Report of the Workshop on Age Reading of North Sea (IV) and Skagerrak-Kattegat (IIIa) Plaice (WKARP)
|Ref.||ICES CM 2010/ACOM: 45.|
|Author(s)||ICES Advisory Committee on Fishery Management|
|Contributor(s)||Mahe Kelig, Elleboode Romain|
|Abstract||The Workshop on Age reading of North Sea (IV) and Skagerrak-Kattegat (IIIa) plaice [WKARP] (Chair: Loes Bolle, The Netherlands) was held 2-5 November 2010 in IJmuiden, The Netherlands. The previous North Sea plaice workshop was held in 2003, and no information is available on when the last workshop was held for Skagerrak-Kattegat plaice. Therefore PGCCDBS called for an otolith exchange (2009-2010) followed by an age reading workshop (2010) for these 2 plaice stocks. The pri-mary aim of the exchange and workshop was to identify and resolve interpretation differences between readers and laboratories.
Nine countries and 20 readers participated in the exchange. The same 9 countries and 14 readers participated in the workshop. The exchange and workshop otolith sets consisted of IIIa and IV otoliths. The readers represented a broader geographical range (III, IV, VI & VII stocks). Some laboratories use whole otoliths, while others use transverse sections for plaice ageing. Therefore both preparation methods were in-cluded in the exchange and workshop sets (i.e. a different method was used for each otolith of a pair).
Comparison of pair-wise age readings and overall age compositions showed no bias related to preparation method if age is less than 10. A small sample of 10+ otoliths indicated an underestimation of age in whole otoliths. A larger sample (from differ-ent stocks) is required for a better evaluation of preparation methods in older fish (for different stocks).
To identify and resolve interpretation differences, the results and annotated images from the exchange were discussed during the workshop. Differences in interpretation mainly stemmed from whether or not to expect regular growth patterns, i.e. whether or not to apply the rule of the thumb that every annulus is wider than the next annulus. This difference in interpretation was most prominent for the first annulus. Growth increment analyses were carried out to examine this issue in more detail, but the differences were not resolved. Consequently, the agreement between readers did not improve for a new set of otoliths which was read during the workshop. The ma-jority view is not necessarily the correct way to interpret growth structures in plaice otoliths (for all stocks). The only way to prove who is right is to carry out validation studies.
WebGR, the web tool to aid ageing and maturity staging workshops, was used to create an agreed age reference collection with annotations. WebGR was considered to be very useful, but the implementation was hampered because it requires experience in using the tool, which most age readers and coordinators do not have. WebGR training sessions for age readers and age coordinators is proposed. It is furthermore recommended to enlarge the plaice reference collection in WebGR, including both agreed otoliths as well as otoliths subject to interpretation differences.
The group also reviewed calibration work done so far, collated information on na-tional procedures, created an international age reading manual and formulated tar-get/threshold statistics and follow-up actions.
In general, regular workshops (at a 3-5 year interval) are recommended. For plaice, the next exchange is proposed for 2012 and the next workshop for 2013. Goal is to pursue the unresolved issues of the 2010 workshop and to maintain and further en-hance international calibration and cooperation between age readers for plaice stocks in ICES areas III, IV, VI and VII.