Key words: fish stocking, impacts, native fish, Murray-Darling Basin
Fish stocking (i.e. hatchery production of fish to a size or stage so that they can be released into an area) has been practiced worldwide for centuries, but it is only recently that the environmental and ecological risks have been recognized. Stocking has been undertaken largely to enhance recreational angling but also to aid conservation of a species. Regardless of the objectives, appropriate and effective management of stocking activities is required because a number of risks exist, which are not always well understood.
Broad aim and methods: This project reviewed the impacts of native fish stocking on fish within the Murray-Darling Basin (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 considered.
A review of literature was undertaken to consider the impacts of native fish stocking on fish within the MDB. Research papers were evaluated using the following criteria: a) whether the research was relevant to native fish stocking; b) whether the design was confounded; and, c) whether the analysis and interpretation of data were correct and the conclusions valid. Studies that did not meet these criteria but provided useful information to generate hypotheses were also considered.
The review focused on four potential impacts of stocking: abundance and behavioural responses; genetic implications; spread of disease; and ecosystem level effects.
Findings: Changes to abundance and behaviour of fish from stocking were reported to arise mainly 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 were noted to 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 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 were noted to have received little attention. The accidental introduction of a disease with the stocking of native species is most likely to have a negative impact on wild populations. Several examples of the spread of pathogens through stocking exist for the MDB.
Ecosystem alteration from stocking fishes is extremely difficult to demonstrate, and has mostly been attributed to introduced species rather than native species.
Lessons learned and future directions: The review concluded that targeted research on MDB species is needed for all potential impacts and highlighted a need for sound baseline data and monitoring programs. Many species of native fish in the MDB are stocked in some way and the potential impacts outlined in this review should be considered when designing or reviewing stocking programs to maximise desired effects (i.e. boost numbers of stocked species) and minimise negative effects on resident native fish species (Figs 1 and 2). The study identified potential benefits in undertaking a risk assessment of potential impacts prior to stocking and conducting experimental evaluation and monitoring of any stocking program. Only with such an approach will the success of stocking programs be evaluated and the risks mitigated.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy, and conducted by Bronwyn M. Gillanders, Travis S. Elsdon and Andrew R. Munro University of Adelaide and Woods Hole Oceanographic Institution.
Contact: Professor Bronwyn Gillanders, University of Adelaide, firstname.lastname@example.org or +61 8 830 36235.