Age readings of Sebastes marinus otoliths: bias and precision between readers and otolith preparation methods

This study presents a comparative age reading on Sebastes marinus from the Icelandic shelf within an otolith exchange program between institutes in Germany, Iceland and Norway. Out of a series of 212 otolith pairs, one otolith of each pair was prepared by the break-and-burn technique, while the other otolith was used for crosssections. Age reading results are compared between readers and otolith preparation methods in terms of bias and precision, using a set of statistical tests and graphical methods. Significant bias was observed for both the comparison between readers and between methods, mainly caused by deviations between age scores in the higher ages (> 20 years). Precision estimates, involving the high longevity of redfish, were relatively good compared to previous age reading comparisons using other species. In contrast, the age dependent percent agreement was poor (< 30%) for a tolerance level of ± 0 years, particularly for the age range 21-30 years. A tolerance level of ± 3 years, however, lead to around 90% agreement for the age range up to 20 years. The fit of age reading scores with the von-Bertalanffy growth curve was relatively good, providing growth parameters comparable to S. marinus from the Norwegian shelf. The observed problems in bias and precision of age readings should to be improved by continuing with similar Sebastes otolith exchange programs and setting up a further age reading workshop to harmonise the interpretation of growth structures