CALIBRATION OF ATLANTIC BLUEFIN TUNA OTOLITH READING CONDUCTED BY AN INDEPENDENT FISH AGEING LABORATORY CONTRACTED BY THE ICCAT RESEARCH PROGRAMME GBYP

Abstract

A calibration exercise was carried out with the objective of ensuring that age readings provided by the Fish Ageing Services laboratory (FAS) follow the ICCAT reviewed reading protocol. There were found differences in band counts between ICCAT expert readers and FAS readings. These differences start from specimens with more than 10 bands and are more pronounced for older specimens. The results of the present calibration are very similar to those of the previous one. These differences in readings appear to be due to the fact that FAS uses the entire section of the otolith to count annual bands, whereas ICCAT readers focus on the inner part of the ventral arm. Analyses conducted to establish which reading is more appropriate: growth function estimation and cohort follow-up analysis, seem to indicate that ICCAT readers are more accurate than FAS readers. We suggest applying a correction vector to otoliths read by FAS and with more than 10 annual bands, in order to incorporate the FAS readings into the GBYP age database. This ageing bias vector has been obtained from the current and previous calibrations.