Brenton P. Zampatti

Can a low-gradient vertical-slot fishway provide passage for a lowland river fish community?

Fishways are commonly used to restore native fish movements in regulated rivers. In the Murray-Darling Basin, Australia, 14 fishways are to be built by 2011 to improve passage along 2225 km of the river. The first of these fishways, constructed in 2003, is a vertical-slot design with low water velocities (0.98–1.4 m s–1) and turbulence (average 42 W m–3). This design was selected to provide passage for individuals between 20 and 1000 mm long. To determine passage success, trapping and a remote automated passive integrated transponder (PIT) tag reading system was used from October 2003 to February 2006. In 57 24-h samples at the exit (upstream end) and entrance (downstream end), 13 species and 30 409 fish were collected at a maximum rate of 4415 fish per day. Fish between 31 and 1030 mm successfully ascended the fishway. However, significantly smaller (<31 mm) fish and small-bodied (<50 mm) carp gudgeons (Hypseleotris spp.), a species previously considered non-migratory, were sampled downstream from the entrance of the fishway. The remote PIT tag reading system revealed that 81% of native golden perch (Macquaria ambigua) and 87% of non-native common carp (Cyprinus carpio) successfully ascended the fishway. These data will help maximise the efficiency of future fishways against a series of pre-determined performance criteria.

Temporal variability in fish assemblage structure and recruitment in a freshwater-deprived estuary: The Coorong, Australia

River regulation can diminish freshwater flows to estuaries and compromise estuarine functionality. Understanding biotic responses to altered flow regimes is imperative to effectively manage aquatic ecosystems. The present study investigated temporal variation in fish assemblage structure and the recruitment of catadromous fish in the Coorong estuary at the terminus of the Murray River, in south-eastern Australia. Over the three-year study period, freshwater inflows to the estuary diminished and ultimately ceased, disconnecting freshwater and estuarine environments. It was hypothesised that these conditions would lead to (1) increases in estuarine salinities and concomitant changes in fish assemblage structure and abundance, and (2) decreased recruitment of catadromous fish. As freshwater inflow decreased, salinities immediately downstream of a series of tidal barrages increased from brackish to marine–hypersaline, species richness and diversity decreased, freshwater and diadromous species became less abundant and assemblages were increasingly characterised by marine species. Furthermore, the abundance of young-of-year catadromous fish decreased dramatically. Excessive regulation of freshwater inflows is resulting in the Coorong estuary resembling a marine embayment, leading to a loss in species diversity. We suggest, however, that even small volumes of freshwater may promote diversity in estuarine fish assemblages and some recruitment of catadromous species.

Freshwater fish conservation in the face of critical water shortages in the southern Murray-Darling Basin, Australia

The lower reaches of the expansive Murray–Darling Basin, Australia, are a hotspot for freshwater biodiversity. The regional ecosystem, however, has been significantly altered by river regulation, including local and catchment-wide water abstraction. Freshwater fishes have suffered from the resultant altered flow regime, together with other threats including habitat degradation and alien species. Impacts reached a critical point (imminent species extinction) during a prolonged drought (1997–2010) that lead to broad-scale habitat loss and drying of refuges during 2007–2010, and urgent conservation measures were subsequently instigated for five threatened small-bodied fish species. A critical response phase included ad hoc interventions that were later incorporated within a broader, coordinated multi-agency program (i.e. the Drought Action Plan and Critical Fish Habitat projects). On-ground actions included local translocation, alien species control, in situ habitat maintenance (e.g. earthworks, environmental water delivery), fish rescues, artificial refuge establishment and captive breeding. Improved river flows signalled an initial phase of recovery in 2011–2012 that included reintroductions. The present paper aims to document the actions undertaken in the Lower Murray, and review successes and lessons from practical examples that will help guide and inform management responses to conserve fish in modified systems subjected to severe water decline.

Within-channel flows promote spawning and recruitment of golden perch, Macquaria ambigua ambigua – implications for environmental flow management in the River Murray, Australia

Restoring fish populations in regulated rivers requires an understanding of relationships between hydrology and population dynamics. In the present study, spawning and recruitment of golden perch, Macquaria ambigua ambigua, were investigated in relation to flow in the regulated lower River Murray. All life stages were sampled in three successive years, with peak flows of 8500 (2004–05), 15 000 (2005–06) and 7000 ML day–1 (2006–07). Larvae occurred only in November/December 2005, and young-of-year fish only in early 2006. Counts of daily increments in otolith microstructure indicated spawning in late October/early November 2005. Back-calculated birth years for adults, derived from otoliths and compared with the hydrograph for the preceding 25 years, revealed the dominance of three year classes spawned in association with increased discharge in 2000, 1998 and 1996. In 2007, an additional year class of 1-year-old fish appeared, following spawning in 2005. In each case, strong recruitment followed spring–summer spawning, when peak flows were >14 000 ML day–1 and water temperatures would have exceeded 20°C. Restoration of within-channel flows of 15–25 000 ML day–1 from late spring through summer would promote spawning and recruitment and improve the resilience of golden perch populations in the lower Murray.

Salinity may cause fragmentation of hardyhead (Teleostei: Atherinidae) populations in the River Murray, Australia

Salinisation in lowland areas of the Murray–Darling Basin, Australia, has had noticeable effects on fish. The endangered endemic Murray hardyhead Craterocephalus fluviatilis is distributed patchily and confined mainly to saline waters (0.4–20 g L–1), whereas the unspecked hardyhead C. stercusmuscarum fulvus has a more continuous distribution but is absent from high salinities (>10). Osmoregulation was compared in these two congeners and an estuarine atherinid, the small-mouth hardyhead Atherinosoma microstoma, over a wide salinity range (0.03–85). All three species are euryhaline, although the osmoregulatory ability of C. s. fulvus falters above ~35 salinity. In low salinity (<1), C. fluviatilis is a better osmoregulator than A. microstoma, but both species tolerate hypersaline conditions (85). These data imply a physiological reason for the predominance of C. fluviatilis in inland saline waters, but the reasons for its absence from freshwater habitats (<0.4) remain unclear. The findings have implications for other freshwater fish, especially populations of closely related species, subjected to the effects of salinisation or other stressors.

