Ian Potter

This study has explored whether restocked fish of a species perform as well as its wild stock, and has estimated their contribution to the fishery and egg production. Approximately 220,000 juvenile Acanthopagrus butcheri, whose otoliths were stained purple with alizarin complexone, were released into the Blackwood River Estuary in 2002/03. The purple stain in their otoliths was still clearly visible in 2014. Data on the biological characteristics and annual contributions to the small commercial fishery of restocked and wild stock A. butcheri were analysed. The growth of restocked fish was only slightly less than that of the wild stock and the differences between their maturity schedules were relatively small. As increasing numbers of restocked A. butcheri attained the MLL of 250 mm for retention by the commercial fisher, their contribution to the fishery increased progressively from 6% in 2005 to 74% in 2010. That contribution subsequently declined to 39% in 2012 and to 10% in 2014, due predominantly to the introduction of the very strong 2008 year class in the commercial catches, the first substantial recruitment of wild stock fish into the population since 1999. Restocked fish were estimated as contributing ~55% to the eggs produced in 2008, suggesting that substantial numbers of the 2008 year class were derived from spawning by restocked fish. The results of this and a previous genetic study imply that restocking is an effective and appropriate way for replenishing stocks of an estuarine species such as A. butcheri.

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.

This presentation will focus on a quantitative scheme for classifying nearshore habitat types and predicting their fish faunas, which has been developed for a range of estuaries in south-western Australia that differ widely in their geographical and geomorphological characteristics. This scheme provides a reliable framework for managers and ecologists to (i) investigate relationships between habitat types and faunal composition at local to regional scales and under different geographic and estuarytype scenarios, (ii) establish benchmarks against which the impact of future environmental change can be assessed and (iii) predict the habitat type and characteristic fauna of any nearshore site of interest within those systems. The habitat classification scheme has been based on the use of enduring environmental criteria that can be easily measured in GIS from mapped data sources such as satellite imagery and bathymetric charts, and has employed novel applications of relatively new multivariate routines. The results of this classification scheme, and the relationships between the derived habitat types and their fish assemblages, will be presented for five estuaries across south-western Australia, namely the permanently open Swan-Canning and Peel-Harvey estuaries, the seasonally-open Broke and Wilson inlets and the normally-closed Wellstead Estuary. Questions regarding the importance of geographical location and estuary type in structuring habitat types and their fish faunas will also be addressed.