Scale pattern analysis is preferable to OTC marking of otoliths for differentiating between stocked and wild dusky flathead, Platycephalus fuscus, and sand whiting, Sillago ciliata

A pilot programme to evaluate the effect of stocking dusky flathead, Platycephalus fuscus (Cuv. & Val.), and sand whiting, Sillago ciliata (Cuv. & Val.), began in the Maroochy River estuary in 1995. Two possible methods of mass marking; oxytetracycline (OTC) marking of otoliths, and scale pattern analysis (SPA) were investigated. A range of OTC trials was conducted, with limited success. High concentrations (1000 mg L^?1) of OTC and low salinities (≤9°/₀₀) produced acceptable marks on the otoliths of both species, but at the expense of fingerling survival and environmental health risks. SPA provided better results, with wild to hatchery misclassification rates of 25 and 11%, respectively, for P. fuscus and S. ciliata. The reverse misclassification rate of hatchery to wild was equal or better for both species. Field trials demonstrated that SPA was able to differentiate between the two sources of fingerlings (wild and hatchery). These results indicated that OTC was impractical, while SPA was acceptable for this pilot programme. The implications of these results are discussed.