Bias, precision and validation of ageing 0+ European barbel Barbus barbus (L.) from their otoliths

The European barbel Barbus barbus L. is considered a ‘flag’ species for river conservation and sport fishing, but it is increasingly threatened in its native range of distribution. To provide accurate age estimates during early life for appropriate management and conservation measures, the bias and precision of otolith (daily) micro-increment counts were evaluated and age determinations validated on laboratory-reared embryos and larvae. Out of the three pairs of otoliths, the lapillus and sagitta provided reliable age estimates for free embryos and larvae up to 17 days of (known) age post-fertilisation, with first micro-increment formation occurring five days postfertilisation. On the other hand, micro-increments on asterisci formed only 16–17 days post-fertilisation. There was agreement in microincrement counts based on lapilli and sagittae, but not between interpreters, indicating that despite consistency between the two pairs of otoliths extensive training and experience are required for reliable age interpretation. The ability to estimate the ages of 0+ B. barbus from their otoliths will contribute to a better understanding of growth rates from both hatchery-stocked and native/introduced cohorts.