Management of alien fishes in the Murray-Darling Basin



Native Fish Strategy 2003–2013 (NFS) investment over the past decade has considerably improved alien fish management for the Murray-Darling Basin. The NFS delivered a wide and varied portfolio of activities on alien fish management – planning, social aspects, on-ground management and a major investment in research. Control of Carp (Cyprinus carpio) was a key feature; however approaches moved beyond this one species to address multiple species and to take a more holistic approach, advocating integrated pest management and linking alien fish with native fish management and broader river rehabilitation. The NFS provided leadership for the management of alien fish in the Basin and established a framework to guide future efforts in this field through the development of a draft Basin Alien Fish Plan.


Fish assemblages in the Murray-Darling Basin (MDB) are now dominated by alien species ─ that is, species originating overseas but now established in Australia (Harris 1995). Twelve alien fish are found in the MDB (Koehn & Lintermans 2012) totalling 60–97% of fish biomass (Davies et al. 2012). The historic sources of introductions of alien fish include deliberate introductions (for recreational species, mosquito control, or aquaculture) and escapes from the aquarium trade (Lintermans 2004). Most native fish, particularly larger commercial and recreational species, had declined before the most dominant alien fish, Carp (Cyprinus carpio), became widely abundant (Koehn et al. 2000). However, alien species contributed to further declines and ongoing suppression of native fish through predation, competition, habitat alteration and spreading diseases and parasites (Ansell & Jackson 2007).

The first ‘explosion’ of Carp numbers in Australia occurred in the MDB, following widespread flooding in the mid-1970s (Koehn et al. 2000) and resulted in an initial focus on this species. The Murray-Darling Basin Commission (MDBC) released a National Management Strategy for Carp Control (NMSCC) in 2000 together with a research plan and guide to planning Carp control at regional scales (Braysher & Barrett 2000; Carp Control Coordinating Group 2000a,b). Complementing these initiatives, Managing the Impacts of Carp (Koehn et al. 2000) synthesised existing knowledge on Carp in Australia. Other work included a risk assessment of riverine pests (Clunie et al. 2002) and development of a draft management strategy for Oriental Weatherloach (Misgurnus anguillicaudatus) (Koster et al. 2002a,b).

Management of alien fish under the Native Fish Strategy

Although the NMSCC was agreed to by all participating governments, it was unfunded and not implemented. The NMSCC principles were considered in the Native Fish Strategy 20032013 (NFS), which recognised ‘controlling alien fish species’ as one of six key management actions to recover native fish populations in the MDB (Murray-Darling Basin Commission 2004). Therefore, the NFS represents a shift from a historically narrow focus of alien species management (Zavaleta et al. 2001), by aligning alien fish management in the MDB with broader restoration goals through ‘whole-of-fish-community’ management (Koehn & Lintermans 2012).

Initially, research mainly addressed key knowledge gaps and control options for Carp (agreed priorities of the NMSCC). Two main factors drove investment: limited understanding of the ecology and biology of alien fish and their impacts in the Australian environment (Koehn & Mackenzie 2004), and a major commitment of nearly $10 million in cooperative research ventures (Invasive Animals Cooperative Research Centre (IACRC) and its predecessor the Pest Animal Control CRC). This expanded during the ten years of the NFS, funding work on many components of alien fish management:

Planning, strategy development and management support

The NFS coordinated planning at multiple levels, recognising separate needs for basin-scale planning and strategic action on priority issues, as well as dealing with regional scale and on-ground management. The need for a basin-wide plan was highlighted at a workshop in 2006 (Ansell & Jackson 2007), with the MDBC subsequently establishing a task force to draft a Basin Alien Fish Plan. On a regional scale, Carp plans were developed to deal with local issues and planning on-ground actions (Braysher & Barrett 2000). The NFS was also able to ‘catalyse actions for priority problems’ (Koehn & Lintermans 2012). Tilapia (Oreochromis spp.) are introduced species from Africa and a major immediate threat due to their potential detrimental impacts and close proximity to the MDB (Hutchison et al. 2011). The Murray-Darling Basin Authority (MDBA – which replaced the MDBC in 2008) established a collaborative work programme with state government fisheries agencies and a regional stakeholders to attempt to prevent Tilapia entering the MDB and to develop a Tilapia exclusion strategy for the northern MDB (the invasion front) (Condamine Alliance 2012a,b).

