Temporal and Spatial Variations in Fish Catches in the Fly River System in Papua New Guinea and the Possible Effects of the Ok Tedi Copper Mine

Abstract

Biological monitoring of fishes in the Fly River system in Papua New Guinea has been carried out in relation to the input into the system of mine wastes from the Ok Tedi copper mine. A total of 86 fish species representing 32 families has been recorded from sites in the main river channel since the commencement of monitoring operations in 1983. Catfish in the families Ariidae (16 spp.) and Plotosidae (9 spp.) were the dominant groups overall, although Nematolosa herrings were the most numerous species, forming over 37% of the catch. However, barramundi, Lates calcarifer, comprised the greatest biomass, forming over 30% of the overall catch. Fish catch biomass at most sites showed considerable temporal and spatial variation over the period of sampling. However, significant reductions in biomass, ranging from 65% to 96%, were recorded at most sites in the Ok Tedi, middle and upper Fly. The greatest declines in biomass were recorded in the Ok Tedi at sites closest to the mine, although reductions up to 73% were also recorded at sites in the middle Fly. Barramundi, which formed a high proportion of catch biomass at many sites, particularly in the middle Fly, declined in number at most sites following peak numbers in the early 1990's. The main causative factors involved in the overall declines in fish catches, including both mine-related and non-mine-related factors, are discussed. It is concluded that 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 dissolved and particulate copper, and other non-mine-related factors, such as introduced species, may also be involved in declining fish catches.