Report of the Workshop on Age reading of Dab (Limanda limanda) (WKARDAB)
Based on the results of a small-scale otolith exchange held in 2013 (Haslob, 2013) the ICES Planning Group on Commercial Catches, Discards and Biological Sampling (PGCCDBS) identified the need for an age reading workshop on dab otoliths (WKARDAB2). This workshop was hosted by the Thünen Institute of Sea Fisheries (Hamburg, Germany) 17–20 November 2015. Eight age readers from five countries (Belgium, The Netherlands, Iceland, Denmark and Germany) participated in the work shop. The workshop was chaired by Loes Bolle (The Netherlands) and Holger Haslob (Germany).Two otoliths sets (exercise 1 and 2), both consisting of whole and sectioned otoliths, were aged before the workshop and on the first day of the workshop. The whole oto-liths were aged before the workshop using WebGR, the sectioned otoliths were aged at the workshop using stereomicroscopes.
Two otoliths sets (exercise 1 and 2), both consisting of whole and sectioned otoliths, were aged before the workshop and on the first day of the workshop. The whole oto-liths were aged before the workshop using WebGR, the sectioned otoliths were aged at the workshop using stereomicroscopes.
After exercise 1 and 2 were completed, differences in interpretations were discussed by projecting images on the screen. Images of whole and sectioned otoliths from exer-cises 1 and 2 were discussed and, in addition, images of neutral-red stained sections, supplied by Denmark and Belgium, were discussed. The main conclusions were:
• Stained sections appear to be a promising way to age dab otoliths.
• The biggest problem in the interpretation of dab otoliths is the edge of the otolith. Especially in the case of a translucent zone on the edge of the otolith in the early part of the 3rd quarter, there were clear discrepancies in the inter-pretation.
• Split rings often occur in dab otoliths, but this did not appear to be a major problem within the current group of experienced readers. In most cases, the whole group agreed on the identification of split rings.
Subsequently, exercise 3 was carried out to examine if the discussions had led to im-provement in the consistency of age reading. For this exercise a new otolith set was used (consisting of whole and sectioned otoliths), that had been prepared prior to the workshop in WebGR. Unfortunately, WebGR failed during the workshop and the group had to switch to real material and stereomicroscopes. The results of exercise 3 did not show an overall improvement in the consistency of age reading.
The discussion on stained sections indicated the need to compare whole and stained sectioned otoliths in a calibration exercise. Images were made available at the work-shop and it was attempted to initiate a 4th exercise. However, this failed again due to problems with WebGR. Therefore this exercise, in an elaborated form (include 3 meth-ods: whole, sectioned and stained sectioned otoliths; include otoliths from 2 periods and several regions/countries), is now proposed as follow-up action.
No validation studies have been carried out for dab age reading yet. We propose a marginal increment study, to validate the timing of the deposition of opaque and trans-lucent material on the edge of the otolith, as a second follow-up action. The results of such a study will help resolve the encountered problems with the interpretation of the edge of the otolith.
Published under the auspices of the following ICES Steering Group or Committee
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