Timing of Increment Formation in Atlantic Bluefin Tuna (Thunnus thynnus) Otoliths

Abstract

Controversies remain regarding the periodicity, or seasonality, of otolith growth band formation, which directly influences a correct age determination of Atlantic bluefin tuna using this structure. The aim of this work was to apply marginal increment analysis and marginal edge analysis to determine the timing of band deposition. The index of completion was analyzed using general additive models to evaluate the importance of variables, such as month, age/size, and reader. Results indicate that the opaque band formation begins in June and is completed by the end of November. From the end of the year to the beginning of the following year, there is minimal marginal edge growth as the translucent band begins to form. The translucent zone then reaches a maximum development in May. The results obtained in this study provide evidence that the annulus formation in the otoliths of Atlantic bluefin tuna are completed later in the calendar year than previously thought. This would mean it is necessary to delay the date of the current July 1st adjustment criterion to November 30.

Otoliths and Their Applications in Fishery Science

“…The authors validated the periodicity of ring formation, examined three age determination methods and calculated the representative growth models using a multimodel approach. The validation of aging is crucial because it directly influences the correct determination of age, as proved by Rodriguez-Marin et al [9]. The results obtained in their study provide evidence that the annulus formation in the otoliths of Atlantic bluefin tuna are completed later in the calendar year than previously thought.…”

Automated Monitoring of Bluefin Tuna Growth in Cages Using a Cohort-Based Approach

“…To substantiate the likelihood that the cohorts identified in July 2020 corresponded, at least partially, to annual cohorts, we compared our modal lengths with the expected body lengths as a function of age according to the well-known Von Bertalanffy growth equation applied to BFT [25,26,[28][29][30][31]. As depicted in Figure 6, the modal lengths aligned with the growth equations, suggesting that they likely represented annual cohorts between 5 and 16 years old.…”

“…Therefore, different results may be obtained depending on operator decisions. In this case, as a first step to obtaining reliable results, available information on BFT growth rates, both in the wild and under farming conditions [16,[25][26][27][28][29][30][31][32][33][34][35][36], were analysed to obtain reference values regarding the expected amplitudes of annual cohorts and the separation index between them throughout the life cycle. Moreover, to ensure the coherence among different analyses, in addition to the basic criteria to identify and characterise modal groups stated in [24], the following criteria were strictly applied:…”

“…As depicted in Figure 6, the modal lengths aligned with the growth equations, suggesting that they likely represented annual cohorts between 5 and 16 years old. Comparison of cohorts identified in July 2020 using the automatic system and Von Bertalanffy growth curves for BFT [25,26,[28][29][30][31] from various studies, aimed at corroborating the correspondence of the cohorts identified in July 2020 with annual cohorts.…”

“…When examining the frequency histogram in Figure 7a, it appears that the distribution shifts towards larger widths (A) from July 2020 to May 2021, i.e., suggesting an Figure 6. Comparison of cohorts identified in July 2020 using the automatic system and Von Bertalanffy growth curves for BFT [25,26,[28][29][30][31] from various studies, aimed at corroborating the correspondence of the cohorts identified in July 2020 with annual cohorts.…”