Workshop on Age Reading of Blue Whiting (WKARBLUE3; outputs from 2021 meeting)

The workshop on age reading of Blue whiting (WKARBLUE3) was conducted to go through the following objectives: review, document and make recommendations on current methods of aging blue whiting (Micromesistius poutassou) to produce the input data for this species stock assessment.

The workshop was preceded by an otolith exchange on SmartDots (Event ID 278). The exchange included 407 otolith images, 190 from the northern areas (ICES divisions 2.a, 4.a, 5.a, 5.b, 6, 6.a, 6.b, 7 and 7.c) and 217 from the southern areas (ICES divisions 9.a and 8.c). A total of 27 readers from 11 countries participate in the exchange, 8 where from the southern areas and 19 from the northern areas. The overall agreement with the modal age from the pre-workshop exercise was 66%. Although, considering only the results from the northern areas by the advanced readers that usually read the otoliths from those areas for the assessment, the percentage of agreement was 69%. In relation to the otoliths from the southern areas, the percentage of agreement on age classifications by the advanced readers that read for the assessment was 79%.

The pre-workshop exchange results reveal that readers are prone to be off in interpreting the age of fish in other areas which they don’t normally read for the stock assessment. The reason to this discrepancy is that blue whiting presents a wide distribution along the Northeast Atlantic with growth variations between areas. The blue whiting from the northern areas of the stock (North of the British Isles) has a slower growth than in the more southern areas (south of the British Isles). The otoliths from some of the northern areas (2.a, 5.b, 6.b and 7.c) were more difficult to read. These results were also observed in the data analysis when only the readers that usually read the otoliths from the northern areas were included. 

During the workshop a new small exchange was also conducted, which included 22 otoliths from the southern areas and 33 from the northern areas (SmartDots EventID 347). In this new exchange the readers only read the otoliths from the areas they usually read from in the assessment. The results showed a slightly increase in the percentage of agreement, with a 78% of agreement in the otoliths from the southern areas and a 71% in the otoliths from the northern areas. 

The other main issues identified during this workshop were the position of the first annual growth ring, false or split rings and interpretation of the edge. These issues are the same as has been mentioned in previous reports and workshops, and thus a reoccurring problem among age readers. A reference collection of images with annotations will be uploaded to SmartDots as soon as the working group on SmartDots governance (WGSMART) has the reference collection module ready. 

The realization of age reading workshops on blue whiting constitute an advantage and are recommended to be conducted on a regular basis, to increase the precision on aging between readers. Nevertheless, since the main issues on aging blue whiting still remain, validation studies applied to this species are also recommended and needed to allow the increasing of accuracy on age classifications.