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- Understanding movement patterns and habitat utilisation is critical for the management of diadromous fishes. An acoustic telemetry array was used to monitor 33 estuary perch, Macquaria colonorum and 39 Australian bass, Macquaria novemaculeata in the freshwater and estuarine reaches on the Shoalhaven River, south-east Australia. On average, tagged M. novemaculeata were detected for a considerably shorter period than M. colonorum, and evidence suggested that fishing pressure may have impacted on their survival. Macquaria colonorum displayed significant shifts in seasonal and size-related habitat use, with fish predominantly residing in deep (>5 m) areas within the middle (mesohaline) reaches of the estuary during the austral spring to autumn months. In winter, M. colonorum individuals made frequent downstream migrations, often to localised areas, within the lower estuary (LE). In contrast, M. novemaculeata were distributed in shallow (<2 m) habitats throughout the year, within the upper (oligohaline) estuarine reaches of the river, as well as in fresh water. Like M. colonorum, M. novemaculeata made extensive downstream and upstream movements, often coincident with reproductive behaviour, water temperature and increased freshwater inflows. It is postulated that the high site fidelity and repetitive homing displayed by both species is influenced by ontogenetic behaviour and prey availability. Furthermore, the extent of instream distribution by both species, and the lack of observed annual spawning migrations by some M. novemaculeata individuals, indicates the once considered 'catadromous' life cycle of these fishes may not be obligatory. A management approach is recommended to ensure that both these species are not over-exploited within a portion of their instream range, thus maintaining their full reproductive potential. [ABSTRACT FROM AUTHOR]