Links to the Coastal Zone

      FISHERIES

      Mangrove forests are functionally linked to the biota and abiotic processes (sediment and nutrient flow, water circulation) of the adjacent coastal zone. Nearly all of the evidence of these linkages in New Guinea comes from the mangroves bordering the Gulf of Papua (Alongi and Robertson 1995; Robertson, Dixon, and Alongi 1998). Three types of mangrove-associated fishing practices occur in the Gulf of Papua: gill netting for Barramundi, trawling for prawns; and spearfishing for lobsters. These practices may, to some extent, be exemplary of other coastal zones of Papua and Papua New Guinea.

      Barramundi (Lates calcarifer) spawn along the coastal strip of the western gulf, in salinities of 30 (Moore 1982; Moore and Reynolds 1982; Reynolds and Moore 1982). Most of the spawning aggregation occurs with the onset of summer, probably triggered by the coincidence of peak spring tides and strong onshore winds. Rich detrital material derived from mangroves and marshes carried offshore by ebb tides may be the trigger for spawning offshore. Eggs hatch and incoming tides carry larvae into shallow wetlands. Most predators are excluded by shallow tidal waters, and food is abundant to promote rapid growth of the young. From these nursery areas, most Barramundi migrate eastward to access mangroves inhabiting the river deltas to feed on the abundant prawn populations (Haines 1979). Localized fisheries take advantage of these predictable movements, especially during the summer spawning (Mobiha 1995). From 1971 to 1984, commercial landings declined dramatically from 394 tons/yr to 139 tons/yr. Annual catch from 1993– 1994 was seven tons in the western gulf while the total catch was 58 tons in the eastern gulf (Opnai and Tenakanai 1986; Dalzell, Adams, and Polunin 1996; Kare 1996). This decline can be partly attributed to overfishing and poor management practices.

      Two prawn fisheries operate along the southern coast of Papua New Guinea. The Torres Strait fishery is dominated by Metapeneaus endeavouri (50%), Penaeus esculentus (40%), and Penaeus longistylus (10%). The fishery in the Gulf of Papua is dominated by Penaeus merguiensis and, to a lesser extent, Penaeus monodon. The prime nursery grounds for the species are seagrass meadows; as the prawns mature they move eastward to deeper waters. For Penaeus merguiensis, the larvae migrate inshore and settle to the bottom as they reach post-larval stage. The principal nursery area for this fishery is the mangrove-fringed islands and channels between the Purari and Kikori rivers (MacFarlane 1980; Evans and Kare 1996). Prawn post-larvae settle in the mangroves in November, grow and recruit to the fishery in February (Evans, Opnai, and Kare 1995). The trawler fleet is one of the largest in the South Pacific, with the annual catch generating up to 1,300 tons/yr (Dalzell et al. 1996). The Gulf of Papua annual average prawn landing was estimated to be 523 tons/yr for 1974–1993 for Penaeus merguinsis, and 844 tons/yr for all the other prawn species (Evans, Opnai, and Kare 1995). The total fish catch off the west coast of Papua in 1997 was 151,133 tons, with most of the catch being tuna, skipjack, and prawns.

      Rock Lobsters (Panulirus ornatus) are a small, but important, fishery species in the region. Lobster larvae develop in the open ocean, taking about six months to grow to juvenile size, and then settle as post-larvae into the seabed of the Torres Strait (Pitcher 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