Flow alteration is one of the most common threats to rivers and streams around the world. Alterations such as weirs, dams and water withdrawal for human uses tend to suppress natural flow variation causing a disconnection between rivers and floodplain wetlands. These changes to the natural hydrology of rivers can have detrimental effects for fish communities because many species of fish rely on periodic access to floodplain wetlands for food, refuge and nursery habitat.
Increasingly, rivers are being managed with environmental flows, which are water allocation policies implemented to protect or restore aquatic ecosystems. In flow-regulated systems, environmental flows are used by managers to provide an array of environmental benefits ranging from specific goals targeted at ‘valuable’ taxa, to more holistic goals that aim to reinstate general ecosystem patterns and processes. However, managers are faced with the challenge of balancing water used for maintaining or restoring ecosystem processes via environmental flows, with water used for other purposes such as providing drinking water, irrigation, and generating power for the public. Fresh water is consequently in great demand and if used for one purpose it cannot be used for others – this presents a substantial challenge for water managers.
Because of the many competing demands placed on freshwater resources, environmental flows are often delivered sparingly and infrequently, leaving wetlands disconnected from the river for long periods of time. However, recent research suggests that this may actually be counterproductive for native fish and beneficial for non-native fish over the longer term. Leah Beesley and colleagues have demonstrated in a recent paper that the effects of environmental flows on wetland fish of the Murray River not only depends on the type of water allocation, but on the flow conditions prior to the delivery. In other words, the benefits for native fish realised from a water allocation can be dependent on the pattern of flows over the previous five years. This is an important finding because it can help managers formulate flow policies that navigate the trade-offs among the many competing demands for water in the Murray-Darling River Basin.
The paper demonstrated how flow policies that inundate floodplain wetlands at regular intervals, such as biannually, can promote native species such as Australian smelt and Flathead gudgeon. Alternatively, the authors also demonstrate that flow policies that allow floodplain wetlands to be disconnected from the main stem of the river for longer periods (e.g. years) can promote non-native species such as Common carp and Eastern gambusia. This suggests that using environmental flow allocations sparingly after long periods of drought, as is a common management practice, can lead to fish communities dominated by non-native species. Unfortunately, implementing flow policies that regularly inundate floodplain wetlands may not be politically or economically feasible given the tremendous demand on fresh water for other purposes. If this is the case, better conservation of water resources by society and additional creative methods to manage fish communities may be needed to meet all the competing demands for freshwater resources.
Guest post and photos provided by Dan Gwinn, who has also written a longer post about this paper “Effects of antecedent flow patterns on juvenile fish of the Murray River.” on his own blog.