Direct ageing of Thunnus thynnus from the eastern Atlantic Ocean and western Mediterranean Sea using dorsal fin spines

Abstract

This study deals with important methodology issues that affect age estimates of eastern Atlantic bluefin tuna Thunnus thynnus using dorsal fin spines. Nearly 3800 spine sections were used from fish caught in the north-east Atlantic Ocean and western Mediterranean Sea over a 21 year period. Edge type and marginal increment analyses indicated a yearly periodicity of annulus formation with the translucent bands (50% of occurrence) appearing from October to May. Nucleus vascularization seriously affected specimens older than 6 years, with the disappearance of 40-50% of the presumed annuli by that age. An alternate sectioning location was a clear improvement and this finding is an important contribution to the methodology of using this structure for ageing the full-length range of eastern T. thynnus. Finally, there were no significant differences between the coefficients of von Bertalanffy growth model estimated from mean length at age data (L∞ = 327.4; k = 0.097; t0 = -0.838) and those estimated from the growth curves accepted for the eastern and western T. thynnus management units.

Growth of bluefin tuna (Thunnus thynnus) in the North-eastern Atlantic and Mediterranean based on back-calculation of dorsal fin spine annuli

“…Specimens caught in the North East Atlantic ranged from 53 to 287 cm SFL whereas specimens caught in the Mediterranean Sea ranged from 45 to 251 cm SFL (Table 1). This extensive data set has been partially used in some ICCAT scientific documents (International Commission for the Conservation of Atlantic Tunas, Collective Volume of Scientific Papers) and some samples were used in the study of Luque et al (2014) for other purposes.…”

“…The procedure for preparation, age interpretation, and measurements of spine sections, described by Rodriguez-Marin et al (2012a) and Luque et al (2014), were followed. An annulus was defined as a wide opaque zone followed by a narrow translucent one (Compeán-Jimenez and Bard, 1983;Hurley and Iles, 1983) and age was estimated by counting the translucent bands.…”

“…spines from young specimens) were used to assign an age to the first inner visible translucent band in vascularized spines (Luque et al, 2014). Following theses authors, when the diameter of at least two of the three innermost visible bands fall within the ranges calculated from the S.D.…”

“…Dorsal fin spines have proven useful in the age determination of T. thynnus, and progress is being made in validating age estimation from this structure in order to use spines as an alternative structure to otolith for bluefin tuna direct ageing. These steps forward consist in cohorts tracking, fin spine radiocarbon age validation and spine edge type and marginal increment formation (Rodriguez-Marin et al, 2009;Rodriguez-Marin et al, 2012b;Luque et al, 2014).…”

“…Nucleus vascularisation of first dorsal fin spiniform rays in ABFT is the major limitation in using this calcified structure for ageing purposes and may result in age underestimation or overestimation of growth, since first growth marks are missing due to nucleus vascularisation, which increases with age or fish length (Kopf et al, 2010). This reabsorption of the central portion of the first dorsal ray begins from 2 years old ABFT (Compeán-Jimenez and Bard, 1983;Rodriguez-Marin et al, 2007;Luque et al, 2014). Therefore, the measurements of the first annuli are essential to ensure the ages estimated from the rest of annuli of ABFT spine sections are estimated accurately.…”