Abstract: Lake Kutubu is a tropical freshwater lake that is internationally renowned for its biodiversity, 12 endemic species of fish, wetlands, and swamp forests. This study reports on a fish kill and the introduction of exotic species, and it assesses impacts on fish stocks and the artisanal fishery. The fish kill, which began in January 2013 and lasted for 6 months, was characterized by fish pathologies consistent with epizootic ulcerative syndrome. Sleeper gobies and gudgeons (i.e., Mogurnda variegata Nichols, Mogurnda furva Allen & Hoese, Mogurnda kutubuensis Allen & Hoese, and Oxyeleotris fimbriata [Weber]) had more obvious signs of disease than did Hephaestus adamsoni (Trewavas), Melanotaenia lacustris Munro, and Craterocephalus lacustris Trewavas. The event coincided with an inflow of a plume of white particulates from the northeast, where hydrocarbon companies carried out extensive horizontal drilling in 2012-2013. Six months after the event, the main species caught by fishers were Cherax papuanus Holthuis (16%), H. adamsoni (55%) and M. lacustris (28%), which is different from the mix of species found in previous surveys of 1995 and 1997. The recent fish kill, as well as socioeconomic changes, substantially reduced the fish catch and fishing effort. Small-scale fish farmers began raising the genetically improved farmed tilapia strain of tilapia (Oreochromis niloticus Linnaeus) and common carp (Cyprinus carpio Linnaeus) in earthen ponds in 2009 and introduced water hyacinth (Eichhornia crassipes [Mart.] Solms) as a food for these fish. Heavy rains in 2010-2012 released farmed fish into the lake, and in May 2015, fishers report that the fish catch is dominated by tilapia. Improved strategies are required to educate stakeholders, assess fish stocks, and protect biodiversity by reducing anthropogenic impacts.

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Lake Kutubu (latitude 06[degrees] 25.79' S, longitude 143[degrees] 20.22 E, altitude 808 m) in the Southern Highlands Province of Papua New Guinea (PNG) has been recognized as one of the most pristine, biologically diverse freshwater lakes in the whole of the Asia-Pacific region (D'Cruz 2008). As evidence, Lake Kutubu was the first subterranean karst wetland type to be added to the Ramsar Classification System by Resolution VI.5 (Ramsar site no. 961). Karst (cave) wetland systems are connected to underground rivers and act as recharge areas when the surrounding water table is low, and as discharge areas when it is too high. Since 1998, Lake Kutubu also has been designated as a wildlife management area (which is the simplest form of protection of an area of land or water, while still retaining full power of landowners to manage their land through an elected committee formed of customary landowners). It has approximately 5,000 ha of open water, 2,000 ha of wetland swamp forest, and a catchment of some 76,000 ha, which is mainly tropical rainforest. The lake has at least 12 endemic fish species, the most well-known is the popular aquarium fish, Lake Kutubu rainbow fish, Melanotaenia lacustris Munro (Coates 1986). The epilimnion (upper layer of water) of Lake Kutubu is normally 10-25 m in depth and has a stable average temperature of 23[degrees]C (D'Cruz 2008). It is...