Report of the Workshop on Age estimation of Whiting (Merlangius merlangus) (WKARWHG2). 22-25 November 2016 Lowestoft, UK
|Type||Technical document (specification, manual)|
|Ref.||ICES CM 2016\SSGIEOM:15|
|Contributor(s)||Mahe Kelig, Chantre Celina|
|Note||ICES WKARWHG2 REPORT 2016 SCICOM/ACOM STEERING GROUP ON INTEGRATED ECOSYSTEM OBSERVATION AND MONITORING|
|Abstract||Based on the results of a full-scale otolith exchange held in 2015 (Smith, 2015) The Working Group on Biological Parameters (WGBIOP) identified the need for an age reading workshop on whiting otoliths (WKARWHG2). This workshop was hosted by Centre for Environment Fisher-ies and Aquaculture Science (Lowestoft, UK 22–25 November 2016. Seventeen age readers from nine countries (Belgium, France, Denmark, Norway, Germany, UK, Northern Ireland, Southern Ireland, and Scotland) participated in the workshop. The workshop was chaired by Joanne Smith (UK) and Suzy End (UK) acted as a Workshop supporting expert.
Two otoliths sets, an exercise set of 105 otoliths and a subsample set of 50 otoliths from the orig-inal exchange otolith set were aged during the workshop. The exercise set were read first, to highlight any issues/disagreements between age readers and the possible reasons for these. Fol-lowing recommendations from WGBIOP otoliths from area ICES Division 4b were included in this exercise as this area was not covered in the original exchange set. Readers had the option of ageing the otolith using both an image and the actual otolith under a stereomicroscope. Only a small number of readers chose to use both methods, most choosing to age using images only. After the exercise set was read, the results were presented and differences between interpreta-tions were discussed.
In addition, a small group of experienced section and whole otolith readers carried out a reading exercise (20 otoliths) to compare the percentage agreement obtained by readers using different otolith preparation methods. Since the results from this small exercise were not encouraging, the conclusion from this exercise was that action needed to be taken to ensure that agreements re-main high regardless of preparation method used. Also, there were few examples from 4a in the original exchange, which is an area used by Norway, Denmark, and Scotland The additional mini exchange will provide them with a more complete dataset to work with which should help with future exchanges. Post workshop, readers who routinely read whiting otoliths, whole or broken, agreed to conduct an otolith exchange in an attempt to clarify, the level of agreement between these readers and where disagreements occur, the possible reasons for these.
The group also carried out discussions on sectioned vs. whole otoliths as reading methods for whiting. The main conclusions from the exercise and the discussion on whole vs. sectioned otoliths were (Appendix VIII):
• There can be difficulties interpreting the first annual ring due to splits and the wide range of growth that can occur;
• The edge can often be misread causing under/over age estimations;
• Misinterpretation of split rings and Humphries shadow can lead to over ageing of the otolith;
• If the otolith is not cut correctly it can often cause readers problems interpreting true rings.
Subsequently, the subsample of exchange otoliths was re-read to examine if the discussions throughout the week had led to improvement in the consistency of age reading. For this exercise it was agreed that only a subsample of the exchange otoliths (50 otoliths) would be read, to allow more time during the week to be spent on discussions. The subsample set was selected using the following criteria:
• The age range which occurred in the exchange was between 1 and 8 years, but since only one otolith was at age 1 and two otoliths were at age 8, all three of so these were included;
• The remaining otoliths were selected across the age range 2–7 years;
• Two otoliths from each quarter were selected, one with high percentage agreement and one with low;
• After this selection, nine otoliths were required to complete the set so these were se-lected from ages 2–3 and 6–7, selected as above, representing problematic ages.
The results of this exercise did show an improvement in age reading compared to the same 50 read in the 2015 exchange.
No validation studies have been carried out for whiting age reading as of yet. We propose an otolith chemistry study to validate the true deposition of opaque and translucent material throughout the otolith. WKARWHG2 strongly recommends such a study, as the results will fa-cilitate resolving the most frequent problem encountered when ageing whiting, namely the split rings/Humphry shadow’.