Synchronous Spawning in a Temperate Diadromous Fish: Implications for Freshwater Inflows

Diadromous fishes are highly susceptible to the effects of environmental and anthropogenic impacts on their assemblages and habitats. A thorough understanding of their migratory requirements is critical for the conservation of these species. The distribution, movement and spawning behaviour of freshwater mullet (Trachystoma petardi) was investigated in response to environmental variables using an extensive acoustic telemetry array in a large tidal river in south-eastern (SE) Australia. Thirty adult T. petardi were monitored for two years covering two consecutive spawning periods between 2009 and 2011. Coincident with a peak in water temperatures (austral summer to early autumn months), both sexes made multiple large-scale movements from their resident freshwater home ranges to localised spawning areas within the lower estuary. The frequency, extent and synchronous nature of these spawning migrations were positively correlated with the magnitude of freshwater inflow events. In particular, all tagged T. petardi migrated offshore on the peak of a significant flood, with 36% of fish never returning to the estuary. This high correlation between river discharge and spawning provides scientific evidence for defensible decisions relating to the provision of environmental flows, whilst identification of key residency and spawning grounds allows for management actions to protect these areas.