Movement and mortality of Murray cod, Maccullochella peelii, during overbank flows in the lower River Murray, Australia

Abstract

Conservation of Murray cod (Maccullochella peelii), a large endangered fish species of Australia’s Murray–Darling Basin, relies on a detailed understanding of life history, including movement patterns and habitat use. We used radio-tracking to investigate the movement of 36 Murray cod in main channel and anabranch habitats of the lower River Murray during a flood and associated hypoxic blackwater event. During a flood peak of ~93 000 ML day–1, dissolved oxygen decreased to 1.2 mg L–1. Four movement types were observed: (1) localised small-scale movement, (2) broad-scale movement within anabranch habitats, (3) movement between anabranch and main channel habitats, and (4) large-scale riverine movement. Murray cod exhibited high fidelity to anabranch habitats but also moved extensively between anabranches and the main channel. Fish were consistently located in the main channel or permanent anabranches, suggesting that use of ephemeral floodplain habitats is limited, and highlighting the importance of connectivity between off-channel and main channel habitats. Mortality of radio-tagged fish was considerable (25%) in association with low dissolved oxygen concentrations, indicating that hypoxic blackwater may have had a substantial impact on Murray cod populations in the lower River Murray.