Validating and corroborating the deposition of two annual growth zones in asteriscus otoliths of common carp Cyprinus carpio from South Africa's largest impoundment.

A total of 816 common carp Cyprinus carpio asteriscus otolith pairs were collected from Lake Gariep, South Africa. Otoliths were interpreted whole, submerged in methyl salicylate and viewed under transmitted light. The precision of growth zone counts of the primary reader was estimated at 5·54 and 7·03% using the average per cent error method and the coefficient of variation, respectively. Age-bias plots indicated no systematic bias between the primary reader and the three secondary readers for up to nine growth zones (95% of the sample). Growth zone deposition rate was validated using a mark-recapture experiment of chemically tagged C. carpio (n = 21) conducted in a large earthen pond under ambient conditions in the vicinity of Lake Gariep. The validation results were corroborated for the wild population by edge analysis and a length-based age-structured model. All three methods suggest that growth zone formation occurred biannually, exemplifying the importance of age validation as a prerequisite for understanding the life history of C. carpio.