Based on the previous work from WKARHOM3 (ICES, 2018), the Working Group on Biological Parameters (ICES, 2020) identified the need for a new otolith exchange followed by an age read-ing Workshop. The Workshop on Age reading of Horse Mackerel, Mediterranean Horse Macke-rel and Blue Jack Mackerel (Trachurus trachurus, T. mediterraneus and T. picturatus) (WKAR-HOM4) had several main objectives; to review the current protocols of ageing Trachurus species, to update the advances in the validation of the ageing criteria (i.e. the annual deposition of one annulus, coherency of the interpretation), to evaluate the new precision of ageing data of Tra-churus species and to update guidelines, common ageing criteria and reference collections of otoliths. An online otolith exchange was performed using SmartDots during 2021 and 2022, and results including the three Trachurus species were published in advance of the meeting, showing a low Percentage of Agreement (PA) both when considering all the readers (44-55%) and the advanced readers only (52-54%) (Massaro and Jurado-Ruzafa, 2022). For T. trachurus the Coef-ficient of Variation (CV) was lower for the sliced samples (22-18%) than for whole otoliths sam-ples (44-38%). Readers participating in the exchange, following discussion during the WKAR-HOM4 meeting, agreed that the main cause of age determination error for T. trachurus was due to the different otolith preparation techniques (whole/sliced). These differences reflect the stunted growth and compactness of the annuli in older specimens (from the 4th-5th annuli on-wards). Anyway, for the three Trachurus species, there are several difficulties in age determina-tion: identification of the first growth annulus, presence of many false rings (mainly in the first and second annuli) and the interpretation and identification of the edge characteristics (opaque/translucent). The second reading exercise was performed during the workshop orga-nized in four different events (i.e. Trachurus trachurus whole otoliths -135 images-, T. trachurus sliced otoliths -95 images- T. mediterraneus whole otoliths -150 images- and T. picturatus whole otoliths –121 images). For T. trachurus no enhancement among readers’ precision was observed in all cases, in terms of PA and CV. Conversely, for T. mediterraneus and T. picturatus a notice-able improvement in terms of PA and decrease of CV occurred compared to the pre-workshop exchange. Finally, this group updated the ageing guidelines and a reference collection of images for all the species, with the aim to employ these tools for all laboratories.

ICES

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