emr_418.fm Page 196 Thursday, December 4, 2008 4:34 PM RESEARCH REPORT doi: 10.1111/j.1442-8903.2008.00418.x EffectsBlackwell Publishing Asia of flooding on recruitment and dispersal of the Southern Pygmy Perch (Nannoperca australis) at a Murray River floodplain wetland By Zeb Tonkin, Alison J. King and John Mahoney ZebECOLOGICAL MANAGEMENTTonkin & RESTORATION and VOL 9 NO 3 DECEMBERDr 2008Alison King are freshwater Summary With limited evidence linking Australia’s Murray-Darling Basin fish species ecologists, and John Mahoney is a senior technical and flooding, this study assessed annual variation in abundance and recruitment levels of a officer with the Arthur Rylah Institute for Environ- small-bodied, threatened floodplain species, the Southern Pygmy Perch ( Nannoperca australis), mental Research, (Department of Sustainability in floodplain habitats (creeks, lakes and wetlands) in the Barmah-Millewa Forest, Murray and Environment, 123 Brown St., Heidelberg, River, Australia. Spring and summer sampling over a 5-year period encompassed large hydro- Vic. 3089, Australia; Tel.: +61 3 9450 8600; Fax: logical variation, including 1 year of extended floodplain inundation which was largely driven +61 3 9450 8799; Email: [email protected], by an environmental water release, and 2 years of severe regional drought. Recruitment and alison.king@dse. vic.gov.au, john.mahoney@dse. dispersal of Southern Pygmy Perch significantly increased during the floodplain inundation vic.gov.au). This work is part of a project funded event compared with the other examined years. This study provides valuable support for an by the Murray-Darling Basin Commission which environmental water allocation benefiting a native species, and explores the link between looks at assessing fish breeding and recruitment in flooding and its advantages to native fish. This suggests that the reduced flooding frequency response to environmental flow management. and magnitude as a result of river regulation may well be a major contributing factor in the species’ decline in the Murray-Darling Basin. Key words: environmental water allocation, floodplain inundation, freshwater fish, Murray- Darling Basin. provision of environmental water to cur- an annual EWA which has been used three Introduction rently over-allocated or overused systems times since its inception in 1993 (King owland river fish communities are to environmentally sustainable levels et al. 2007). It was not until the latest of L thought to rely on regular floodplain and improve environmental management these events, the 2005 EWA, that an intense inundations which provide a range of practices is a key objective of the National fish monitoring programme was under- benefits such as spawning cues and an Water Initiative (NWI 2005). Although taken to assess the response of the forest’s abundance of food and habitat (Junk these EWAs are being increasingly targeted fish community to such an event. King et al. 1989; Bayley 1991). The flood pulse at improving native fish populations, there et al. (in press) documented increases in concept is often extrapolated to Australia’s are unfortunately only a few examples of a spawning and/or recruitment of larger, Murray-Darling Basin (MDB) fish com- positive response by fish to an environ- native riverine fish species Golden Perch munity (e.g. the ‘flood recruitment model’, mental flow (Poff et al. 2003; Arthington (Macquaria ambigua), Silver Perch Harris & Gehrke 1994) despite limited et al. 2006; King et al. in press). (Bidyanus bidyanus), Murray Cod evidence of any benefits floodplain The Barmah-Millewa Forest is a 70 000- (Maccullochella peelii peelii) and Trout inundation has had on any MDB species ha highly complex floodplain wetland Cod (Maccullochella macquariensis) (Humphries et al. 1999; King et al. 2003; system located on the Murray River in during this EWA-driven flood year. As part Mallen-Cooper & Stuart 2003; Graham & south-eastern Australia’s MDB. The forest is of this programme, this paper assesses Harris 2005). With most rivers in the MDB one of the six recognized icon sites under annual variation in abundance and recruit- now regulated, there has been a significant the Murray-Darling Basin Commission’s ment of a small bodied, cryptic floodplain loss of low to medium level floods, which is ‘Living Murray Initiative’ (COAG 2004), and species, the Southern Pygmy Perch believed to be a major contributing factor is internationally recognized as an import- (Nannoperca australis). to the decline in the abundance and ant wetland under the Ramsar Convention The Southern Pygmy Perch is the most distribution of the Basin’s native fish fauna (Ward 2005). The forest’s hydrology has widespread of the six nannopercid species, (MDBC 2004). As a consequence, much been severely altered as a result of the occurring from coastal south-west Western attention has been given to the recovery of regulation of the Murray River, and poses a Australia to southern Queensland (Hum- a more natural flow regime as a