Methods for marking stocked fish

Methods for marking stocked fish

More than three million native fish are produced in private and government hatcheries and stocked into waterways of the Murray-Darling Basin each year. Little is known of the fate of stocked fish or their impact on wild populations, in part because of a lack of suitable methods for marking small juvenile fish. The increasing use of wild fish populations as indicators of river ‘health’ was also potentially compromised by the inability to distinguish between hatchery-bred and wild fish. This suite of projects developed techniques for chemically marking hatchery bred fish. National chemical registration requirements for the chemicals involved in the marking were investigated and these techniques were then trialled in a number of streams to examine the contribution of stocked fish to ‘wild’ fish populations.


Three marking methodologies were successfully trialled and developed using Golden perch as the model fish species:

  • Osmotic induction marking of fingerlings (which enhances uptake of the marker chemical by immersing fish in a 5% saline solution) with alizarin red S and calcein;
  • Marking of otoliths (ear bones) via immersion of fingerlings in enriched stable isotopes of strontium and barium; and,
  • Trans-generational marking of otoliths through the injection of enriched stable isotopes of barium into maternal broodfish.

The feasibility of each of the methods was examined with particular emphasis on practicality and economic considerations for government and private hatcheries. Each of the methods was found to have strengths and weaknesses, with the choice of method dependent upon the aims and limitations of the marking program. Techniques that only marked otoliths (i.e. stable isotopes) require the fish to be killed and the otoliths analysed in a laboratory to determine whether they are marked. However, alizarin and calcein leave external marks that can be detected in the field without having to sacrifice the fish, which is a distinct advantage for most river health monitoring programs or projects involving threatened species.

A wide variety of statutory controls exist around the use of chemicals in veterinary medicine, agriculture, pest control, food production, the environment, and the use of chemicals in products that could end up being consumed by the public (e.g. eating marked fish caught by recreational fishers).

Investigation of whether the chemical marking techniques developed in this project were captured by the relevant legislation at a national and state level revealed that there were no registration requirements for any of the marking techniques and chemicals, and that the chemical marking techniques developed during the project can be legally applied in hatcheries provided specific processes are undertaken.

In the final project, the objectives were to:

  • develop and evaluate practical mass marking techniques to allow for accurate discrimination of hatchery-produced and wild fish;
  • facilitate and encourage broad-scale uptake of the marking techniques by government agencies and commercial hatcheries; and
  • apply the marking techniques to quantify the contribution of stocked fish to populations in experimentally stocked rivers within the MDB.

A field detection kit for calcein marks was developed which included a field fluorometer for quantitative measurement of calcein fluorescence, with a specialised torch and glasses set for visual identification of marked fish.

As a first step towards understanding the impacts of native fish stocking on riverine fish populations in the MDB, chemically marked fish were stocked into three rivers of the southern MDB – Billabong Creek, Edward River, and the Murrumbidgee River. Post-stocking surveys were then conducted over a 5-year period and the proportion of marked fish determined. The results showed that at least a proportion of the stocked fish survived to reach the legal minimum size in all three rivers. However, the impacts of stocking on population structure were very different among rivers. In the Edward and Murrumbidgee Rivers, the age classes corresponding to the years of stocking were comprised of 18-38% experimentally stocked fish, and these fish made only a relatively minor contribution to the total catch of Golden perch. In contrast, stocked fish comprised up to 100% of age classes corresponding to stocking years in Billabong Creek, and stocking resulted in a four-fold increase in the catch rates of Golden perch.

Implications for native fish

Osmotic induction marking with calcein had no detectable effects on fish health and is
relatively quick and easy – it takes only 15 minutes to mark up to 20,000 fingerlings. The simplicity and cost-effectiveness of osmotic induction marking make it feasible for widespread adoption in hatcheries.

Large-scale calcein marking of hatchery fish commenced in 2009, and agencies from all State and Territory jurisdictions within the MDB have initiated processes to incorporate calcein marking into their stocking and/or research programs.

The results of the experimental stocking study demonstrate that stocking has the potential to strongly affect population structure and abundance of the stocked species.


R5003: Crook, D., Munro, A., Gillanders, B., Sanger, A, O’Mahony, D. and Thurstan, S. (2006). Improved methodologies for discriminating stocked and wild Fish – Final Report. MDBC Publication No. 38/06, Murray-Darling Basin Commission, Canberra.

MD697: Sanger, A. and Crook, D. (2007). Chemical Marker Registration – Final Report. MDBC Publication No. 18/07, Murray-Darling Basin Commission, Canberra.

MD741: Crook, D.A, Gillanders, B.M., , Sanger, A.C, Munro, A.R., O’Mahony, D.J., Woodcock, S.H., Thurstan, S. and Baumgartner, L.J. (2010). Methods for discriminating hatchery fish and outcomes of stocking in the Murray-Darling Basin. Final Report to Murray-Darling Basin Authority.

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