A review of direct ageing methodology using dorsal fin spine from Atlantic bluefin tuna (Thunnus thynnus).

Abstract

This paper aims to fill the limited literature description of important areas of concern that may influence age estimates of ABFT using spine cross-sections methodology. A total of 3862 ABFT were sampled from the North East Atlantic and the Mediterranean Sea over 21 year period. The first spiniform ray of the first dorsal fin (spine) was removed and cross-sectioned near the condyle base in order to count annulus for age determination. The regression between spine diameter and SFL showed high determination coefficient indicating that the size of the spine and fish body were closely related. Results from edge type and marginal increment analysis confirmed an annual periodicity of annulus formation. In assessing spine nucleus vascularization, approximately half of the total number of inner annulus were missing. A new spine sectioning level showed a clear improvement respect to the “traditional” location reducing the influence of nucleus remodelling. Mean lengths at age obtained in this and previous studies using spines, show similarities, appearing a small disparity of only one year difference from age 7. Differences were observed when fitting the growth model to observed length at age or mean length at age data, illustrating the sensitivity of parameters estimation to the method employed. The estimated von Bertalanffy growth parameters ( L¥ =382.7; k=0.070 and t0=-1.334) were compared to those reported in the literature, showing some differences. The estimated L¥ is more realistic in relation with the maximum length found in the ABFT catches for the eastern stock and hence more reliable than the currently asymptotic length adopted for this stock. Our growth rate results indicate that spine sections can be used as an alternative to otoliths for ABFT age estimations.