Ageing of Adult Snapper (Pagrus auratus) from Otolith annual ring counts: Validation by tagging and Oxytetracycline Injection

Abstract

Of 4657 snapper (Pagrus auratus) that were tagged, injected with oxytetracycline (OTC), and released in 1986-87, 1113 fish were recaptured after periods at liberty from 18 days to 3 years. An OTC mark was clearly visible in thin sections of all but four of a sample of 125 otoliths. In 90% of the remaining 121 otoliths the position of the OTC mark was correctly inferred from age readings (made without seeing the OTC mark) based on annual ring counts in otolith sections. It is concluded that the otolith rings are deposited annually, and estimates are made of error rates in age estimation-both in identifying annual rings and in interpreting the otolith edge. Further support for the ageing method is contributed by an analysis of the growth of tagged fish, and by year-to-year consistency in the relative strength of cohorts. The growth analysis showed a large (more than twofold) interyear variation in growth rates. Snapper (and other sparid) age estimates based on scale readings may be underestimates. It is recommended that age validation studies clearly distinguish between three separate factors: periodicity of marks, identification of marks, and edge interpretation.