Performance and contribution to commercial catches and egg production by restocked Acanthopagrus butcheri in the Blackwood River Estuary

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