Investigating growth information for yellowfin and bigeye tuna from the IOTTP tag-recapture data

Previous growth models that were estimated for yellowfin (Thunnus albacares) and bigeye (T. obesus) tuna using tag-recapture data from the Indian Ocean Tuna Tagging Programme (IOTTP) suggested both species have a phase of slow growth as juveniles, followed by a phase of faster growth (Eveson et al. 2015; Dortel et al. 2015). One of the drawbacks of using tag-recapture data to model growth is that the age of a fish at release is unknown. These models deal with the problem by modelling the age at release as a random effect.
In 2020, the European Union and the Indian Ocean Tuna Commission (IOTC) supported a project to develop new estimates of age and growth for yellowfin and bigeye tuna in the Indian Ocean. The aim was to follow methods recently developed by Farley et al. (2017; 2020) for bigeye tuna in the Western Central Pacific Ocean (WCPO) to estimate the age and growth of yellowfin and bigeye tuna from counts of daily and annual growth zones in otoliths. Using the relationship between the daily age estimates obtained from this project and fish length, the age at release for fish in the tag-recapture data could be estimated from their release lengths. The resulting age estimates are very different than those obtained from the random effects models. Here we present these new findings and discuss potential reasons for the differences.