1991). When they are about two and one-half years old they begin a mass migration in August from the strait (MacFarlane and Moore 1986) to coastal reefs in the eastern gulf near Yule Island. Most of the lobsters arrive in poor condition and most apparently die after the breeding season (Dennis et al. 1992). However, it is suspected that some lobsters move to other, unknown, breeding grounds such as deep reef habitats on the edge of the continental shelf (Evans 1996).
The lobster exhibit great plasticity of habitat use due to their complex reproductive, migratory, and settlement processes. They are found in a wide range of environments from sheltered, turbid waters to very silty areas near rivers and mangroves. Their diet consists mostly of mollusks and crustaceans (Joll and Phillips 1986). The lobsters are fished at both ends of the migration route and, until recently, along the route as well. Because this species is susceptible to being caught by trawlers, lobster by-catch was marketed at up to 200 tons/yr, but concerns about the reduction of spawning stocks and subsequent effects on recruitment led to a ban on keeping the by-catch (Pitcher 1991). In the Daru area, the lobster catch peaked at 92 tons in 1994 with a minimum catch of 57 tons in 1987 (Evans and Polon 1995). The annual catch at the Yule Island end of the migration route is usually two to three metric tons (Dennis et al. 1992). There is a Bêche-de-Mer (Holothuria scabra) fishery in the Daru area which in 1995 had a total yield of 55 tons, but there is little known about holothurian biology or the sustainability of this fishery.
According to estimates of fish resources in the Arafura Sea along the southern coast of Papua (Dalzell and Pauly 1989), potential yields for small pelagic and demersal fisheries are equivalent to those in the Gulf of Papua, with small pelagic yields on the order of 2.8 and 2.5 tons per km2 per yr and demersal yields averaging 1.1 and 1.5 tons per km2 per yr for the Gulf of Papua and Arafura Sea, respectively.
NUTRIENT AND SEDIMENT FLUXES
Mangroves are often considered to be accumulation sites for particulate nutrients and sediments, and this appears to be the case for the mangroves of New Guinea. Geological studies of mangroves bordering the Gulf of Papua and the Ajkwa and Tipoeka Rivers in west Papua (Thom and Wright 1983; Barham 1999; Brunskill et al. 2004; Walsh and Nittrouer 2004; Ellison 2005) have indicated that the man-groves are accreting. In west Papuan mangroves, Ellison (2005) estimated sedimentation rates in the range of 0.6–1.5 mm/yr, a rate in the same range as that measured by Thom and Wright (1983) in the Purari mangroves. In a series of contiguous cores, Brunskill et al. (2004) measured sedimentation rates in the Ajkwa mangroves on the order of 4.5–13 kg sediment per m2 per yr, which are well within the range of rates measured in other mangroves. Walsh and Nittrouer (2004) examined sedimentary history of the mangroves bordering the Gulf of Papua and measured accumulation rates ranging from 1.3–7.5 cm/yr (11–65 kg sediment per m2 per yr). Greatest rates of accumulation were observed on accreting banks in the mid-tidal zone, with lower rates above and below this tidal horizon. These figures are higher than those measured in the Ajkwa estuary, but remote sensing indicates that areas of accretion co-exist with areas of erosion in many mangrove/estuarine regions of the Gulf of Papua. Nevertheless, the net accumulation of coastal mangroves of the western Gulf of Papua is estimated to account for 2–14% of the total sediment load of the gulf. All of these sedimentation rates are testimony to the large volume of river sediment discharged from the land to the coastal zone of New Guinea. The discharge of water and sediment from the New Guinea highlands to the coastal plain translates not only into the deposition of particulate material in mangroves, but also into the export of nutrients to the adjacent coastal zone. It is believed that this export of dissolved and particulate material stimulates pelagic and benthic food webs in coastal waters, and supports the fisheries described above.
