Key words: Population model, Murray Cod, fisheries management, Native Fish Strategy
Threats and Impacts: Murray Cod (Macculochella peelii) is a key recreational fishing target species as well as being a nationally listed threatened species (Fig 1). Management action is required to rehabilitate populations of this species in the Murray-Darling Basin. Fish population models are a simple description of a fishes life cycle and try to incorporate any external factors that may affect the individual and population. The main use of these models is to hypothetically assess the impacts (negative or positive) of different management or environmental scenarios to provide managers with predictive power to better manage fish populations. Prior to this project no such model had been created for any species in the Murray-Darling Basin.
Broad aim and specific objectives: The objectives of this project were to:
- Develop a computer model (or models) to represent the population dynamics of Murray Cod under alternative management options.
- Develop various management scenarios in relation to size and bag limits and potential recovery times from overfishing, fish kills and other management or environmental scenarios which may affect Murray Cod populations.
- Document the findings of this work, and the implications for developing management options for Murray Cod and the research on Murray Cod biology and ecology required for improving the model (or models).
Methods: A review was undertaken initially to summarise relevant scientific, management, angler and aquaculture literature on:
- Murray Cod biology and ecology;
- Management options for Murray Cod and similar fish in the Murray-Darling Basin and elsewhere; and,
- Population and other (climate and GIS) models for fish or other fauna which will allow alternative management options to be tested;
Conceptual models of Murray Cod biology and ecology were then developed, and information gaps which needed to be addressed were identified. A workshop was also held to bring together a range of technical experts and jurisdictional representatives (SA, QLD, VIC, NSW, ACT and Commonwealth) to determine the key management actions to address the sustainable management of Murray Cod as well as the knowledge requirements necessary to develop the appropriate model(s) to assess the key management actions.
A population model for Murray Cod was developed as a key output of this project, which would enable different management actions/scenarios to be assessed and compared on the basis of their relative benefit and level of risk.
Findings: Modelled management scenarios for Murray Cod indicate that the risk to populations can be reduced substantially by appropriate changes to the size limits on angler take. The implementation of a slot size (minimum and maximum size limit) that protects both smaller and larger fish reduced population risk considerably. While habitat changes are difficult to quantify, it was illustrated that reductions in amount of habitat can place additional risk on populations, particularly when combined with angler take. Importantly, the collective impacts of less recognised threats such as thermal pollution, fish kills and mortalities to larvae over weirs and losses into irrigation off-takes can be explored and need to be recognised as having the potential to contribute significantly to mortalities at certain sites.
Lessons learned and future directions: The methods outlined in this study offer a formalised, rational, modelling approach that can form the basis for the assessment and prioritisation of management options for Murray Cod to minimise the risk to populations. Such modelling also highlights data gaps and monitoring requirements and can become an integral part of the conservation and fishery management process (Fig 2) and provides a tool for exploring the outcomes of management scenarios at both the regional and local scale. The modelling process has helped facilitate interagency Murray Cod management and emphasises the need for coordination between fishery managers and water/environmental protection/conservation agencies.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy.
Contact: Dr. Charles Todd, Arthur Rylah Institute, (02) 60519920, Charles.Todd@depi.vic.gov.au, 23 Brown St, Heidelberg, Victoria, Australia, +61 3 9450 8600.