Carp (Cyprinus carpio) are a worldwide, pervasive and very successful alien pest fish species that has invaded most of the Murray–Darling Basin (MDB) in less than 50 years. They are a highly visible fish, widespread and abundant, with biological attributes (e.g. high fecundity; highly mobile) that allow their populations to expand rapidly. Carp spawning and recruitment can be enhanced by flooding (especially onto floodplains), and as Carp are very abundant in MDB river systems, population responses can result in large increases in Carp numbers.

One of the inherent challenges in managing flows for native biota is that there may also be unavoidable benefits to Carp or other unwanted species. Carp already occur in very high numbers in the MDB river systems; therefore, their population responses are likely to be large, being intrinsically linked to the existing high abundances. These high abundances mean that Carp populations in the MDB will be ongoing, with or without environmental water. In contrast, the abundances of many native fish species are often much lower than Carp, and hence they may not exhibit the same initial magnitude of population response.  However, improvements in the long-term viability of many native fishes will rely on these smaller cumulative responses to flows.

Carp biology:

Our understanding of Carp biology indicates that we may intuitively expect some types of water management options in the MDB to increase Carp numbers. There are a range of different types of flow events in the MDB, from in-channel pulses to those that flood habitats such as wetlands and floodplains. Flow events that result in prolonged inundation of preferred Carp breeding and nursery habitats will significantly increase the risk of strong recruitment events and the subsequent increase in populations. Importantly, since the Millennium Drought there has also been an increased emphasis on the construction and use of infrastructure such as pumps and regulators to deliver water to maximise the floodplain area inundated per volume of water used. These initiatives pose risks for native fish as well as having the potential for increased production of Carp.

This study used up-to-date biological and ecological knowledge of Carp  to formulate conceptual models for key aspects of Carp life history, which have then been used to develop an age- and abundance-based stochastic population model. The model has been developed for the southern connected MDB (Murray, Murrumbidgee, Lachlan and Lower Darling river systems), with the option of adapting it for other areas, especially for the northern MDB.

A review of flow regimes and watering objectives, options and delivery mechanisms was undertaken to inform the flow scenarios to be modelled. Modelling that linked flows to habitat inundations (availability) was then undertaken for a range of habitat types and watering scenarios. Outcomes from this modelling could then be compared and assessed within a risk framework in order to guide management.  Key messages from this work are summarised as follows:

• Priority objectives for environmental water management in the MDB are to benefit native biota, and this focus must be maintained.
• Carp are a highly visible and abundant invasive fish species that can readily respond to flows, especially overbank flooding. The long potential spawning season for Carp overlaps with that of many native fishes and also with likely watering times for other biota; hence, careful management is needed.
• Natural flooding does promote Carp and native fish population growth, but water managers have little control over these flows.
• Carp are now a major component of MDB fish fauna, and their recruitment may be an inevitable by-product of some watering activities, including those for environmental objectives. The responses observed in Carp populations are influenced by existing high abundances. In general, however, in-channel environmental flows will have minimal impacts on Carp populations, but will have benefits to native fish populations. Furthermore, existing large reproductive Carp populations in the Lower Lakes of the Murray River mean that environmental flows into South Australia will have limited further impact on Carp numbers in the lower Murray River.
• Habitats and flows that result in high population growth rates pose the highest risk of increases in Carp populations, and these involve the inundation of floodplain, wetland or lake habitats.
• Artificial floodplain inundation using regulators is likely to pose a significant risk of increasing Carp populations. Such inundations may export Carp from floodplains and substantially increase the river metapopulation. Frequent, sequential inundations of the floodplain and the cumulative impacts from multiple large-scale sites constitute the greatest risk of increasing the Carp populations in the Murray River. Nevertheless, water managers have high levels of control over this type of management action and, hence, have the ability to manage such inundations carefully.
• Watering for non-fish outcomes could be considered during winter months (water temperatures <16°C) to minimise Carp recruitment. This may mean, however, that positive outcomes for native fish should not necessarily be expected. Winter watering events may not produce some of the desired outcomes for native biota, so the use of winter flows should be carefully considered in the context of ecological objectives.
• There is a need for Carp to be managed in conjunction with watering through the development and implementation of adequate Carp management plans for high-risk watering activities and sites (e.g. large wetland/floodplain areas), with actions based on pest management principles. These site plans would benefit from being set within the context of a coordinated, MDB-wide Carp management plan.
• In order to quantify the responses of Carp to flows and to manage populations, data from regular monitoring is needed. These data can also be incorporated into population models that can be used to forecast potential changes in Carp and native fish abundances over the appropriate temporal (decadal) timescales.
• There is a need to evaluate the benefits of flow management actions to native species so that these can be balanced against any impacts from any potential increases in Carp populations. A step towards this has occurred through the initiation of a project for developing native fish population models that will allow the benefits of environmental flows for fish to be explored.

The full report by  John Koehn, Charles Todd, Leigh Thwaites, Ivor Stuart, Brenton Zampatti, Qifeng Ye, Anthony Conallin, Lauren Dodd and Kasey Stamation is here: Managing flows and Carp report.

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