Applying mixed-effects growth models to back-calculated size-at-age data for Atlantic bluefin tuna (Thunnus thynnus)

We fit growth models to back-calculated size-at-age data for Atlantic bluefin tuna (Thunnus thynnus) captured in the western Atlantic management area to inform alternative growth scenarios within the current management framework of the species. The Modified-Fry function was the best-performing of three back-calculation functions based on a leave-one-out cross-validation for within-cohort comparisons. We fit multiple growth models to the back-calculated growth trajectories, and demonstrated that growth parameters were highly sensitive to whether the lack of independence between back-calculated lengths from each individual was accounted for by the model. Non-linear mixed-effects modelling provides a suitable approach for accounting for this lack of independence and the autocorrelation between back-calculated lengths from the same individual. We further demonstrated the utility of mixed-effects models for predicting future growth of individuals using a forecasting test. We used this modelling approach to demonstrate that male bluefin tuna had greater estimated asymptotic length than females, and found that there was no significant difference in growth parameters between individuals assigned to genetically distinct spawning stocks.1,58

REPORT OF THE 2021 ICCAT SWORDFISH BIOLOGY WORKSHOP (Online, 22-26 March 2021)

“The results, conclusions and recommendations contained in this Report only reflect what was discussed by the Group of scientists that participated in the 2021 Swordfish Biology Workshop. Therefore, these should be considered preliminary until the Swordfish Species Group and the SCRS discuss and possibly adopts them at its annual Plenary meeting and the Commission revise them at its Annual meeting. Accordingly, ICCAT reserves the right to comment, object and endorse this Report, until it is finally adopted by the Commission.”En prens

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

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

Report of the ICCAT GBYP international workshop on Atlantic bluefin tuna growth

In the last Atlantic bluefin tuna assessment, an age-length database coming from direct ageing was presented for the first time. It was observed that otolith age estimates for fish younger than 8 years old had a smaller size at age compared to spine (first dorsal fin radius) age estimates. This difference, although small, was enough to misallocate the year class. This misallocation was solved when introducing a vector of bias corrected aged otoliths based on paired otolithspine samples. We have identified two possible causes for over-estimating age in the otolith agelength data: the current age adjustment criterion (to convert the bands counting into ages) and a reading bias in age estimations from some laboratories. Otolith preparation and reading protocols have been reviewed. The edge type and marginal increment analysis showed that the formation of opaque zones would seem likely to occur primarily between December through to June, contrary to what was thought until now, for which a new criterion for age adjustment has been proposed

New protocol to avoid bias in otolith readings of Atlantic bluefin tuna juveniles

dentification of the first annual increments in otoliths of Atlantic bluefin tuna juveniles is difficult due to the frequent appearance of numerous sub-annual bands which can be easily misinterpreted. Instead, the pattern of deposition of the annuli on the first dorsal fin rays (spines) is very distinct during the first years. Furthermore, the bias vector for correcting otoliths age, and developed from paired otolith-spine samples, allowed a better tracking of cohorts in the 2017 assessment. With this background, 262 paired structures with identical age estimates were used to revise reading criteria for annual increments of juvenile bluefin otoliths and to create a template. A table with first annuli measurements was built to serve as a reference and help identifying the first otolith annual bands. For this, it was necessary to establish the location of origin of the measurements (anchor point) and define them. In addition, otoliths from this set of paired structures can be used as a collection that will serve to test the new reading criteria and template

REPORT OF THE 2021 ICCAT SWORDFISH BIOLOGY WORKSHOP (Online, 22-26 March 2021)

“The results, conclusions and recommendations contained in this Report only reflect what was discussed by the Group of scientists that participated in the 2021 Swordfish Biology Workshop. Therefore, these should be considered preliminary until the Swordfish Species Group and the SCRS discuss and possibly adopts them at its annual Plenary meeting and the Commission revise them at its Annual meeting. Accordingly, ICCAT reserves the right to comment, object and endorse this Report, until it is finally adopted by the Commission.”S