Report of the Workshop on Age estimation of European hake (WKAEH)

In 2008, ICES PGCCDBS recommended conducting a Workshop on Age Estimation of European Hake (WKAEH). The WKAEH took place in Vigo, (IEO) Spain, from 9 to 13th November, 2009; (Chair: C. Piñeiro (IEO, Spain), Co-chair: H. de Pontual (IFRE-MER, France). The workshop was preceded by an otolith exchange between laboratories involved in the assessment of the hake stocks, in order to identify the current ageing problems arising between readers and to evaluate the state of art of age estimation according to the validation studies conducted so far. Readers from 8 research institutes (AZTI, IPIMAR, CEFAS, IM, IFREMER, IEO, AFBI NI, and VTI-DF) participated in the exchange, for which the aims were:

1. To evaluate the age estimation errors (accuracy and precision) based on a reference collection (otoliths marked with oxytetracycline).
2. To subsequently evaluate the relevance of the ageing method traditionally used to provide ALKs for stock assessment purposes.
3. To inter-calibrate readers, specifying the interpretation differences (ring positions).
4. To progress in the implementation of quality control and quality assurance(QC/QA) in the labs.
   

The exchange collection consisted of digital images of otolith sections from 104 tagged fish recovered during all seasons and for which size at recapture ranged be-tween 25 cm and 67 cm. The exchange program and workshop were carried out following the recommendations of the EFAN (European Fish Ageing Network) Report 3-2000 on Guidelines and Tools for Age Reading Comparisons (Eltink et al., 2000).
Given the relevance of the WKAEH for the assessment of hake stocks, and its terms of reference (TORs), a presentation session was organized during the workshop to promote a wide discussion on ageing and growth issues that are presently compromising the quality of Northern and Southern hake assessment (2010 Benchmark).The workshop was divided into four parts: (i) Reporting on the results of the exchange programme 2009; (ii) A presentation session of recent studies related with growth and complementary information on the species conducted so far; (iii) Working in two subgroups based on the expertise: Age Readers (G1) and Growth, Modelling /Assessment experts (G2); (iv) Plenary sessions for presenting and discussing the results of both subgroups
A number of six calibration exercises were undertaken for and during the workshop where a group of 15 readers participated. The overall results of the workshop confirm that the previous internationally agreed ageing criteria are neither accurate nor precise and provide overestimation of age. Interpretation of tagged material resulted in a general shift towards younger ages (from 0-10 to 1-5 years) for the same otolith/fish collection. This raises concern about the use, for previous stock assessments, of ALKs that were inaccurate and demonstrates the need to develop approaches allowing the integration of a "validated" growth model or age reading errors into the stock assessment model.
In general the comparison among the results of the exercises shows that irrespective of experience in reading hake otoliths, the age interpretation of hake otoliths was made easier when tagging information was available, improving the precision and agreement among readers in contrast with the age interpretation without tagging information. The complexity of age estimation of European hake remains and validated material is essential to further develop a new method of otolith interpretation for the age estimation of the species. A preliminary set of guidelines have been established to help the interpretation of otoliths but it will required further refinement using younger and older marked fish to study the structural growth pattern of the otolith.
Results on different approaches considered to incorporate estimated ageing errors in Age-Length keys (ALK) due to the wrong criteria of otolith interpretation for the species showed some promise but further work on growth models will be necessary and it was suggested to continue this work until the 2010 ICES benchmark. It was recommended to continue working on the analysis of tagging data, ‘daily’ ring counting and age readings to: (i) estimate a growth model or, (ii) develop an error transition matrix between ages identified with previous protocol and ages identified with tagged otoliths or daily ring counts. Both approaches would allow the integration of a growth model or age reading errors into the stock assessment model.
The workshop achieved quite a lot in terms of demonstrating that hake is a much faster growing species than was previously believed and recognised the necessity of working together towards a solution to improve the accuracy and precision of ageing for the assessment. The calibration exercises and general discussions proved positive, by bringing stock assessors, otolith readers and research scientists together, in order to identify the issues and associated consequences of age estimation of hake and to propose some clues to settle this matter.
Another workshop was recommended in three years to continue promoting standardization of methodologies and practices for age estimation of hake built on the work undertaken during the WKAEH 2009. It should be noticed that the availability of otoliths from a wider size/age range of tagged material is very necessary for this work.