restoration major threat to the ecological values of the phries 1995). Although relatively common strategy, particularly through environ- forest (Ward 2005). In an attempt to return in coastal Victoria, the Southern Pygmy mental water allocations (EWA; Poff et al. a more natural hydrological regime to the Perch is patchily distributed in northern 1997; Arthington et al. 2006). Indeed, the forest, the Barmah-Millewa Forest receives Victoria and is now listed as Endangered in 196 ECOLOGICAL MANAGEMENT & RESTORATION VOL 9 NO 3 DECMEBER 2008 © 2008 Ecological Society of Australia emr_418.fm Page 197 Thursday, December 4, 2008 4:34 PM RESEARCH REPORT New South Wales (established under part conditions resulted in the complete drying possessed clear banding. Although daily 7a of the NSW Fisheries Management Act of many of the floodplain habitat sites increment deposition has not been vali- 1994 ) and South Australia (SA Fisheries Act towards the end of the study. dated for the species, daily deposition has 1982; Action Plan for South Australian Sampling was conducted using modified been confirmed for another member of the Freshwater Fishes 2007). The species quatrefoil light traps (Floyd et al. 1984; genus, Nannoperca oxleyana (J. Knight, reaches a maximum size of 85 mm Secor et al. 1992), which are a useful unpubl. data 2008). Furthermore, back (although rarely >65 mm) and prefers passive technique for collecting fish from a calculation of increment counts did corre- slow flowing or still waters containing range of meso-habitat types and depths. spond to the length frequency distributions dense habitat such as aquatic vegetation Light traps were set on dusk with a yellow and approximate birthdates of the species or complex woody debris and is therefore 12-h light stick, and were retrieved as early presented by other authors (e.g. Humphries commonly associated with floodplain as possible the next day. Active sampling 1995), suggesting that increments are environments such as wetlands and billa- was also conducted in all habitat types deposited on a daily basis for the species. bongs (see also Kuiter et al. 1996; Linter- using the Sweep Net Electrofishing (SNE) Sagittal otoliths were removed with the aid mans 2007). The species is able to tolerate method (King & Crook 2002). The SNE of a stereo microscope and mounted in a broad range of temperatures and method is a modified standard backpack thermoplastic cement (proximal face down). extremely low dissolved oxygen levels electrofishing unit (Smith-Root Model 12, Otoliths were then polished to the level of (McNeil & Closs 2007), which is typical NE Salmon Creek Ave., Vancouver, WA, USA), the primordium across the sagittal plain of such environments. Submerged and with a 15-cm diameter anode ring, and using 3 μm lapping film and 0.5 μm alumina aquatic vegetation are also the preferred fitted with a moulded plastic rectangular slurry. Using a compound microscope spawning site of the species where females frame (25 × 30 × 2 cm) with an attached (400–1000 × magnification) fitted with a randomly disperse their small demersal eggs 250 μm mesh sampling net. A replicate SNE digital camera and the image analysis (Llewellyn 1974). Unsurprisingly, habitat sample involved approaching the selected system IMAGEPRO EXPRESS (version 5.0.1.26, alteration such as loss of aquatic vegetation habitat, activating the anode and moving Media Cybernetics Inc., Bethesda, MD, USA) and seasonal flow changes/reductions are at a constant speed in a forward zigzag the number of increments from the first listed as possible reasons for the species’ motion for 20 s of electrofishing time (see major increment outside the primordium decline (Lintermans 2007). The aim of this King et al. 2007). Five replicate samples of to the edge of the otolith was counted three paper was to assess annual variation in both light trap and SNE were taken at each times, after which the mean ring count was abundance and recruitment levels of site (only three replicates were taken in determined (see Sweatman & Kohler 1991). this species in floodplain habitats of the smaller wetland habitats), throughout the The mean ring count for each fish was then Barmah-Millewa Forest over a 5-year period duration of the study. The 2006/2007 and used as the estimate for daily age; however, encompassing large variation in hydrological 2007/2008 seasons consisted of extremely given the timing of deposition for the first conditions including an environmental low water levels, resulting in a number of increment is unknown, ages have been watering event. the wetland sites drying. presented as ‘estimated age’ throughout. After collection, fish were preserved No attempt was made to assess the age in 95% ethanol and later measured for structure of older fish given the difficulties Methods standard length (Ls – nearest 0.1 mm) and of annuli interpretation reported by As part of a larger fish monitoring pro- weight (nearest 0.001 g). These measure- Humphries (1995). gramme (see King et al.