Population models to inform aquatic species management

Two-spined Blackfish

ARI assessed the potential effects of reservoir expansion on population persistence on the threatened Two-spined Blackfish and identified management actions to reduce extinction risk. The work demonstrated how changes to habitat availability and connectivity can influence long-term population viability and helped inform reservoir planning and mitigation measures.

Publications

• Todd et al. (2017). Assessing the impacts of reservoir expansion using a population model for a threatened riverine fish. Ecological Indicators.

Australian Lungfish

More recently, metapopulation models were developed for the threatened Australian Lungfish in Queensland to ass ess the long-term impacts of dams, altered flow regimes and habitat loss. Lungfish models have been developed for the Burnett, Mary, North Pine and Brisbane rivers and, more recently, for a Pumped Hydro Energy Storage system to assess the potential effects of operations on Australian Lungfish.

The model showed that water infrastructure can reduce population viability by inundating key spawning habitats, disrupting movement and altering recruitment processes. It also demonstrated how population modelling can be used to test the benefits of management actions such as environmental flows, habitat restoration, improved fish passage and changes to dam operation. This work highlights the value of population models for assessing the cumulative effects of reservoir management on long-lived species where population responses may take decades to become apparent through monitoring alone.

Publications

• Todd et al. (2025). A metapopulation model to assess water management impacts on the threatened Australian Lungfish, Neoceratodus forsteri. Fishes.

Thomson Reservoir;  A Two-spined Blackfish

Understanding how fish populations respond to flows is central to planning how best to deliver water for the environment. ARI has developed fish population models that include modelling a range of flow conditions to allow quick exploration of likely outcomes to these complex management scenarios.

This enables comparisons of different management options to maximise benefits to fish. Models are based on the latest knowledge and understanding of fish species' lifecycles and their ecological needs. The models incorporate options to assess other impacts such as catch by fishers, barriers to movement, habitat reduction or cold-water releases from dams.

ARI has developed population models for multiple native species: Australian Bass, Estuary Perch, Golden Perch, Macquarie Perch, Murray Cod, Murray Hardyhead, Olive Perchlet, Silver Perch, Southern Pygmy Perch and Trout Cod.

Murray Cod and Golden Perch

ARI developed population models for Golden Perch and Murray Cod to assess how native fish populations may respond to river restoration and environmental flow scenarios under the Reconnecting River Country Program. The models predicted that increased floodplain connectivity and environmental flows would provide substantial benefits for Golden Perch populations, increasing opportunities for recruitment and movement.

In contrast, Murray Cod populations were predicted to show limited responses to increased flows, suggesting other factors may have a greater influence on long-term population outcomes. The findings have helped inform assessments of environmental benefits associated with river restoration and environmental water management in the southern Murray-Darling Basin.

Publication

• ARI Technical Report 341 (2022). Population modelling of native fish outcomes for the Reconnecting River Country Program: Golden Perch and Murray Cod (PDF, 13 MB), (DOCX, 0.6 MB).

ARI has also developed a population model for the introduced pest species Carp, to quantify how this species may respond to a variety of flow conditions.

ARI’s environmental water research and Lower Snowy River fish monitoring provides further information on population modelling work undertaken under particular research and monitoring programs.

Publications

• Stoessel et al. (2023) Assessing the use of environmental flows and stocking for the persistence of a flow-dependent spawner in a drying climate. Marine and Freshwater Research.
• Stoessel et al. (2022) Assessing outcomes of environmental flows for estuary-dependent fish species using a novel stochastic population model approach. Estuaries and Coasts.

Releasing a Murray Cod; Releasing a Golden Perch

Many population models have been developed to assist conservation efforts for a range of threatened fish.

Trout Cod

The successful reintroduction of Trout Cod into the Ovens River was guided by a population model that identified optimal age classes for release. Fingerlings released into the river from 1997 to 2006 have contributed to what is now a thriving population.

