many parts of the island and its proximal archipelagoes remain among the most pristine on earth.
New Guinea’s general ecological integrity notwithstanding, the presence of exotic freshwater fishes is an increasing problem throughout the island. Allen (1991) reported the presence of 22 species representing 19 genera, 11 families, and all six continents. Since then at least six more introductions have been noted, and more can be expected, especially on the Indonesian side of the island. In the present chapter we provide details of the more recent introductions as well as a general overview of the invasive problem.
Most of the introductions have had a negative impact, either by competing for space and limited food resources, or by feeding on native species, including their eggs and fry. Tilapia (Oreochromis mossambica) has adversely affected the environment, creating turbid conditions in formerly clean lakes, and badly overcrowding the indigenous fauna due to its prolific breeding. Several species including tilapia, walking catfish, carp, and climbing perch appear to be undergoing rapid population increases and therefore pose a serious threat to native fishes.
The current distributional pattern of introduced fishes is closely tied to transmigration areas of Papua Province, particularly the larger population centers such as Jayapura and Timika. The transmigration program of the Indonesian government is no doubt responsible for many of the introductions. Newly arrived settlers often bring their pets and fish-pond stock from other parts of the archipelago. Thus, there is a major potential for further introductions.
Of primary concern is the relatively recent appearance of four invasive species (tilapia, snakehead, climbing perch, and walking catfish) in the Bensbach River system of southwestern Papua New Guinea (Hitchcock 2002). At least some of these possibly entered the river via drainage ditches associated with the building of the Trans-Irian Highway, which in 1982 crossed the international border in two locations on the Upper Bensbach. Tilapia and walking catfish are more recent introductions, having been first noticed in the area in about 1995.
Of equal concern is the appearance of two South American fishes, Prochilodus argenteus (Prochilodontidae) and Colossoma bidens (Characidae), and Barbonymus goniotus (Cyprindiae) from western Indonesia in the Ramu system of Papua New Guinea. The origin of these introductions remains a mystery, but they may have been species that were experimentally raised for potential introduction during an ill-conceived fish stock enhancement program sponsored by the Food and Agriculture Organization of the United Nations (FAO) in the 1980s.
Allen et al. (2002) noted that the Mamberamo River in Papua Province had the highest percentage (17.1) of introduced fishes of any major river system in New Guinea. The appearance of species such as tilapia, walking catfish, snakehead, and three species of cyprinids is particularly alarming, given the relative isolation of this system and lack of major population centers.
Another problem area is the Timika region of southern Papua Province. Prior to the opening of the Freeport gold and copper mine, there were no invasives in the region. But a huge influx of transmigrants has seen the introduction of tilapia, climbing perch, walking catfish, and snakehead (Allen et al 2000). In addition, the Blue Panchax (Aplocheilidae) from southeast Asia was introduced in the 1990s, apparently for mosquito control.
Across New Guinea as a whole, invasive species appear more concentrated in lakes and wetlands, although certain lowland streams and river systems, particularly in the Mamberamo and Sepik-Ramu basins, are badly contaminated. The amazingly intact character of New Guinea’s wetland systems in a physical sense may in fact be limiting the spread of invasives, due to a lack of canals and periodically flooded agricultural field systems, coupled with natural seasonal drying. By contrast, the introductions of invasive fish into lotic (i.e., flowing water) environments is of great concern, since this enables highly vagile invasives such as tilapia, mosquitofish, or snakeheads to penetrate repeatedly both riverine and ephemeral riparian wetland habitats after seasonal flooding. Particularly problematic in this regard has been the introduction and continuing spread of snakeheads (Channa spp.) because of their ability to survive buried in the mud of ephemeral wetlands for months utilizing their accessory breathing organ (Courtenay and Williams 2004). This predaceous invasive has the potential to spread throughout the entire coastal wetland zone of southern New Guinea, from Etna Bay 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