Nicole McCasker

Adaptive Management of Environmental Flows: Using Irrigation Infrastructure to Deliver Environmental Benefits During a Large Hypoxic Blackwater Event in the Southern Murray–Darling Basin, Australia

Widespread flooding in south-eastern Australia in 2010 resulted in a hypoxic (low dissolved oxygen, DO) blackwater (high dissolved carbon) event affecting 1800 kilometres of the Murray–Darling Basin. There was concern that prolonged low DO would result in death of aquatic biota. Australian federal and state governments and local stakeholders collaborated to create refuge areas by releasing water with higher DO from irrigation canals via regulating structures (known as ‘irrigation canal escapes’) into rivers in the Edward–Wakool system. To determine if these environmental flows resulted in good environmental outcomes in rivers affected by hypoxic blackwater, we evaluated (1) water chemistry data collected before, during and after the intervention, from river reaches upstream and downstream of the three irrigation canal escapes used to deliver the environmental flows, (2) fish assemblage surveys undertaken before and after the blackwater event, and (3) reports of fish kills from fisheries officers and local citizens. The environmental flows had positive outcomes; mean DO increased by 1–2 mg L−1 for at least 40 km downstream of two escapes, and there were fewer days when DO was below the sub-lethal threshold of 4 mg L−1 and the lethal threshold of 2 mg L−1 at which fish are known to become stressed or die, respectively. There were no fish deaths in reaches receiving environmental flows, whereas fish deaths were reported elsewhere throughout the system. This study demonstrates that adaptive management of environmental flows can occur through collaboration and the timely provision of monitoring results and local knowledge.

Contrasting patterns of larval mortality in two sympatric riverine fish species: A test of the critical period hypothesis

Understanding the causal mechanisms that determine recruitment success is critical to the effective conservation of wild fish populations. Although recruitment strength is likely determined during early life when mortality is greatest, few studies have documented age-specific mortality rates for fish during this period. We investigated age-specific mortality of individual cohorts of two species of riverine fish from yolksac larvae to juveniles, assaying for the presence of a “critical period”: A time when mortality is unusually high. Early life stages of carp gudgeons (Hypseleotris spp.) and unspecked hardyhead (Craterocephalus stercusmuscarum fulvus)—two fishes that differ in fecundity, egg size and overlap between endogenous and exogenous feeding—were collected every second day for four months. We fitted survivorship curves to 22 carp gudgeon and 15 unspecked hardyhead four-day cohorts and tested several mortality functions. Mortality rates declined with age for carp gudgeon, with mean instantaneous mortality rates (-Z) ranging from 1.40–0.03. In contrast, mortality rates for unspecked hardyhead were constant across the larval period, with a mean -Z of 0.15. There was strong evidence of a critical period for carp gudgeon larvae from hatch until 6 days old, and no evidence of a critical period for unspecked hardyhead. Total larval mortality for carp gudgeon and unspecked hardyhead up to 24 days of age was estimated to be 97.8 and 94.3%, respectively. We hypothesise that life history strategy may play an important role in shaping overall mortality and the pattern of mortality during early life in these two fishes.

Subfossil chironomid head capsules reveal assemblage differences in permanent and temporary wetlands of south-eastern Australia

Subfossil chironomid head capsules have been used extensively as proxies to characterise past environmental conditions of waterbodies. To date, their potential to distinguish between temporary and permanent waterbodies has not been determined. This study set out to assess if subfossil chironomid head capsules could be used to distinguish between temporary and permanent floodplain wetlands from the Ovens River, south-eastern Australia. Twenty-six taxa were collected in both wetland types: one taxon (Paracladopelma spp.) was found exclusively in permanent wetlands; and five taxa (Cladopelma spp., Cryptochironomus spp., Microchironomus spp., Microtendipes spp. and Cricotopus spp.) were found exclusively in temporary wetlands. The overall concentrations of chironomid head capsules were greater in permanent than temporary wetlands. Furthermore, eight taxa were found in significantly higher concentrations in permanent than temporary wetlands but, apart from the unique taxa, the concentrations of no other taxa were significantly greater in the temporary than permanent wetlands. The temporary and permanent wetlands had distinct chironomid assemblages, as evidenced by the abundance and presence/absence data. This study highlights the importance of heterogeneity in the environment to maintain chironomid diversity and suggests that chironomid subfossils have the potential to be useful in palaeoecological studies aiming to reconstruct past changes in hydrology.