Calibration of the fish ageing services readings carried out in GBYP Phase 7, to estimate age of bluefin tuna from the eastern Atlantic stock

Abstract

The Fish Ageing Services laboratory (FAS) was contracted by ICCAT GBYP in Phase 7 to provide age estimates from 2000 Atlantic bluefin tuna otolith samples. With the objective of ensuring that age readings provided by FAS follow the ICCAT reviewed reading protocol, a sub-sample calibration exercise was carried out. The findings show that band counts are similar between FAS and the group of laboratories involved in direct ageing. This is reflected by an acceptable precision between both readings. However, there is a one-year bias in the count of bands in older specimens, starting from 10-13 years of age, with a lower count by FAS compared to the rest of the laboratories. This bias seems to be due to the fact that FAS counts the bands in a different area of the otolith ventral arm than other laboratories. This counting discrepancy, although small, is significant and it would be necessary for FAS to reread the samples of specimens older than 10 years using the area close to the sulcus margin of the ventral arm.