The migrations of diadromous fishes are highly susceptible to anthropogenic alteration of connectivity and freshwater flow. In regulated systems, an understanding of migration in association with environmental conditions (e.g. river flow), and in the context of anthropogenic modification (e.g. flow-regulating structures), is necessary to support species management. We investigated the downstream catadromous spawning migrations of adult female congolli (Pseudaphritis urvillii) in a novel ecosystem in southern Australia, the Southeast Drainage network and Coorong. In this system, migration of congolli to the ocean requires passage past multiple flow-regulating structures and an atypical salinity gradient that progresses from brackish in the drainage system, to hypersaline in the Coorong Southern Lagoon, to marine near the estuary’s mouth.
Two separate groups of fish (n = 32 and 35) were implanted with acoustic tags in years associated with: 1) high freshwater inflow and less pronounced salinity gradient (2023), and 2) low freshwater inflow and highly pronounced salinity gradient (2024). Most detected individuals initiated rapid downstream migrations in winter. In 2023, 80% of detected individuals passed flow-regulating structures and half of these successfully traversed ~100 km of estuarine lagoon and salinities of up to ~60 g.L-1 before reaching the river’s mouth. In 2024, most individuals (60%) that initiated migrations were obstructed by flow-regulating structures and remained within the drainage network. Those individuals that passed the regulators were faced with a fragmented lagoon system (low water level) with salinities approaching 100 g.L-1. Just one individual successfully reached the river mouth.
This study provides important data to inform local management of diadromous fishes in the Southeast Drainage system and Coorong, and more broadly, highlights the incredible physiological and navigational capacity of diadromous fishes, and the risk-and-reward trade-off of migration.