Chapter 12 Use of Changes in Fish Assemblages in the Fly River System, Papua New Guinea, to Assess Effects of the Ok Tedi Copper Mine

Abstract

Abstract Biological monitoring of fishes in the Fly River system in Papua New Guinea has been carried out to assess the effects of mine wastes from the Ok Tedi copper mine. Since the commencement of monitoring operations in 1983, 86 fish species representing 32 families have been recorded from sites in the main river channel, and 66 species from 33 families in floodplain habitats (oxbow lakes, blocked valley lakes, and seasonally inundated grassed floodplain). Catfish in the families Ariidae and Plotosidae were the taxonomically most diverse groups overall, although Nematolosa herrings were the most numerically abundant species, forming 37 and 66%, respectively, of fish caught from riverine and floodplain sites. Barramundi, Lates calcarifer , had the greatest biomass, constituting over 30% of the overall weight taken. Fish catch showed considerable change over time. At riverine sites, there were highly significant declines in biomass over time, ranging from 57% decline at Obo (1987–2002) to 92% decline in the Ok Tedi at Ningerum (1983–1996). The greatest declines in biomass were recorded from sites closest to the mine within the Ok Tedi, although reductions of up to 79% were also recorded at sites in the middle Fly. Within the Fly River, the extent of declines in biomass decreased with increasing distance from the mine. Changes in assemblage structure were also recorded at riverine sites, with a decline in frequency of occurrence of some species and in some cases, loss of species from parts of the system. The main causes of fish declines in riverine reaches, including both mine-related and non-mine-related factors, are discussed. The loss of fish habitat through increased river bed aggradation, due to the input of mined waste rock and tailings, is likely to be the main causal factor. However, other mine-related factors, such as elevated levels of suspended sediment, and dissolved and particulate copper, and non-mine-related factors, such as introduced species and artisanal fishing, may also contribute to declining fish catches. Reduced catches have also been recorded from a number of floodplain sites over the sampling period, although changes in fish assemblages at lakes Pangua, Bosset, and Daviumbu do not appear to be mine related, but are probably associated with natural climatic factors, particularly El Nino-La Nina drought-flood cycles and algal blooms. Fish populations were shown to recover only slowly in shallow lagoons affected by severe drought conditions, with extensive mats of floating grasses likely hindering fish recolonization. Fish stocks in a lake affected by an algal bloom recovered more quickly than stocks in lakes affected by drought. Recent analysis suggests reductions recorded from floodplain sites in close proximity to the Ok Tedi may be mine related (viz. OXB06). Suggested causes relate to increased flooding and forest die-back due to riverbed aggradation, which result in elevated turbidity, infilling of oxbows, and elevated dissolved metals. Introduced species and increased commercial and artisanal fishing may also be affecting floodplain assemblages.