A range of other work assisted decision-making and assessments to improve alien fish management and included the following: alien species establishment risk models (Bomford 2008); reviewing efficiency of existing response arrangements (Ayres & Clunie 2010a,b); decision support tools (Acevedo et al. 2013); guidelines to assess the likely effectiveness of Gambusia (Gambusia holbrooki) removal (Tonkin et al. 2012) and complex ecosystem models tracking changes in response to Carp removal (Gehrke et al. 2010).

Research and On-ground control

The NFS provided research tailored to management and policy and tested and refined outcomes in on-ground trials; aspects that traditional alien species management programmes often lack (Hulme 2006). The NFS made major contributions to research on alien fish in Australia as a key funding partner to the IACRC ( and with research delivered by the MDBA (Table 1 and Supplementary Table S1).

Table 1. Examples of NFS-funded research related to alien fish management (the examples were chosen to illustrate all areas of alien fish management covered by the NFS and include examples of IACRC projects)
Planning, Strategic Development and Management Support
Regional Carp Management Plans
Northern Basin Tilapia Exclusion Strategy
Rapid response planning (IA CRC) (Ayres & Clunie 2010a,b)
Impact assessment of potential incursion of Tilapia in the MDB (Hutchison et al. 2011)
Integrated pest management trials for Carp (Gehrke et al. 2010; Vilizzi et al. 2013)
Experimental control of Gambusia and decision support tool for removal of Gambusia (Macdonald et al. 2012)

Research and On-ground Control

 (a) Prevention, Preparedness and Response

Environmental DNA for detection (IACRC 2012)
Targeted surveys for Tilapia in MDB
Baseline incursion genetics of Tilapia (Ovenden et al. in press)
 (b) Control(Short-term Studies)
Carp Separation Cages; Jump and push traps. (Stuart et al. 2006; Smith et al. 2009; Thwaites et al. 2010).
Carp Screens (Hillyard et al. 2010).
Carp Attractants (IACRC) (Smith & Thwaites 2007; Elkins et al. 2009; Lim & Sorensen 2010).
  Review of potential fish-specific biocides. IA CRC (Invasive Animals CRC)
  Assessment of Carp fish-outs/fishing competitions in Carp management programmes (Norris 2011)
  Carp recruitment hotspots (IACRC 2012)
  Carp movement and migration patterns (IACRC 2012)
  Carp growth and aging in the northern Basin (Hutchison et al. 2012)
  Habitat preferences and assessment of efficiency of common fish capture techniques for Tilapia in South East Queensland. Potential exploitation of temperature as a control tool (Norris et al. 2012)
  Integrated management for Tilapia in north Queensland, including biological investigations (IACRC) (Russell et al. 2012a,b, 2013).
Social Aspects
Tilapia Training Package and ‘Train the Trainer’ workshops (The State of Queensland 2011a).
Pest Fish ID Guide for NSW/Queensland (The State of Queensland 2011b).

Numerous research projects investigated, tested and refined a range of tools and techniques, complemented by species-specific studies (e.g. habitat associations and behaviour), to improve prevention, detection, containment, eradication and control of alien fish (Table 1). Investigation of tools and techniques had a dual focus: using and refining existing tools to manage alien fish at localised scales (e.g. Carp cages and screens) while also investing in long-term and innovative technology with a goal of future broad-scale control. The NFS promoted Integrated Pest Management (IPM) trials for Carp (Gehrke et al. 2010; Vilizzi et al. 2013), Gambusia (Tonkin et al. 2012) and Redfin Perch (Perca fluviatilis) (Boys et al. 2014) as part of Demonstration Reach programmes under the NFS and demonstration sites under the IACRC (IACRC 2012).

Ultimately, basin-scale control may only be possible by developing new biological control measures to complement other existing techniques. Two options were investigated by the IACRC – daughterless technology and a Carp virus (Koi herpesvirus – KHV). Daughterless technology alters fish genes to create carriers that produce only male descendants (Thresher 2008). Preliminary trials have started on Carp, with some success, and the technology could be adapted for other invasive fish. However, the eventual release of carriers would require long-term stocking programmes (Thresher et al. 2013), and there are a number of constraints that need to be overcome (Hayes et al. 2013). No future commitments have been made by government agencies towards pursuing this technology. However, investment in KHV is continuing through the IACRC. Initial evaluation of this control agent is very promising with a high species specificity and death rates for Carp (Saunders et al. 2007; Sunarto et al. 2011; IACRC 2012).

