The influence of tidal cycles and freshwater inflow on the distribution and movement of an estuarine resident fish Acanthopagrus butcheri

Movement patterns and habitat utilization by black bream Acanthopagrus butcheri (Sparidae), an estuarine resident species, were investigated using acoustic telemetry in a small estuary on the east coast of Tasmania, Australia. Thirty-four adult A. butcheri were tracked for periods of up to 187 days between August 2005 and January 2006. Although able to tolerate a wide range of salinities, the fish spent most of the time within the upper and middle regions of the estuary, where brackish conditions dominated. The species exhibited extensive movements linked to tidal cycles, with small-scale upstream movements during incoming tides and downstream movements during out going tides. The extent of these movements was positively correlated with the tidal height difference between consecutive tidal peaks and troughs. Freshwater inflows and resultant changes in salinity also significantly influenced distribution and movement patterns. Fish moved downstream during the periods of heavy inflows, returning upstream as salinities increased to c. >10. During the peak of spawning period (November to December) fish moved into the upper region of the estuary, where they aggregated to spawn. Periodic increases in freshwater discharge, however, resulted in fish leaving the spawning grounds and moving downstream. Towards the end of the spawning season (January), the fish became more dispersed throughout the entire estuarine system.