Understanding fish movement is critical for managing population resistance and resilience in the variable rivers of the northern MDB. We used otolith 87Sr/86Sr profiles and growth annuli to reconstruct movement history in golden perch, bony bream, and carp captured during 2020–2023, following the severe 2018–2020 drought. Freshwater mussel shells from capture sites provided isotopic baselines for interpreting fish provenance and movement.
All three species exhibited partial migration, highlighting the importance of local recruitment to nMDB population persistence. Movers comprised 21% of golden perch, 16% of bony bream, and 28% of carp. Movement participation varied spatially and was influenced by local hydrology. Higher movement rates were associated with larger waterhole area at the drought peak, and more frequent barrier drownout events. Most movements were localised, but rare long-distance movements of up to 600 km linked Qld tributaries, including the Moonie and lower Balonne, with NSW tributaries including the Barwon, upper Darling, and lower Macquarie.
Movement in native species was concentrated in the first two years of life, whereas carp moved across a wider age range, up to 17 years, although movement was also concentrated in younger years. Random forest models identified barrier drownout as the strongest predictor of movement participation for natives, whereas carp movement responded primarily to flow-event duration.
Despite movement, post-drought recovery was strongly dominated by local recruitment. Resident fish in waterhole refuges maintained population continuity through drought, while the limited numbers of mobile individuals supported dispersal, recolonisation, and demographic mixing when flow returned and barriers were drowned-out. Managing resilience in the nMDB therefore requires protection of resident populations while maintaining pathways for the mobile minority through temporary flows that drown out instream barriers.