Report of the Workshop on Age Reading of Turbot (WKART)

The Planning Group on Commercial Catch, Discards and Biological Sampling (PGCCDBS) meeting in Valetta, Malta in March 2007, identified turbot (Psetta maxima) as a species requiring an ageing workshop to evaluate and improve the age interpretation based on stained slides of the otoliths. The Workshop can build on the results of the otolith exchange organized in 2004 and will be the first ageing workshop for turbot.
Two otolith sets were included in the workshop: a North Sea turbot set (N=110), and a Baltic turbot set (N=96). Thirteen readers out of six countries attended the workshop namely Belgium (3), France (2), Germany (2), The Netherlands (2), UK (1), Sweden (2) and Latvia (1). Unfortunately, the Baltic otolith set has been lost after the exchange and only the photographs remained as the basis for further investigation.
For turbot, one of the main difficulties is the interpretation of the first annual ring, causing uncertainty among the readers during the exchange. Because validated otoliths or agreed reference collections do not exist at present, the final debate of whether or not the first ring is indeed the first annual ring is still ongoing. The work‐shop therefore dedicated its effort to conclude to a common interpretation of this particular ring and thus improve the agreement among readers.
A manual on the preparation of turbot 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 turbot.
The two regions, North Sea and Baltic Sea, are treated separately as the particularities between the datasets from the regions are too different. Also, these areas comprise different stocks so that the combination is clearly to be avoided.
The overall agreement rate of the North Sea sample was 82.8%. The range of agreement with the modal age was 70.5–91.1%. The overall agreement rate of the Baltic sample was 71.6%. The range of agreement with the modal age was 55.8–87.4%. The lower score for the Baltic area originates mostly from the poor quality of the image set. Furthermore, the Workshop participants were formed out of two clearly separated Expert Groups i.e. the North Sea and the Baltic Sea. Using results from age readers who are not familiar with the selected area, clouds the agreement within the area.
The overall results for this first turbot workshop are positive. For the North Sea area, expert readers for that area can reach an agreement of more than 90%. This indicates that the age estimation of turbot can be highly precise when the agreed interpretation is used. For the Baltic area, the results are more in the range of 70–80% but this is probably caused by the poor quality of the images and the reduced quality of the dataset especially the lack of younger ages.
The WK used ORAcle for analysing the results which is an improved version of the Eltinck spreadsheet and has been evaluated by the PGCCDBS 2008.