Fine-scale vertical distribution of black bream Acanthopagrus butcheri larvae explains poor recruitment in the Blackwood River Estuary

The black bream Acanthopagrus butcheri is an important recreational fish species in the estuaries of temperate Australia and contributes to the commercial fishery in some of these systems. As this species completes its life cycle within its natal estuary, the stock of this species in any given system is particularly susceptible to overfishing and other anthropogenic effects. There is evidence that over the past three decades there has been a marked decline in black bream stocks in the Blackwood River and this is often attributed to fishing pressures. However, there has also been a significant decline in freshwater discharge over the same time period. This study investigates the link between the distribution of black bream eggs, larvae and various environmental factors that could explain the reduction in recruitment. We used a plankton net to sample the ichthyofauna at ten sites, during day and night, at 1m depth intervals. Black bream eggs were distributed throughout the mid-water column along the length of the salt-wedge. In contrast, the greatest concentrations of larvae occurred at water depths >3 m that were hypoxic. We explore the possible reasons, including water clarity, prey availability and feeding competition, as to why larvae are moving to the deeper, less favourable, hypoxic waters. This movement of larvae into deeper hypoxic waters as a result of declining freshwater discharge must presumably lead to mortality among the larval assemblage. It is proposed that the above movements contributed to the decline in the abundance of black bream in the Blackwood River Estuary