Report of the Workshop on Age Determination of Redfish (WKADR)
The WKADR held in Nanaimo/Canada 2-5 Sep 2008 was a follow-up of the 2006 workshop with a specific focus to analyze the results of the otolith exchanges carried out in 2007-2008 and to identify the sources of error in age determination. In addition, improvements in age reading methodology and quality assurance/quality control of redfish age reading were discussed based on working examples. The workshop showed significant progress in the correspondence between readers,
The workshop showed significant progress in the correspondence between readers compared with previous workshop results. Also there was a clear improvement in agreement between the exchange exercise and the readings during the workshop after discussions, especially when using only clear-pattern otoliths and comparing the most experienced readers. Good quality otoliths with good readability showed only small differences among readers and demonstrated that in general there are no differences in age reading criteria among readers. This fact highlighted the importance of using a confidence index for each reading. The workshop encourages to continue the implementation of QA/QC in the different laboratories involved in age reading of redfish. For stock assessment and regular precision monitoring, a confidence index is proposed.
Only otoliths with good or fairly good confidence index should be used for comparisons and discussion on ageing criteria among readers. However, it remained unclear if only those otoliths should be used for production ageing, especially for stock assessment purposes, as these clear-pattern otoliths may correspond to given ages or cohorts and hence may bias the age-length key and the stock age composition if only those otoliths are used. It was also clear that several of the observed differences are considerably reduced when using a high-quality binocular equipment, prepared for large magnification and with high quality light especially for older otoliths (>30 yrs). Standardization of the equipment, and its quality, promotes reproducibility between readers and agencies.
Some differences for certain criteria were detected in the position of the first few annuli, the identification of several checks, especially during the first 3-4 years, and the interpretation of the transition zones. Although discrepancies could be due to species/stock-specific differences, it was agreed that each country should measure the first three annuli in 10 otoliths with very clear patterns for a number of stocks, and containing several year classes. The workshop recommends to create a reference col-lection within each lab for their specific stocks, and their pictures should be transferred to an image collection to be uploaded on a webpage with sufficient metadata. These differences in interpretation very likely are derived from each reader being familiar with some species or stocks. To overcome this problem, it is recommended to continue with the harmonization of the age reading within and across stocks and species following the guidelines, and through short-term exchanges among labs. To harmonize age reading within stocks, short-term exchanges should also be conducted among labs determining age of the same or related stocks
The break-and-burn technique was established as preferred otolith preparation method for redfish. Several recommendations are given with regard to the improvement of lab procedures, the continuation of age reading harmonization and small-scale otolith exchanges. An updated glossary and protocol for handling and age determination of redfish otoliths is given in the annexes.
Published under the auspices of the following ICES Steering Group or Committee