Movement behavior of a threatened native fish informs flow management in a modified floodplain river system

Movement is a key driver of the distribution of animals and the structure of populations, communities, and ecosystems. Habitat loss and fragmentation can compromise movement and contribute to population declines. However, there is often insufficient knowledge about when, why, and where animals move, particularly in highly modified environments. We present results from an 8-year study on the movement behavior of Murray cod Maccullochella peelii, an Australian freshwater fish species that has undergone major declines due in part to river flow regulations. We studied movement within and between different habitat types in a highly modified floodplain ecosystem in the lower Murray River to (1) identify the key environmental conditions associated with movement, (2) examine how a new regulating structure can be managed to influence movement behavior, and (3) explore movement mediated recovery following a hypoxic event. Movement within and between an anabranch and main river channel habitats increased during the core spawning period and during elevated discharge. The likelihood of Murray cod moving to an anabranch system from the Murray River declined substantially following construction of a new flow regulating structure (a weir and vertical slot fishway). Managed flows delivered through the anabranch after regulator construction in accordance with targeted recommendations (time-of-year and magnitude of discharge) increased the movement of adult fish within and between habitats. Finally, a hypoxic event caused not only high mortality but also resulted in a high proportion of fish migrating outside of the study reach, before returning to the system over several years. These results demonstrate how flow management can help a keystone species access habitats required to complete critical life history requirements including recovery from disturbance events. Importantly, the work provides an example of how timely and robust applied research has informed a major intervention program aimed at enhancing ecological outcomes.