Workshop on age reading of Norwegian spring-spawning herring (Clupea harengus) (WKARNSSH)

The assessment working group (WGWIDE) for Norwegian spring-spawning (NSS) herring requested an age reading exchange to calibrate age reading. Issues that can bias the age reading were identified in a previous workshop, e.g. different calcified structures are used (otoliths and scales), stock mixing occurs during specific periods and areas, different interpretations of the first winter ring, or different interpretations of the new annual growth occurring in May. Therefore, the aim of this age reading workshop was to address potential age reading issues and thus minimize the bias associated with the age data. Further, this workshop aimed to evaluate if these issues have been resolved following the provided guidelines. Prior to this workshop, two exchanges with NSS herring otoliths (SmartDots event 447) and scales (SmartDots event 448) of the same individuals were conducted.

In total, 254 individuals were aged by 9 scale readings (6 advanced, 3 basic) and 18 otolith readings (10 advanced, 8 basic). Modal ages of otoliths and scales were compared directly as well as individual readings combined. The modal age of the independent exchanges resulted in 77.95% percentage agreement (PA) and an average percentage error (APE) of 3.71%. The comparison of all individual readings resulted in a PA of 76% and APE of 6%. The PA of all readers decreased from above 75% to below 50% at age 9 and older, where the otolith age is typically younger than the scale age. An age error matrix is provided that could be used in the stock assessment.

No differences have been observed between samples from varying quarters. Thus, the earlier proposed issue with identifying new growth in May has been resolved. The general understanding of age reading for both structures, otoliths and scales, is consistent among readers. However, other potential issues are highlighted in the report. Disagreement between scales and otoliths occurred especially for older individuals. It needs to be checked if this discrepancy will impact the stock assessment of NSS herring. Furthermore, in terms of age reading, stock mixing seems to be a minor issue and consequently, the age reading of stock-mixed samples will have no direct impact on the assessment of NSS herring. However, this needs to be investigated in more detail in the near future.