Report of the Workshop on Age Reading of Dab (WKARDAB)

The  Planning  Group  on  Commercial  Catch,  Discards  and  Biological  Sampling  (PGCCDBS) meeting, in 2008, identified dab (Limanda limanda) as a species requiring an exchange and ageing workshop to evaluate and improve the age interpretation based on whole otoliths. The Workshop can build on the results of the otolith exchange organized in 2009 and will be the first ageing workshop for dab.
One otolith set were included in the workshop: whole otoliths from 160 fish of the North Sea recovered during all seasons (January, April, July and November) of 2008 (40 fish per Quarter). Only digital images (Reflected and Transmitted light) are used.
6 countries (Germany, Belgium, Denmark, Netherlands, Irish Republic, France and UK England), including 12 readers, were participated at the exchange.
For dab, one of the main difficulties is the interpretation of the first annual ring, causing uncertainty among the readers during the exchange. Others difficulties were identified as the identification of few rings closed to the edge during the third quarter; Confusion with the hyalin zone and opaque zone as growth rings; Difference of used light (transmitted and reflected) and misinterpretation of the occurrence of split rings. During this workshop, the first ring was identified well with an average of 674 μm and a standard deviation of 146 μm.
A manual on the preparation and reading of dab otoliths has been compiled, documented with a reference set of annotated images. These documents can be used as a guideline and can form the template for discussion when refining the interpretation of the growth pattern and for identifying gaps and opportunities concerning the current knowledge of the age estimation of dab.
Mean precision of age estimate for individual fish were Coefficient of Variation (CV) of 12.0% and percent agreement to modal age of 79.3%. There were variations in precision of age estimate between individual fish, with CV ranging from 0 to 49% and percent agreement range from 45 to 100%.