Defining the stock structure of northern Australia's threadfin salmon species
James Cook University (JCU)
Threadfins form an important component of barramundi fisheries and are likely to play a significant ecological role in northern inshore habitats. Despite their importance and the expansion of coastal fisheries, the status of the threadfin resource in most parts is unknown whereas in WA they are considered fully or over-exploited (Pember et al, 2005). This uncertainty arises from the limited understanding of threadfin biology, stock structure and a lack of available data on resource exploitation (Welch et al. 2002). The inshore net fisheries across northern Australia are currently managed separately and under vastly different management regimes. However, without knowledge of threadfin stock structure the appropriate spatial scales of management is not known. In August 2003, the Northern Australian Fisheries Management Forum (NAFM) signalled its intention to move from single jurisdiction-based fishery assessment and management towards a more integrated approach that reflected the management needs of species across their northern Australian range. Elucidation of threadfin stock structure is vital for their management at an appropriate ecosystem scale. Attending to these critical issues for threadfins will also provide a framework for addressing management of other inshore species that are fished in adjacent State and Territory waters. Consequently, this project addresses research priorities outlined by QFIRAC, the NT FRAB, Fisheries WA and Sunfish. Industry has also expressed major concerns for the sustainability of the threadfin fisheries. These concerns are based on fishers’ personal experience’s, whereby large concentrations of king threadfins usually associated with inshore fishing grounds, especially in the south-east Gulf of Carpentaria, are now being encountered much less frequently, and their movement on and off the grounds is much more erratic than previous years (G. Ward, pers. comm.).
1. To determine the stock structure of king and blue threadfin salmon across northern Australia.
2. To use these findings to define the appropriate cross-jurisdictional management framework for sustainable use of king and blue threadfin resources in northern Australia.
Principal Investigator: D J Welch, A C Ballagh, S J Newman, R J G Lester, B R Moore, L van Herwerden, J B Horne, Q Allsop, T Saunders, J M Stapley and N A Gribble
Key Words: Blue threadfin, King threadfin, Eleutheronema tetradactylum, Polydactylus macrochir, stock structure, spatial dynamics, otolith isotope ratios, population genetics, parasites, fisheries, management
Summary:The project has determined that the appropriate spatial scale for management of blue and king threadfin fisheries is likely to be by state/territory and at local scales within these jurisdictions. The project identified that both king and blue threadfins show limited adult and larval movement between localised stocks separated by as little as several tens of kilometres. This means that the likelihood of straddling stocks of either species in northern Australia is unlikely; however there is evidence for the exchange of genetic material among adjacent stocks within the Gulf of Carpentaria. This information will greatly assist compliance with the Commonwealth Environment Protection and Biodiversity Conservation Act 1999 for net fisheries in northern Australia by providing the necessary basis for robust assessment of the status of threadfin stocks, thereby helping to deliver sustainable their harvest.
The project provides the spatial framework for monitoring and stock assessment of threadfins in northern Australia. Fine spatial scale stock separation will dictate the scale at which future monitoring and stock assessments for threadfins need to be carried out. Also, although not central to the project, the estimation of life history parameters for many of the stocks identified provide critical input parameters that will improve the certainty in future stock assessments. These parameters include growth and size at sex change. This has delivered considerable value to the project and further enhances management outcomes.
The project provided further evidence for the utility of using a holistic approach in stock structure studies. The approach taken in this study followed that of recent FRDC studies in using multiple techniques simultaneously to identify fish stocks, and confirmed the value in adopting such an approach in providing greater certainty in the scale of stock separation and in interpreting the possible mechanisms that influence this separation.
The project provided significant human capital development opportunities. The samples collected during the project provided material for two PhD student projects to be carried out on the life history characteristics of both blue threadfin (Aaron Ballagh, James Cook University) and king threadfin (Bradley Moore, James Cook University / University of Queensland). These significantly value-added to the stock structure study here (Chapters 5 and 9) and to the outcomes of the project. The analyses of parasites have also contributed to the PhD carried out by Brad Moore (Chapters 2 and 6).
The project further enhanced links between research, industry and management. Due to the interjurisdictional nature of this project fisheries managers from Western Australia, the Northern Territory and Queensland were formally consulted and included on the project team to facilitate the timely management uptake of research outcomes. This meant that contact with each of these managers was regular and they were kept reliably informed of project results. Sample collections were also largely reliant on the willing assistance of commercial fishers and required the establishment and maintenance of good relationships between the project team and industry.