Social aspects

Effective management of alien species must cover social aspects (Zavaleta et al. 2001). Communication and awareness, knowledge exchange and capacity building all received attention from the NFS, as well as efforts to better understand the influence of social factors on alien fish management (Table 1). The NFS had strong communication and engagement networks; NFS coordinators played a vital role in knowledge exchange (Koehn & Lintermans 2012), and their efforts were augmented by various forums (e.g. NFS annual fish forum, Gambusia forum (Jackson & Bamford 2011)). The broader work of the NFS also benefited from alien fish management actions. For example, Carp fishing competitions were popular in regional MDB areas, and although not effective as a Carp control tool (Norris et al. 2013b), they were valuable for engaging with people about alien species management and river rehabilitation (Norris & Ballard 2013a).

Evaluating progress under the NFS

The NFS has an overall goal of rehabilitating native fish communities back to at least 60% of pre-European settlement levels within 50 years. Quantitative evaluation of the contribution of alien species management to the improvement of native fish populations at the Basin level during the first 10 years of the NFS could not be meaningfully determined. The MDBA's Basin monitoring programme (Davies et al. 2012) did not provide sensitive enough measures to attribute ecological improvement/decline of the Basin's native fish community to the actions of the NFS. This was particularly difficult due to the sustained, basin-wide drought coincident with the implementation of the NFS, followed by widespread floods. Individual projects, however, were able to demonstrate significant improvements and with limited resources. For example these include: (i) the identification of Carp ‘hotspots’, demonstrating that a majority of Carp larvae originate from a relatively small number of locations (Gilligan & Rayner 2007); (ii) the development of Carp push traps that catch large Carp and can be retrofitted to existing separation cages in fishways and incorporated with Carp exclusion screens fitted to wetland inlet/outlet structures (Thwaites et al. 2010); and, (iii) investigations of the benefits to native species from controlling Gambusia in floodplain habitats (Tonkin et al. 2012).

Measureable progress has been made in laying the groundwork for the effective management of alien fish into the future, and significant advances were made in implementing actions identified within the NFS (see also Table 1); however, a ten-year timeframe is not long enough to conduct research (especially into biological control measures) and test and implement management actions over such a broad scale.

Managing alien fish into the future

Without coordinated and adequately funded actions, alien fish will continue to affect environmental, social and economic values in the MDB. Little data are available on the economic losses associated with alien species. The IACRC has estimated the costs associated with invasive vertebrates in Australasia to be at least $720 million per annum (Harris 2013). Conservative estimates of the detrimental impacts of Carp have been put at nearly $16 million per annum (McLeod 2004). Based on this, Carp has cost the Australian community $308 million in damages over the 13 years since the NMSCC was released. Social and environmental impacts are even less well understood but are likely to be significant, as is evident by the ongoing public concern related to Carp numbers in the Basin.

The draft Basin Alien Fish Plan (BAFP), developed under the NFS, provides a framework for managing alien fish into the future. It focuses on three key areas:

Preventing or minimising future risks and threats

Preventing new problems is paramount, both in limiting the spread of established alien fish and preventing new species from establishing (Wittenberg & Cock 2001). In the MDB context, Tilapia is an acknowledged threat, and Oriental Weatherloach and Redfin Perch are still spreading (Ansell & Jackson 2007). New introductions are a high risk with southern Australia being amongst the world's hotspots for fish invasions (Leprieur et al. 2008). Many species that established in recent decades originated in the aquarium and ornamental trade (Corfield et al. 2008), with over 1000 alien fish species present in Australia, many in aquaria (McNee 2002). This is a major potential source for new introductions.

Humans are the main cause for the transfer of alien fish, whether by accidental or deliberate release (Lintermans 2004). People are also a key part of the solution and can actively participate in prevention, eradication and control efforts (Witmer et al. 2009). Targeted public awareness is vital to modify attitudes and behaviours that spread alien fish and to raise support and participation in alien fish management (Clunie et al. 2002; Garcia-Llorente et al. 2008). Improved early detection is essential, and increased effort should be placed on understanding where and how alien fish enter river systems (García-Berthou 2007). Public sightings can be the first report of new incursions (Lodge et al. 2006), and this can be useful to augment formal surveillance programmes (Cacho et al. 2010). More sensitive and cost-effective monitoring for aquatic species (Hayes et al. 2005) is needed for early detection and control. Genetic tools have potential to aid surveillance, particularly those able to detect DNA of subject species in environmental samples (eDNA) (Darling & Mahon 2011). These techniques are potentially more efficient and sensitive than traditional methods as they are: able to detect low densities and different life stages of target species (Dejean et al. 2012); and are already useful in surveillance and monitoring programmes for alien fish (Jerde et al. 2013). Preliminary investigations into eDNA for the early detection have commenced, and these will continue through the IACRC.

