Updated estimate of the growth curve of Western Atlantic bluefin tuna

Abstract

The curve used until recently by the International Commission for the Conservation of Atlantic Tunas (ICCAT) to represent the growth of western Atlantic bluefin tuna, Thunnus thynnus, was estimated using tagging information and modal sizes that corresponded primarily to very young fish (ages 1–3, primarily). The estimated maximum average size from this curve is very large (382 cm), which could be a result of the scarcity of large bluefin in the data used. Recently, scientists have developed techniques for reading ages from bluefin ear bones (otoliths); the accuracy of the age readings has been validated with bomb radiocarbon dating. These age readings are primarily for large bluefin (ages 5 and older), and indicate slower growth and older ages than was previously assumed. However, an analysis of these data resulted in growth curves that predicted very small mean sizes for the youngest age group, which could be a result of the lack of small fish in the data used. In this study, we combine the otolith-based age readings with the size frequency distributions of small (ages 1–3) bluefin caught by purse seiners in the 1970s where the age groups are distinctly statistically as well as visible to the eye. We analyzed the two datasets jointly using a maximum likelihood approach and assumed that variability in length-at-age increases with age. The resulting growth curve predicts sizes at young and old ages that are very consistent with observed data such as the maximum sizes observed in the catch and the modal sizes for very young bluefin. The resulting curve is also very similar to the curve used by ICCAT for eastern Atlantic and Mediterranean bluefin.