Assessing the impact of the growth on estimates of fishing mortality for the Indian Ocean bigeye tuna

In 2021, a new growth estimate for bigeye tuna in the Indian Ocean was derived based on otolith aging studies. The new growth estimates represent a size-at-age that is significantly larger than the growth currently used for bigeye tuna stock assessment. This is expected to have a significant impact on the assessment results if included in the model. This report aims to assess the potential impact of the new growth on the estimates of fishing mortality for bigeye tuna by performing an analysis of length composition data based on the assumption that the length distribution is primarily determined by fish growth and mortality. Assuming that growth and natural mortality are known, the analytical method estimates fishing mortality rates and selectivity parameters from the longline length freqeuncy dataset (assuming each length abundance is a steady distribution). The performance of the estimator was validated using simulated data. The analysis shows that longline length frequency data suggest that estimates of annual fishing mortality for new growth are 2–3 times higher than the current growth estimates for bigeye tuna.