Norman Hall

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.

Data on the length and age compositions and lengths at ages of King George Whiting (Sillaginodes punctata) collected throughout south-western Australia during this and several previous studies were collated and then analysed. The resultant information was used to 1) compare the size and age ranges of fish caught by commercial and recreational fishers in different regions of south-western Australia, 2) explore whether there is evidence that the offshore movements known to be undertaken by this species are related more to length than to age, 3) ascertain whether an adjusted form of the von Bertalanffy growth model, which can account for unidirectional length-related movements of a species between habitats, provides statistically better descriptions of growth of this species than the traditional von Bertalanffy model, and 4) from growth curves reported for S. punctata in this and other studies, elucidate whether the lengths of this species at corresponding ages vary among different regions of Western Australia and in South Australia. Evidence as to whether there is a consistent pattern of differences in growth of S. punctata with latitude (and water temperature) are explored. The growth characteristics of S. punctata in different regions are considered in the context of findings reported elsewhere for other fish species and predictions of the Metabolic Theory of Ecology.