Evaluating migratory fish behaviour and fishway performance: testing a combined assessment methodology

To provide passage for migratory native fish, a series of 14 vertical-slot and lock fishways are being constructed on the Murray River in south-eastern Australia. Three of these vertical-slot fishways, at Locks 7, 9 and 10, have a conservative slope (1V : 32H) and are designed with internal hydraulics suitable for the passage of a broad size range of fish (30–1000 mm long). An assessment of these fishways was performed using a combined trapping survey and passive integrated transponder (PIT) approach to determine fishway effectiveness at passing an entire fish community. Fish were trapped within the three fishways between 2004 and 2006, where a total of 13 626 individuals comprising 13 species were collected from 48 sample days (24 h each). Trapping data revealed that the three fishways successfully passed fish within the target size range, though significantly greater numbers of individuals smaller (10–29 mm long) than the target size range could not ascend. PIT tagging revealed important information on fishway ascent times, descent times, seasonality and diel behaviour of medium and large fish. Although each method alone had advantages and disadvantages, the dual assessment approach was useful as it permitted an assessment of fishway success and also provided insights into migratory fish behaviour.

Movement and mortality of Murray cod, Maccullochella peelii, during overbank flows in the lower River Murray, Australia

Abstract. Conservation of Murray cod (Maccullochella peelii), a large endangered fish species of Australia’s Murray–Darling Basin, relies on a detailed understanding of life history, including movement patterns and habitat use. We used radio-tracking to investigate the movement of 36 Murray cod in main channel and anabranch habitats of the lower River Murray during a flood and associated hypoxic blackwater event. During a flood peak of ∼93 000 ML day–1, dissolved oxygen decreased to 1.2 mg L–1. Four movement types were observed: (1) localised small-scale movement, (2) broad-scale movement within anabranch habitats, (3) movement between anabranch and main channel habitats, and (4) large-scale riverine movement. Murray cod exhibited high fidelity to anabranch habitats but also moved extensively between anabranches and the main channel. Fish were consistently located in the main channel or permanent anabranches, suggesting that use of ephemeral floodplain habitats is limited, and highlighting the importance of connectivity between off-channel and main channel habitats. Mortality of radio-tagged fish was considerable (25%) in association with low dissolved oxygen concentrations, indicating that hypoxic blackwater may have had a substantial impact on Murray cod populations in the lower River Murray.

Fish productivity in the lower lakes and Coorong, Australia, during severe drought

Anthropogenic modification of catchments and river flow can significantly alter estuarine habitats, hydrology and nutrient delivery with implications for fisheries productivity. The Coorong estuary at the terminus of Australia’s River Murray supports an economically important fishery as well as being recognised internationally as a critical site for migratory birds. Salinity near the Murray Mouth varies between fresh and marine depending upon river flow, but the Coorong becomes increasingly saline along its 120 km length. Freshwater flow to the Coorong is naturally variable but has significantly reduced by extraction for irrigated agriculture and domestic use upstream. Extreme drought from 2000 to 2010 and over-allocation of water resources resulted in the cessation of freshwater flow to the Coorong, significantly increasing salinity. During this period the diversity and abundance of organisms in the Coorong declined which reduced food web complexity. During lower flows the system generally becomes less productive as evidenced by: lower nutrient concentrations and loads, lower chlorophyll and primary productivity, a decrease in the abundance of fish-prey items (zooplankton, macroinvertebrates and small fish), a decrease in fish abundance, although this is not well reflected in fishery catch data because of the concentration of fishing in available habitat. The maintenance of flow is the only management strategy that stimulates recruitment, delivers nutrient resources to the estuary and ensures maintenance of habitable area by maintaining appropriate salinity.

Advances in the study of River Murray ecology and the legacy of Keith Forbes Walker (1946−2016)

The rivers of the Murray-Darling Basin are central to the lives of many in south-eastern Australia, through provision of water for human consumption and agriculture, through ecosystem services and by providing enormous recreational and cultural capital. The development of the River Murray as a water resource has fundamentally altered the integrity of the river’s aquatic and floodplain ecosystems. Many researchers, conservationists, managers, and policymakers have devoted their careers to a better understanding of the ecology of this river system, with the ultimate aim of rehabilitating it. A central character in River Murray ecological research was Keith Forbes Walker. Keith was born in Aberdeen, Scotland, on 20 June 1946. In 1951, his family emigrated to Australia and lived in Melbourne. Keith completed a Bachelor of Science (Honours) at Monash University and then continued on to a PhD at Monash where his supervisor (and mentor) was WD (Bill) Williams, a pioneer in Australian limnology. After completing his PhD, he undertook Post-Doctoral studies at the University of Washington with the famous limnologist Tommy Edmundson who developed our understanding of lake eutrophication, and more importantly how to manage it. Keith returned to Australia to take up a postdoctoral position with Professor Ralph Slatyer at the Australian National University (ANU). While at ANU, Keith took part in the first ecological review of the River Murray at Albury-Wodonga with a small TRANSACTIONS OF THE ROYAL SOCIETY OF SOUTH AUSTRALIA, 2017 VOL. 141, NO. 2, 87–91 https://doi.org/10.1080/03721426.2017.1373456