Magnitude, frequency and duration of instream flows to stimulate and facilitate catadromous fish migrations: Australian bass (macquaria novemaculeata perciformes, percichthyidae)
The migratory response and behaviour of catadromous Australian bass with regard to hourly mean river flows and water temperatures was assessed over 15 months. Fish movement was assessed using a 75-km passive acoustic telemetry array in the regulated Shoalhaven River below Tallowa Dam, NSW, Australia. The majority (62%) of downstream pre-spawning migrations from freshwater to estuarine habitats were stimulated by a series of flow pulses from April to September, but a proportion of fish (38%) commenced downstream migrations under regulated baseflow conditions after a sustained decrease in water temperature to below 15°C in late autumn. Equal numbers of fish undertook post-spawning upstream return migrations during flow pulses and during regulated baseflow conditions, with regulated baseflow migrants exhibiting a preference for dusk–dawn passage through freshwater pool–riffle sequences. The median magnitude of flow pulses at the time of commencement of downstream and upstream freshwater migrations by Australian bass was not large, equivalent to natural (in the absence of river regulation) flows equalled or exceeded for 56% and 48% of time, respectively. There was no evidence for increased numbers of migrants with increasing flow pulse magnitude, with individual fish ignoring some flow pulses but responding to subsequent events. In regulated rivers, the release of more frequent flow pulses with peak magnitudes approximating the natural 50th flow duration percentile may be more effective in stimulating greater numbers of Australian bass to undertake pre-spawning and post-spawning migrations between freshwater and estuarine habitats than the release of a single, larger event. The propensity of Australian bass to also undertake spawning migrations under regulated baseflow conditions emphasizes the need for provision of baseflow regimes in regulated rivers that can facilitate migrations by large bodied fishes.
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