Report of the 3rd Workshop on European hake age reading
A series of workshops and exchanges have been conducted in the last years in order to increase the precision of age estimation of European hake (1997, 1999, 2001, 2003). The last exchange in 2003 was undertaken mainly due to uncertainty in age estimation of older fish. In 2002 the ICES Working Group on Hake, Monk and Megrim (WGHMM) had shown difficulties in the assessment of hake stocks because of older ages primarily. This has led the WGHMM to continue to use a 8+ group. To address these problems a hake otolith exchange programme focused mainly on older fish was recommended between readersand subsequently an international workshop to discuss the results. Ages were estimated using internationally agreed ageing criteria, which have not been validated. The results of the 2003 otolith exchange programme indicate that the precision of age readings has decreased compared with the precision of the 2001 exchange and a strong bias has been found in estimates of older fish. The overall Average Percent of Error (APE) and coefficient of variation (CV) obtained in 2003 were 35 and 48% respectively while those obtained in 2001 were 19 and 25%, respectively. This loss of precision highlights the problems associated with applying ageing criteria, which are not validated. A second reading of a subset of the otolith collection used in the 2003 was done during the 2004 workshop. The results indicate that ageing fish older than 3 years is not possible with an acceptable level of precision. In addition, recent results from mark-recapture experiments have provided evidence that ages estimated using the internationally agreed criteria are overestimated (De Pontual et al., 2003). Taking into account the two aspects referred, the used of age readings data in stock assessment can introduce high uncertainty. Therefore, the workshop recommends to interrupt the supply of age reading data to elaborate Age Length Keys (ALK) for the WGHMM until validated ageing criteria become available.
