4 Natural flows drive the boom and bust’ ecology of fish in Cooper Creek 45 of abundance: silver tandan (∼54% of total catch) and Hyrtl’s tandan (~14%), bony bream (~16%), two grunters (Welch’s grunter (∼8%), Barcoo grunter (∼6%)) and yellowbelly (~2%). A typical fish sample may include several of these species, as well as turtles (Fig. 4.3). Fish ecology during the bust The fish fauna spans a range of body sizes, physiological tolerances, habitat and dietary preferences, and breeding strategies (Table 4.1). These adaptations equip all species to tolerate the ‘bust’ conditions in drying waterholes, and to reproduce opportunistically, seasonally or on rising flows and floods, tracking changing habitat conditions and food resources (Fig. 4.1). Although all 12 native species are adapted to survive bust conditions, fish abundances and assemblage composition in waterholes change markedly during dry periods. Individual species suffered 50–100% reduction in numbers per waterhole, and across the entire fish fauna 93% of individuals were lost from 15 drying waterholes between April and September 2001 (Arthington et al. 2005). Each species responded to different environmental pressures during this dry period (Arthington et al. 2010). For the desert rainbowfish, low physical habitat diversity and susceptibility to decreasing water temperatures drove losses from 11 of 15 waterholes. The yellowbelly was lost from six of 15 waterholes, particularly those on the fringes of the floodplain that are rarely rewetted and reconnected. The Barcoo grunter did not persist in 11 waterholes with low levels of bed, bank and habitat complexity and less cover to protect them from predators. This grunter was also less likely to persist in waterholes on narrow floodplains, possibly because fish coming off narrow floodplains may not have accumulated the body lipids that govern condition, and probably survival, in isolated, drying waterholes (Puckridge et al. 2000). These natural pressures drive adaptation to flow variability over successive generations. The fate of the uncommon Cooper Creek catfish in drying waterholes is particularly interesting. Waterholes that experienced increases in primary production over the dry season were able to sustain this catfish, which is the only species not found on the floodplains of Cooper Creek when they are inundated (Balcombe et al. 2007). During dry periods, a conspicuous, shallow ‘bathtub ring’ of benthic algae forms around the margins of isolated waterholes and this productive food resource sustains the entire fish community (Bunn et al. 2003). This is especially important for large species like the Cooper Creek catfish that feed on large-bodied invertebrates (crustaceans and snails), themselves sustained by high levels of algal production (Balcombe et al. 2005). However, without an energy subsidy from the food- rich floodplain, the Cooper Creek catfish would be totally reliant on the waterhole food web, and this is not very productive in steep-sided waterholes where the bath-tub ring of algae is usually less well developed (Arthington et al. 2010). Fish ecology during the boom High fish mortalities during the dry times in Cooper Creek are balanced by episodes of spawning and recruitment. Fish exhibit three main recruitment strategies in Cooper Creek (and other rivers of the Lake Eyre Basin), termed no-flow recruitment, seasonal recruitment, and flow or flood-dependent recruitment (Kerezsy et al. 2011). At least six of the 12 native
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