Macquarie Perch

ARI’s population model  for Macquarie Perch explored a variety of scenarios for translocation and provided useful insights into the age class, quantity of fish and timeframes likely required for successful establishment of new populations.

It provides a distinct example of the benefits of conservation models in threatened species recovery. It also provides a basis for future modification of the model to investigate other aspects of Macquarie Perch.

Publications

• Yen et al. (2022). Establishing new populations in water-secure locations may benefit species persistence more than interventions in water-stress locations. Biological Conservation.
• Todd and Lintermans (2015). Who do you move? A stochastic population model to guide translocation strategies for an endangered freshwater fish in south-eastern Australia. Ecological Modelling.
• Todd  et al. (2004). Density-dependence uncertainty in population models for the conservation management of trout cod, Maccullochella macquariensis. Ecological Modelling.

A Trout Cod; Releasing a Macquarie Perch

When developing models for recreationally angled fish it is important that relevant stakeholders are involved in the process, and that models consider the impacts of fishing.

Murray Cod

ARI developed a population model for the threatened Murray Cod with input from a range of managers, agencies and other stakeholders, such as recreational fishers, across the Murray-Darling Basin.  Model development focussed on balancing conservation management, reducing population risks through fishery management and enhancing recreational harvest by fishers.

The model identified that establishing a harvest slot length limit (HSLL) of 550-750mm, rather than a minimum legal limit, would provide a lower risk of species’ decline as well as greater catch rates for fishers. This work was part of a review of Murray Cod catch limits and led to the implementation of new fishing regulations for this species.

• Koehn and Todd (2012). Balancing conservation and recreational fishery objectives for a threatened fish species, the Murray cod, Maccullochella peelii. Fisheries Management and Ecology.

For more information see the Victorian Fisheries Authority website on Murray Cod catch limits for Victoria.

Catching a Murray Cod; Releasing a large Murray Cod

Silver Perch

ARI developed a metapopulation model for Silver Perch to assess the species' viability across the Murray-Darling Basin and support conservation planning. The model integrated species life-history information, river flows, stocking history and recreational fishing to predict population trajectories under a range of environmental and management scenarios.

The modelling identified Silver Perch as being in long-term decline across the Murray-Darling Basin, with northern basin populations predicted to be highly imperilled and approaching functional extinction. The results highlighted the importance of flow-dependent processes, connectivity and recruitment for population recovery, and indicated that even low levels of recreational harvest can negatively affect populations. The model provides a tool for prioritising recovery actions, including environmental flows, fish passage restoration and other interventions to improve population viability.

Southern Pygmy Perch

ARI used a Southern Pygmy Perch population model to assess the viability of a population in NSW, where very few populations still occur. This small species generally occurs in small, often isolated populations in a variety of habitats. It is vulnerable to drought, habitat loss and fragmentation, and predation.

By using the model, ARI discovered that the oldest age classes had the highest reproductive value: identifying that large old fertile fish are particularly valuable in sustaining natural populations. When exploring species persistence scenarios, the model found that if in a stable habitat, a population of about 2000 female adults would likely be viable and able to withstand some disturbance.

Such a population could possibly be used as a source for reintroductions elsewhere, an option often considered for fish that have significantly declined.

This information can be applied when considering how to achieve conservation aims for this and other similar species, such as increasing numbers and the viability of populations.

Publications

• ARI Technical Report 374 (2023). Reintroduction strategies for Southern Pygmy Perch to Gunbower Island wetlands using a metapopulation model.
• Todd et al. (2017). Forgotten fishes: what is the future for small threatened freshwater fish? Population risk assessment for southern pygmy perch, Nannoperca australis. Aquatic Conservation.

Murray Crayfish

ARI has investigated key characteristics of Murray Crayfish biology and threats, to better inform management and species recovery.  This work has included the use of population models to examine the risk of harvest pressure and blackwater events.

Releasing a Silver Perch; Southern Pygmy Perch; A Murray Spiny Crayfish