Jennifer Chaplin

Snapper is an iconic recreational and commercial fish species across its range in southern Australia and northern New Zealand. In many of these locations, stocks have been reduced to low levels by fishing. The ability to economically restock or enhance the species, while ensuring genetic diversity, would be improved if fertilised eggs could be collected from the wild during their spawning aggregations. This project developed effective techniques to collect snapper eggs from the extensively studied spawning aggregations in Cockburn Sound in Western Australia and to assess the genetic diversity of the offspring produced. Optimum weather conditions, times of day, methods and locations for egg collection were identified during the project. The selection of snapper eggs based on their diameter and oil globule size proved to be a reliable method of separating potential snapper from other eggs within the collections. Samples of these eggs were confirmed to be snapper using real-time PCR, comprising a snapper specific probe and a general fish probe. A microsatellite-based comparison of a sample of juveniles raised in the hatchery from wild spawned eggs with a sample of wild-caught individuals from Cockburn Sound tested the genetic integrity of this collection and culture method. A number of snapper cultured from this project were marked with alizarin complexone and retained in the hatchery to test for mark integrity over time.

Historically western school prawns (Metapenaeus dalli) were an important and iconic catch for commercial and recreational fishers in the Swan-Canning Estuary in Western Australia. However, the abundance of this penaeid declined dramatically since the 1960s, resulting in the closure of the commercial fishery and essentially the cessation of the recreational prawning. Furthermore, despite the large reduction in fishing effort, stocks of this species did not recover. In 2012, a restocking, research and community engagement program was undertaken to 1) investigate the feasibility of increasing the abundance of this key species through restocking; 2) determine the factors that may have contributed to its decline; and 3) assess the effectiveness of the restocking. This presentation outlines the components of the project, including the aquaculture procedures used to cultivate M. dalli, an assessment of the genetic consequences of the restocking, the biology and ecology of this species in the Swan-Canning Estuary and the methods used to engage recreational fishers and the broader community in the project.

A number of elasmobranchs across different families exhibit sex-biased dispersal. However, the selective pressures that favor the evolution of sex-biased dispersal in elasmobranchs are not well understood and sex-biased dispersal has been assessed in too few species for a clear pattern(s) to emerge. This study provides the first evidence of sex-biased dispersal in sawfishes and demonstrates how such dispersal may vary with habitat usage in Indo-West Pacific species. The Freshwater Sawfish, Pristis microdon, which utilizes freshwater rivers as juveniles and marine/estuarine waters as adults, was found to have male-biased dispersal in Australian waters. In contrast, P. clavata and P. zijsron, which spend their entire lives in marine and/or estuarine waters, are genetically structured in northern Australian waters. The use of freshwater rivers as juveniles by P. microdon suggests that the evolutionary history of this species in Australian waters was potentially very different to those for P. clavata and P. zijsron and may have influenced the evolution of sex-biased dispersal in the former, but not the latter, species.

Pairs of closely related species on either side of a barrier provide an opportunity to test evolutionary hypotheses. In southern Australian seas, many species pairs occur on either side of the Bass Strait, which temporarily becomes an isthmus during glacial stages. We used mitochondrial and nuclear approaches to examine recent demographic changes in one such species pair, Arripis trutta (east of Bass Strait) and A. truttaceus (west of Bass Strait). Data from the two remaining members of the family Arripidae, A. xylabion and A. georgianus, were included to provide context for interpreting patterns in the focal species pair. All Arripis species were characterised by low genetic diversity and there was evidence for a more recent and/or severe population contraction in A. truttaceus compared to A. trutta. These different demographic signals support the a priori hypothesis that environmental conditions to the west of the Bass Strait were more severe than to the east during glacial stages and, in particular, the last glacial maximum. The results are considered in the context of the biology of the species and the historical marine biogeography of temperate Australia.

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.

This study describes the results of a long‐term investigation into the biological performance and genetics of restocked and wild black bream (Acanthopagrus butcheri) in the Blackwood River Estuary in south‐western Australia. The restocking was done in 2002‐03 and involved the release of juveniles of black bream, cultured using broodstock from the Blackwood River Estuary, in an attempt to replenish a heavily depleted population of this species in this system. The results of an investigation into the biological performance of the restocked fish for 3.5 years after their release into the estuary have already been published. This study builds upon this early work by providing information about the growth rates of restocked and equivalent wild individuals for eight years post‐release, the proportion of restocked individuals reaching maturity, and the contribution of the restocked individuals to the gill‐net fishery for this species in the estuary. It also includes the first information on the genetic consequences and implications of this restocking. The results demonstrate that the restocking of the black bream in the Blackwood River Estuary has been very successful in most respects and highlight the value of long‐term monitoring in fish restocking programs.

Studies in the Fitzroy River commenced in 2001 and have been ongoing. Strong collaboration between indigenous organisations and communities has resulted in a number of important projects being completed. Projects have resulted in fish posters of the river’s fishes in five Aboriginal languages and an increased awareness of the importance of the river to endangered species, such as Freshwater Sawfish, Dwarf Sawfish, Northern River Sharks, Barnett River Gudgeon and Greenway’s Grunter. Species distributions and habitat associations have been completed and the biology of a number of species, including Barramundi, Freshwater Sawfish, Dwarf Sawfish, Bull Sharks and Lesser Salmon Catfish, determined. The diets and trophic relationships for many of the river’s fishes have been examined. Genetic work has compared the Fitzroy River populations of Barramundi, Freshwater Sawfish and Dwarf Sawfish to those elsewhere. Tagging (conventional, acoustic and satellite) studies for Freshwater Sawfish are ongoing.