Fish stocking has been practiced worldwide for centuries, but it is only recently that the environmental and ecological risks have been recognised. Stocking in the Murray-Darling Basin (MDB) has been largely undertaken either to enhance recreational angling or to aid conservation of a species. This project reviewed potential impacts of native fish stocking on fish within the MDB, and also provided a summary of stocking activities within the MDB. Potential impacts on abundance and behaviour, genetics, disease and ecosystem level effects were discussed.
The review focused on 4 potential impacts of stocking:
- abundance and behavioural responses,
- genetics implications;
- spread of disease and;
- ecosystem level effects.
Changes to abundance and behaviour of fish from stocking mainly arise through competition between stocked and wild fish. These effects can be either direct (for food and habitat) or indirect (habitat alteration, behavioural changes, expansion of species range and displacement of wild stocks). Generally, there has been a lack of research on abundance and behavioural responses to fish stocking on native Australian species.
Genetic impacts of hatcheries and hatchery fish on wild populations have received a lot of attention, but the literature is mainly theoretical in nature. Genetic effects can be direct (e.g. hybridisation) and indirect (e.g. reduction in population size and genetic diversity caused by predation, competition and diseases). Artificial breeding of fish also alters the genetics of captive bred populations. At the time of this project, very little was known about the genetic structure of native fish populations in the MDB.
Impacts of introducing diseases, parasites and exotic organisms unintentionally when stocking fish have also received little attention. The accidental introduction of a disease with the stocking of native species will most likely have a negative impact on wild populations. There are thought to have been several examples of the spread of pathogens through stocking in the MDB.
Ecosystem alteration from stocking fishes is extremely difficult to demonstrate, and has mostly been attributed to introduced species rather than native species.
The review concluded that targeted research on fish species found in the MDB is needed to better understand all potential impacts. Sound baseline data and monitoring programs are necessary to track changes over time. Stocking will likely result in changes to the receiving system and therefore it is important to ensure that the benefits outweigh the costs and ensure that the goals of any stocking program are met.
Implications for native fish
Many native fish species in the MDB are stocked and the potential impacts outlined in this review should be considered when designing or reviewing stocking programs. This will ensure that the desired effect of stocking programs (i.e. to boost numbers of focus species) will not have negative effect on resident native fish communities. A risk assessment of potential impacts should be undertaken prior to stocking, and experimental evaluation and monitoring of the stocking program considered. Only with such an approach will the success of stocking programs be evaluated and the risks mitigated.
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