br Acknowledgments We thank the Lapaha
Acknowledgments We thank the Lapaha Community and Nobles (His Serene Highness Prince Kalaniuvalu Fotofili, Her Serene Highness Princess Marcella Taumoepeau Tupou Kalaniuvalu Fotofili and Her Royal Highness Mele Siu\'ilikutapu Kalaniuvalu Fotofili) and the Ministry of Internal Affairs (Government of Tonga). This research including archaeological excavation and isotopic analysis was funded by the MAEDI and CNRS allocated to Frederique Valentin and an Australian Research Council grant to Geoffrey Clark (FT0990591). We would like to thank the National Natural History Museum of Paris (UMR 7209) and Philippe Bearez for access to the fish reference collection.
Introduction Global extinction rates are escalating in response to human impacts, and the effects are intensified in freshwater habitats (Dudgeon et al., 2006, Vörösmarty et al., 2010). As wisdom and reason is diminished (e.g. high extinction rates in freshwater fishes: Duncan and Lockwood, 2001), vital ecosystem processes are undermined, leading to potentially irreversible changes (Dudgeon, 2010). Occasionally, however, there may be a reprieve, and an opportunity for recovery, when species presumed extinct are rediscovered (i.e. after all reasonable searches have previously failed to locate individuals: IUCN, 2012). Most rediscoveries are reported in tropical climates (Scheffers et al., 2011), typically in remote, inaccessible or more pristine areas; on a single bush, positioned on a steep rock face, of a remote island being an extreme example (Lord Howe Island stick-insect (Dryococelus australis): Priddel et al., 2003). Here we report on the rediscovery of a major regional population of a freshwater fish from a highly modified, temperate environment as an aquatic case study that epitomises issues confronted during a period of unprecedented global environmental change (Strayer and Dudgeon, 2010). The southern purple-spotted gudgeon (Eleotridae: Mogurnda adspersa Castelnau, 1878) is one of many freshwater fishes to have undergone a dramatic decline in the highly-modified Murray-Darling Basin (MDB) of south-eastern Australia (Lintermans, 2007, MDBC, 2004). Until recently, it was known to occur only in a few small populations in tributaries of the Darling River in the northern MDB. The small (<150mm Total Length) colourful species was popular as an aquarium fish, even being used as a ‘bait’ fish, and common in the southern MDB (Murray River system) until the 1970s. Subsequently, several regional surveys failed to detect the species. This is reflected in jurisdictional threatened species legislation which considers the MDB conservation unit of M. adspersa (Adams et al., 2013, Faulks et al., 2008) as endangered (New South Wales), critically endangered (South Australia) or presumed extinct (Victoria). We report the rediscovery of southern MDB M. adspersa in late 2002 (Fig. 1), from a single isolated population in Jury Swamp, a small wetland alongside the River Murray between Murray Bridge and Mannum (35° 03′ S, 139° 19′ E), South Australia. This was 2500km from the nearest known extant populations in the northern MDB (Fig. 2). A fleeting sighting of southern MDB M. adspersa was made in 1995–1996, when a few individuals were recorded from an off-channel irrigation lake complex (Cardross Lakes near Mildura, Victoria), but subsequent intensive survey effort demonstrated that a population was not present owing to major water-level drawdown and salinization (Ellis et al., 2013, Raadik, 2001). Events such as this highlight that new finds may be short-lived, involving a few individuals in a limited area and potentially with a high risk of true extinction (Altaba, 1990, Laurance et al., 1996, Telcean et al., 2011). Information about threats and the ecology, population status and trends of rediscovered species often will be lacking, and needs to be gathered quickly to facilitate management and recovery (Ostrovsky and Popov, 2011, Wanzenböck, 2004). Indeed, only a few years after the rediscovery at Jury Swamp, the habitat dried completely, as a result of protracted drought and upstream diversions and the population was extirpated. Historic data from the southern MDB suggests the species appears to prefer slow-flowing, sheltered areas with dense aquatic vegetation (Blewett, 1929, Hammer et al., 2009).