In the Gulf of Papua region, there is sufficient evidence to show that mangroves utilize significant quantities of dissolved riverine materials to sustain their high rates of primary production (Liebezeit and Rau 1987; Alongi, Christoffersen, and Tirendi 1993). A budget of carbon gains and losses in the Fly delta (Table 5.4.8) indicates that of the approximately 22.1 10 11 g C that comes into the delta from both river discharge and production by mangrove forests, roughly half is consumed in the delta and the other half is exported to the Gulf of Papua (Robertson and Alongi 1995). Roughly 25% of the organic carbon that is exported is man-grove-derived. Most of this material is low-quality detritus, such as leaves, roots, and bark. A study of the carbon-isotope composition of the sediments in the Gulf of Papua (Bird, Brunskill, and Chivas 1995) confirms that this material is exported from the Fly delta, but is limited to within a few kilometers of the coastline. The rapid decline of dissolved nutrient concentrations from the rivers to the adjacent coastal waters off the Fly, Purari, and Mamberamo deltas (Viner 1979; Robertson et al. 1993; Muchtar 2004) suggest similarly important, but geographically limited, export of mangrove material to the coastal ocean bordering the entire island.
Extrapolating the sediment and carbon discharge rates for rivers draining into the Gulf of Papua to the rest of New Guinea suggests total sediment and carbon discharge rates for the island similar to those of the Amazon River (Milliman 1995). There is circumstantial evidence that mangrove litter reaches the deep Coral Sea. Considering the much narrower distance from the rivers to the deep sea along the north coast of New Guinea, it is likely that proportionally more mangrove-derived matter reaches the deep ocean in the north (Kuehl et al. 2004).
Human Impacts
Most of the mangrove forests of Papua are still relatively pristine as human population density is low, and most human use is on a small scale. The only mangroves that have been subjected to a substantial degree of human impact are those near development projects and industries, such as copper mining, capture fisheries, wood chip extraction, and several oilfield projects. Mangrove losses in Papua have been small (< 10%) but for the islands fringing the Timor Sea, the losses of man-grove forest range from less than 5 to 50% (unpublished references cited in Morrison and Delaney 1996). Table 5.4.9 lists examples of the various human uses of mangrove forest in Papua.
The best-documented areas of human impact are the Ajkwa River estuary and Bintuni Bay. The main source of impact in the Ajkwa River estuary is the tailings from a copper-gold mine located some 3,700 m above sea level in the Moake Range. This mine is operated by PT Freeport Indonesia (PRFI) and has been open since 1972. Mine tailings are discharged into the estuary at a rate of about 125,000 tons/day. These tailings consist of sand and small pieces of ground rock, and deposit in a 130 km2 area contained by levees above the salt wedge of the estuary. Despite the levees, recent geochemical measurements (Brunskill et al. 2004) have found that copper accumulation rates have been enhanced 40-fold in mangrove sediments since the introduction of mining. The biological impact of these copper concentrations is unknown.
In Bintuni Bay, the mangroves have been increasing affected by wood extraction, fisheries, and oil and gas development (Brotoisworo 1991; Ruitenbeek 1992, 1994). Most of the primary forest bordering the bay has been allocated for timber concessions (Petocz 1987) with at least seven companies holding concessions of about 300,000 ha of the total mangrove area of 618,500 ha (Erftemeijer et al. 1989; Ruitenbeek 1994). An economic analysis of mangrove management options for Bintuni Bay (Ruitenbeek 1992, 1994) indicates that traditional non-commercial uses of mangroves have an estimated value of US$10 million/yr; commercial prawn fisheries are valued at US$35 million/yr and mangrove wood extraction are worth US$20 million/yr. Ruitenbeek (1994) suggested that the optimal management strategy was selective cutting of 25% of the harvestable mangrove for a total return of US$35 million/yr.
Oil concessions to four companies have resulted in extensive oil and gas production, with an estimated recoverable reserve of about 12.2 million barrels (Brotoisworo 1991). With the extensive logging concessions and the fact that the penaeid prawn production of Bintuni Bay was 1,375 tons/yr or about 20% of the total prawn production for Papua, there is significant overlap in resource use in the bay. At present, there does not appear
