Report of the International Workshop on the Ageing of Skipjack Tuna from Indian Ocean

A recent study found differences in age estimates of skipjack (Katsuwonus pelamis) (SKJ) based on otoliths and dorsal fin spines obtained from the same fish caught in the Indian Ocean. This difference highlighted the need to develop standardized and validated aging criteria for both hard structures. In response, an international workshop was conducted in February 2023, with the objective of discussing and reviewing preparation protocols, reading criteria, validation methods to ensure consistent and comparable age data across laboratories. This document summarizes the key points of discussion and the progress achieved during the workshop on the following topics: 1) what constitutes annual growth zones in both the spine and otolith sections, 2) where are the inconsistencies between otoliths and fin spines readings, 3) assessment and adoption of edge type criteria for fin spine and otolith readings, and 4) age interpretation differences between readers. Protocols for fin spine and otolith preparation were reviewed and found to be consistent with previous studies on SKJ aging, with a few exceptions. The group agreed to revise the fin spine and annuli diameter measurements approach following an agreed age interpretation method. Results from a previous OTC mark and recapture study in the Indian Ocean suggested that daily increment counts in SKJ otoliths were unreliable. As the marked OTC otoliths have been prepared for daily age reading, they are not useful in directly validating the annual increments. However, the group suggested that the measurement of the amount of otolith growth after the OTC mark may be useful in verifying the annual increment widths and these samples have been selected for further investigation. A novel age algorithm was discussed to determine the fractional age using counts of opaque zones and otolith measurements. This algorithm uses the relationship between daily age and otolith size to estimate the age of the fish when the first opaque zone is deposited, which needs further investigation.  The group recognized the difficulty of assigning the edge type, particularly in otolith sections.  A more simplified set of edge type criteria was suggested. In addition, as a general criterion, only fully formed translucent and opaque areas on fin spines and otoliths, respectively, would be counted as rings. Participants identified crucial follow-up actions (tasks) that must be addressed as a stepwise process before any age estimates obtained from hard structures can be included in SKJ age-structured stock assessment models. The group also concluded that further age validation studies (e.g., using post bomb C¹⁴radiocarbon chronologies) would be highly beneficial for this species.