Abstract
Juvenile mulloway Argyrosomus japonicus (54·6 ± 4·6 mm total length, mean ± s . e .) were immersed in a range of oxytetracycline (OTC) solutions ranging between 0–600 mg l −1 in salinities of 5 (diluted sea water) and 35 (undiluted sea water), and alizarin complexone (ALC) solutions ranging between 0–60 mg l −1 in undiluted sea water, for 6, 12 and 24 h. Optimal marking conditions were 600 mg l −1 OTC for 24 h in a salinity of 5, and 30 mg l −1 ALC for 12 h respectively. Mark quality (MQ) was assessed using a score of 0–3 in both otoliths and anal fin spines, with a score >2 found to be acceptable for adequate mark identification. Acceptable marks were not produced using OTC in undiluted sea water. Immersion in OTC or ALC, or reduced salinity had no effect on survival relative to controls. Transverse sections of vertebrae from the ALC and OTC treatments with the highest otolith mark quality showed no discrete marks. Optimal marking techniques were used to produce double marks with a 3 day interval between marking, and marking techniques were applied to 130 000 juvenile mulloway in batch mode with minimal mortality. A numerical model of the chemical behaviour of OTC in sea water describes the decline of available OTC in increasing salinity, so that a species’ salinity tolerance and successful marking can be optimized.