Optimising Calcein Marking Techniques for Juvenile Murray Cod Stocked into Riverine Systems

Murray cod (Maccullochella peelii) are Australia’s largest freshwater fish, reaching a maximum of 1.8m and 112kg. Murray cod are an important recreational species that have declined substantially in abundance since the 1950’s. The decline has required the implementation of strict management strategies to control angler harvest and resulted in the subsequent closure of a commercial fishery for the species. Re-stocking is increasingly used to improve recreational catch and over 12 million fish have been released into the Murray-Darling Basin since hatchery techniques were refined. To date, there has been no quantitative assessment of stocking success for this species largely because techniques to distinguish between hatchery-reared and wild fish are not available. Chemical marking is a useful technique to determine natal origin of fish and is increasingly used to determine success of fish stocking programs. This study sought to optimise an osmotic-induction batch marking technique, using the calcium-binding chemical, Calcein, to enable future identification of hatchery-marked fish. Success of osmotic induction techniques is highly dependent on immersion into a saline bath to increase osmotic pressure and maximise chemical uptake. It was hypothesised that higher saline concentrations would create a more reliable mark but it was unknown whether saline exposure would influence fish welfare. A replicated tank and pond trial was undertaken to determine the optimum saline concentration required to maximise survival of Murray cod whilst maximising marking of bony structures. Results  have already been implemented into standard practices for both government and private hatcheries. This has enabled Calcein signatures to be detected in fish collected from Australia’s waterways following release. This information will be a major tool to inform stocking success for this species.