Scotte D. Wedderburn

Shifts in small-bodied fish assemblages resulting from drought-induced water level recession in terminating lakes of the Murray-Darling Basin, Australia

Over-abstraction of water places unsustainable pressures on river ecosystems, with the impacts amplified under drought conditions. Freshwater fishes are particularly vulnerable due to associated changes in water quality, and habitat availability, condition and connectivity. Accordingly, fish assemblages are ideal indicators of the impacts of drought and over-abstraction. The Murray-Darling Basin (MDB), south-eastern Australia, terminates at the Ramsar listed Coorong and Lower Lakes, which comprise Lake Alexandrina and Lake Albert. Over-abstraction and extreme drought during the last decade has placed these lakes under severe environmental stress. The purpose of this study was to investigate shifts in fish assemblages caused by substantial water level recession and salinization in the Lower Lakes. Small-bodied fish assemblages were sampled at the beginning and several years into the drought. Off-lake habitats held diverse fish assemblages in 2003, but most sites were dry by 2009. Remaining habitats were disconnected, salinities increased substantially, and aquatic vegetation shifted from freshwater to salt-tolerant species. There was a substantial decline in the proportion of specialist species, especially diadromous and threatened species, and an emerging dominance of generalist freshwater and estuarine species. The findings warn of the inevitable ecological impact of over-allocating water for human use in drought-prone regions, and highlight the need for adequate environmental water allocations. This study also emphasises that understanding the ecological attributes of a fish species, and the subsequent assignment to a functional group, will help predict vulnerability to decline and extirpation.

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.

Conservation of an inauspicious endangered freshwater fish, Murray hardyhead (Craterocephalus fluviatilis), during drought and competing water demands in the Murray–Darling Basin, Australia

Approximately 40% of Australian freshwater fish species are of conservation concern, largely because of the impacts of river regulation, habitat fragmentation and alien fishes. Murray hardyhead is a threatened fish endemic to the southern Murray–Darling Basin in Australia, which has declined significantly in range and abundance since European settlement. Conservation of the species has relied largely on environmental watering of off-channel wetlands where isolated populations persist. This became problematic during recent drought (1997–2010) because of competing demands for limited water, and resentment towards environmental watering programs from communities that themselves were subject to reduced water entitlements. In response, emergency conservation measures prioritised the delivery of environmental water to minimise applied volumes. Captive maintenance programs were established for fish rescued from four genetically distinct conservation units, with varying levels of breeding success. Several translocations of wild and captive-bred fish to surrogate refuge sites were also conducted. Future recovery of the species should secure existing natural and stocked populations and translocate fish to additional appropriate sites to spread risk and reinstate natural pathways for dispersal. The approach to the conservation of Murray hardyhead during extreme environmental conditions provides insights to inform the management of fishes in other drought-prone regions of the world.

Shifts in fish assemblages indicate failed recovery of threatened species following prolonged drought in terminating lakes of the Murray–Darling Basin, Australia

Freshwater fishes are vulnerable to changes in water quality, physical habitat and connectivity resulting from drought, particularly in regulated rivers. When adequate river flows return, the recovery of populations might depend on the duration and consequences of drought. Rivers of the highly regulated Murray–Darling Basin in south-eastern Australia terminate at two large, shallow lakes that are separated from the estuary by tidal barrages. Over-abstraction of water and widespread prolonged drought (1997–2010) placed the lakes under severe environmental stress, culminating in critical water level recession from 2007 to 2010. Concurrently, most freshwater fish populations collapsed. We investigate shifts in fish assemblages resulting from habitat inundation in the lakes following the drought. The inundation and re-connection of the lakes and fringing habitats led to a substantial reduction of salinity throughout the region, and aquatic vegetation shifted from salt-tolerant to freshwater species. Fish assemblages became increasingly characterized by common freshwater taxa (ecological generalists), including high proportions of alien species. There were no indications of population recovery for three threatened species. The findings emphasize that short-lived fishes with specialized habitat requirements are vulnerable to severe population declines during prolonged drought in regulated rivers, which might restrict their recovery when adequate flows return.

