Regional extinction, rediscovery and rescue of a freshwater fish from a highly modified environment: The need for rapid response
Graphical abstract
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 biodiversity 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 (< 150 mm 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 2500 km 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).
A further quandary concerning rediscoveries arises in habitats modified and frequented by humans, where rediscovered species may occur through accidental or deliberate introductions (Metcalf et al., 2007) rather than having persisted despite adverse conditions. Confusion regarding origin could evoke conflicting management priorities ranging from urgent conservation action, a ‘do nothing’ approach, to invasive species control (Crees and Turvey, 2015). As a response to presumed regional extinction, in circa 1997, M. adspersa were translocated from northern MDB tributaries to the southern MDB at a small, isolated artificial wetland, the Murray Bridge ‘Army Range Wetland’ (Pierce, 1997) (Fig. 2). A population was established and plans were made to release some fish to the wild, but it is unknown if these were implemented (Hammer et al., 2012, Wager and Jackson, 1993). The Army Range Wetland is only 10 km from Jury Swamp, suggesting that the rediscovered fish might have been derived from the translocated population. Mogurnda species are also sold as aquarium fish in nearby Adelaide (population of 1.25 million people) and could have easily been transported to the Murray.
The possibility of translocation provided reason to question the origin of the rediscovered population. Furthermore the rediscovery occurred in an area of intense human activity (angling, boating, houses, dairy farms, drains, levees and introduced plants). As a consequence, government agencies were not persuaded to implement a formal conservation programme. Fortunately, during wetland drawdown some of the last remaining fish were rescued into captivity by a non-government organisation as the basis of a captive breeding programme, on the assumption they could be native to the area (Hammer et al., 2013).
This paper documents a research programme that ran parallel to, and informed, evolving conservation measures for southern MDB M. adspersa. Our aims were to (1) assess the population status of M. adspersa in the field, and (2) investigate genetic divergence and population heterogeneity using both nuclear and mitochondrial DNA markers in order to determine population origin (Hickley et al., 2004, Miller et al., 1989, Waters et al., 2002). In retrospect, we consider how an effective precautionary management response might be developed for application to comparable situations in the future.
Section snippets
Study region and environmental change
The MDB is an expansive river system that covers an area of 1.06 million km2 in south-eastern Australia (Fig. 2). The Murray and Darling rivers join and then flow to the Southern Ocean via the 830-km ‘Lower Murray’. The Lower Murray includes four zones, namely: (1) broad floodplain tract (the ‘Riverland’), (2) limestone gorge tract with a narrow floodplain, (3) swampland tract with only sparse wetland remnants in an area now reclaimed for pasture, and (4) the terminal ‘Lower Lakes’ region,
Ecological observations
Contemporary records of M. adspersa in the southern MDB, following rediscovery in 2002, are restricted to Jury Swamp in the swampland tract of the Lower Murray. Their local distribution was limited to two inlets to the wetland and a nearby drain entrance, all connected to the main channel (river length 600 m; estimated area of occupancy < 0.05 km2). Searches at other locations (including known historic sites) in the swampland and gorge tracts were not successful, although critical water shortage
Discussion
Extirpation of freshwater biota is the trend for degraded river systems, and second chances for conservation are rare (Matthews and Marsh-Mathews, 2007, Miller et al., 1989, Taylor et al., 2001). Native fish populations of the MDB have undergone major declines, with M. adspersa symptomatic of this change, shifting from being a common widespread lowland species to one presumed extinct from the southern MDB for 30 years (Hammer et al., 2009, Lintermans, 2007, Walker and Thoms, 1993). Our chance
Acknowledgements
Numerous people provided field assistance or other support. Special thanks to S. Angley, L. Beheregaray, C. Bice, D. Bray, G. Doyle, S. Doyle, R. Foster, A. Frears, D. Gilligan, A. Goodman, D. Goodman, A. Hall, M.F. Hammer, J. Higham, A. Kessel, K. Mason, L. Piller, T. Raadik, D. Short, D. Sortino, M. Treanor, M. Tucker, Q. Ye, S. Wedderburn, S. Westergaard and S. Wright. Permits for field research were obtained from PIRSA Fisheries with approval of the Animal Ethics Committee at The University
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