Adopting efficient emergency preparedness and response arrangements

Well planned, and regularly tested arrangements for fish responses need to be in place to deal with new incursions. In Australia, national emergency preparedness and response arrangements are in place at a generic level, but lack sector-specific provisions to support effective fish responses (Ayres & Clunie 2010b). Furthermore, state arrangements currently vary in effectiveness and can be ad hoc (Acevedo et al. 2013). Ayres and Clunie (2010a,b) outline measures to significantly advance response arrangements for freshwater fish incursions in Australia and bring them closer in line with other, more advanced, sectors.

Addressing the negative effects from established species

Cost/benefit considerations need to be applied to planning control operations – control should focus on areas where there are high impact levels from pest fish (e.g. aggregation sites) or where the return for control effort is greatest (e.g. areas of high value) (Tonkin et al. 2012). Sites with characteristics such as natural barriers to re-invasion, ease of accessibility and supportive landowners also improve the probability of success. Planning should also consider how to fit alien fish management within broader system objectives and to help achieve their goals (e.g. catchment rehabilitation objectives) (Koehn & Lintermans 2012).

Additional alien fish management technologies will need to be developed to support control programmes, and biological control measures need to remain on the agenda. In particular, there needs to be continuing support for investigating the potential release of KHV as a control agent for Carp in Australia (IACRC 2012).


Under the NFS, significant progress has been made in laying the groundwork for the effective control of established alien species in the MDB and the prevention of additional introductions, such as Tilapia. A number of overarching accomplishments have also been attained under the umbrella of the NFS:

  • A coordinated response between the multiple governments managing alien fish in the MDB has been achieved.
  • IPM plans for Carp have been developed and tested.
  • The NFS has built community interest and active participation in alien species management (e.g. stakeholder ownership of Tilapia actions, natural resource management driven IPM plans for Carp, community-driven Gambusia management).
  • Alien fish management is now part of the broader context of natural resource management.
  • Control tools (e.g. Carp separation cages, Carp screens) have been developed, tested, and are being utilised in the field.
  • Research has improved knowledge of key aspects of the biology of alien fishes.
  • Long-term research into biological control measures has commenced.

Despite such progress, there remain many challenges. Carp remains a perennial issue in the MDB, and the problem is likely to be exacerbated if expected population increases due to recent flooding events eventuate. Likely range extensions of Oriental Weatherloach, the threat from Tilapia and the potential introduction of other new alien species highlight the need for management and coordination at a Basin scale.

A coordinated approach is essential, and the draft BAFP provides the blueprint for basin-scale alien fish management. The roll-out of the BAFP would be best achieved under a broader NFS programme where all threats to native fishes are considered in a natural resource management context. The significant advantages of such an approach have been highlighted throughout this paper and include the ability of the NFS to catalyse actions in relation to priority problems across the Basin and the very strong communication and engagement networks established. Ultimately, broad stakeholder ownership and involvement remains essential to effective alien fish management.

Without the NFS, the BAFP on its own at least provides a template for managing alien fish in the Basin into the future with a set of prioritised actions that could easily be adopted by Basin jurisdictions. The implementation of the BAFP and a coordinated approach between jurisdictions could be facilitated by an agreement to reconstitute the BAFP Task Force with an appropriate mix of expertise and jurisdictional authority.

New biological control measures that have basin-scale potential should continue to be supported. In particular, the IACRC work on the Carp virus KHV shows significant promise and should be maintained into the future. Unfortunately, in the experience of the NFS, the greatest challenge may be securing long-term commitment and funding to complete programmes for technology development, plan formulation, the implementation of prevention and control measures, and adequate monitoring. There still remains poor recognition of the impacts of alien fish in Australia and consequently, inadequate resourcing to address the issues. There is an urgent need for the development of a national strategic framework for alien fish management to help raise the importance of alien fishes on the national agenda to the same level of importance given to other pest fauna.


The authors wish to thank all the people that have worked on or supported work on alien fish over the last 10 years. Particular thanks go to the members of the Basin Alien Fish Task Force for their cooperative spirit and hard work in contributing to the development of the BAFP. The comments of the reviewers are also acknowledged.


  • Jim Barrett is the principal of Jim Barrett Consulting (36 Canning St, Ainslie, 2602, Australia

  • Heleena Bamford is the Assistant Director, Environmental Watering Plan Implementation, Murray-Darling Basin Authority, Canberra, ACT, 2601, Australia

  • Peter Jackson is a consultant specialising in river ecology and freshwater fishes (3 Donnybrook, Queensland, 4510, Australia