Common carp (Cyprinus carpio) are implicated globally in the degradation of freshwater habitats. In Australia, carp movement studies are often focused on lateral spawning movements from the river to wetlands. These studies are typically in temperate systems where riverine and wetland habitats are permanently connected, and where high flow events are common. Carp in Australia however are mostly distributed in intermittent dryland systems where longitudinal movement is the norm and opportunity for lateral connection though flooding is rare and sporadic. In these dryland systems, fish are restricted to in-channel habitats and experience extended periods of no-flow and confinement to refuge waterholes, interrupted by small and short-lived in-channel flows. Movement behaviour of carp in these systems is largely unknown.
We used passive acoustic telemetry to track 91 carp over 3 years of drought across 340km of intermittent dryland river in the semi-arid lower Balonne system. All flow events, though small, initiated some carp movement, but sedentary behaviour was very common with up to 70% of carp not moving on any flow at one site. Movement was not predictable by season or age class. Distance moved and fish size were not correlated, and probability of movement did not significantly increase with increasing flow. On proportionally high flow magnitudes, carp moved equally upstream and downstream; but upstream movement dominated with lower magnitudes.
We found that during drought carp can be both mobile and sedentary at both the population and individual level. Large variations in movement extent on even the largest of flows suggests there are individual behavioural responses to flow that we are only beginning to understand. Flexible and complex movement behaviours make the management of this pest species in intermittent rivers incredibly challenging as carp populations in these regions are likely to respond to management intervention in unpredictable and non-linear ways.