Zooplankton response to flooding of a drought refuge and implications for the endangered fish species Craterocephalus fluviatilis cohabiting with alien Gambusia holbrooki

Disruption to a river’s natural flow regime changes its ecological character, which becomes unfavourable for previously adapted biota. The zooplankton particularly are affected, and survival of larval and juvenile fish is largely determined by their availability. Alien fishes can also impact on recruitment in native fishes, sometimes through competition. In this regard, the invasive eastern Gambusia Gambusia holbrooki is linked to the decline of several fish species. It can have a substantial influence in shaping plankton communities, which implies that it competes with native fish that rely on the microfauna. The effects of river regulation and over abstraction of water in the Murray–Darling Basin, south-eastern Australia, were exacerbated by drought from 1997 to 2010. Consequently, the endangered Murray hardyhead Craterocephalus fluviatilis underwent substantial population decline and extirpations. The purpose of this study is to determine if a link exists between zooplankton response to flooding of a drought refuge and the recruitment success of C. fluviatilis in the presence of G. holbrooki. Flooding triggered sharp and substantial increases in the zooplankton and their eggs, which was the sole food of C. fluviatilis. This apparently benefitted the recruitment of C. fluviatilis, and sometimes alleviated diet overlap with G. holbrooki. Conversely, the zooplankton in a nearby non-flooded refuge was low in abundance and diversity, and all fish species were extirpated. The findings indicate that the flooding of drought refugia with relatively small volumes of water can be timed with ecological cues that would otherwise be desynchronized in highly regulated rivers, particularly during drought.

Piscivory by alien redfin perch (Perca fluviatilis) begins earlier than anticipated in two contrasting habitats of Lake Alexandrina, South Australia

Abstract. Redfin perch (Perca fluviatilis) introduced to the Southern Hemisphere has contributed to the decline or localised extirpations of native fishes, principally due to predation. It has been widely recorded in the Murray–Darling Basin, south-eastern Australia, since the 1920s but the ecological consequences are largely undetermined. The purpose of this study was to examine the diet of juvenile redfin perch in Lake Alexandrina to assess its potential impacts on native biota in two distinct habitats (channel and lake). We proposed that the broad dietary composition of juvenile redfin perch matches that of its natural range (small decapods and insects). Most juvenile redfin perch with prey items in their guts, however, had consumed native fish. There was variability in the diet of redfin perch between the channel and lake where gudgeons and gobies, respectively, were targeted. Unexpectedly, otolith ageing revealed that the redfin perch were larger and shifted to piscivory at a much younger age compared with populations in its native range. Among other ecological issues, the findings are pertinent to threatened small-bodied fish populations in the Murray–Darling Basin. More broadly, they suggest that a generalist feeding behaviour can lead to the early onset of piscivory in alien fish populations.

Flow regulation simplifies a lowland fish assemblage in the Lower River Murray, South Australia

ABSTRACT Regulation has profoundly altered the ecological character of many rivers in the world, with pronounced effects in dryland systems subject to competing demand for water between the environment and society. Pervasive changes to flow regimes have cascading effects on ecological processes and biota attuned to natural variability. The Lower River Murray in south-eastern Australia occupies a curious landscape, being a large lowland river flowing through semi-arid environs fed by a vast inland basin from upstream temperate (River Murray, seasonally predictable) and subtropical-to-desert (Darling River, episodic and highly variable) regions. Extensive regulation via upland dams, water abstraction and serial lowland weirs has reduced mean annual discharge to the sea by two-thirds and transformed riverine hydraulics, leading to dramatic changes in the composition and abundance of biota, including the simplification of a formerly biologically and functionally diverse fish assemblage. Research since the 1980s, often led by the late Keith Forbes Walker, has sought to characterise ecological response to regulation on the Lower River Murray to understand pattern and process and aid in rehabilitation. This paper synthesises a significant body of work relating to freshwater fishes in the river to set a foundation for future monitoring, research and management.

Prey selection and diet overlap of native golden perch and alien redfin perch under contrasting hydrological conditions.

Abstract. Many freshwater fishes have been introduced outside their natural range. The consequences have included the decline or extinction of native fishes, principally due to competition and predation. Redfin perch (Perca fluviatilis) is a highly efficient predatory fish species that was introduced to Australia in the 1800s. It now has a broad distribution in the Murray–Darling Basin, but its impacts on native fishes are largely unstudied. It often cohabits with native golden perch (Macquaria ambigua ambigua), which is similar from a trophic ecomorphology perspective. We examine prey selection and diet overlap of adult redfin perch and golden perch under contrasting hydrological conditions in terminating lakes of the Murray–Darling Basin. Prey selection by both species varied substantially between drought and flood conditions. Diet overlap of redfin perch and golden perch was significant only during flood, and was apparently related to pelagic prey availability. There were dietary differences during drought that imply that resource partitioning occurred between the perches, possibly because competitive interactions were intensified. Conversely, the promotion of pelagic prey fishes during flooding apparently facilitated resource sharing. The findings suggest that redfin perch can directly compete with native piscivores for prey. The potential impacts on native piscivores and small-bodied fish populations warrant further experimental and field investigations.