Million Murray Cod Program: Identification of hatchery origins in adult Murray Cod
Recreational Fishing Grants Program Research Report.
Over one million extra, hatchery-raised, Murray cod (Maccullochella peelii), were stocked into Lake Eildon between 2010/11 and 2012/13. Evaluation objectives include a monitoring program to determine how these large stockings of juvenile cod survive and contribute to the fishery.
Calcein has been used to reliably mark golden perch and Australian bass, both species closely related to Murray cod, using the method known as 'osmotic induction'. The present study evaluates whether similar results could be obtained in Murray cod. If so, this will make surveys of hatchery-reared Murray cod both cost-effective and non-destructive. Murray cod juveniles aged 6-8 weeks were marked with calcein. Observation under UV light clearly showed fluorescent marks on the skin and fins of the young fish (Figure 1). A sample of marked fish and unmarked control fish were retained in two small ponds in January 2010. Fish were allowed to grow undisturbed in each pond and sampled in June 2013 at age-2+.
No consistent differences in the fluorescence signal were observable between external readings taken from calcein marked fish and unmarked control fish. Average fluorescence in control fish was higher than marked fish at seven out of eleven anatomical locations.
Examination of otolith thin-sections confirmed that fluorescent annular marks were clearly visible near the primordial of all three age-2+ Murray cod sampled from the farm dam, that were marked as fingerlings. No fluorescent marks were visible in otoliths of unmarked control fish.
Murray cod fingerlings can clearly be successfully marked with calcein using 'osmotic induction'. At age-2+ years, marks cannot be reliably identified externally on freshly caught Murray cod, at any of the eleven anatomical locations examined, using a hand-held GFP-meter.
Based on the present evidence, identifying Murray cod marked as fingerlings requires destructive sampling and microscope examination of otolith sections.
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