Multiple-Phase Biometric Relationships and Sexual Maturity in the Atlantic Bluefin Tuna, Thunnus thynnus (Osteichthyes: Scombridae)

Simple Summary Fish undergo distinct growth phases during their life. Passages from one growth phase to the following one correspond to a dramatic change in body proportion and/or growth rate. These changes can be detected as change-points in biometric relationships, such as that between length and weight. In this paper, we checked whether any change-points could be detected in the somatic proportions of the Atlantic bluefin tuna during its growth in order to better define some life history traits, including size at sexual maturity, that represent the essential basic knowledge for the effective management of this species. Fork length-weight relationship, length-age relationship (von Bertalanffy growth equation), and the relationship between the surface of the cross section of the first dorsal spine (a measure indicative of spine bone thickness) and the fork length were examined. All of the somatic relationships showed a change-point between 101 and 110 cm fork length. The present results corroborate the disputed hypothesis that Atlantic bluefin tuna from the eastern stock reproduce for the first time at 3-4 years of age. Most fish undergo distinct growth phases during ontogenesis. An extremely important passage from the juvenile to adult phase occurs at the onset of sexual maturity, which shows in body proportion and/or growth rate changes. These can be detected as change-points in biometric relationships. In this paper, the Atlantic bluefin tuna was analyzed to verify whether its somatic proportions show any sign of discontinuity during growth, i.e., whether any change-points may be detected in its somatic proportions. This fish has never been examined in this respect, and single-phase models, which are indeed easier to both compute and apply, are used in stock analyses. The following somatic relationships were analyzed in Atlantic bluefin tuna captured in the Mediterranean Sea between 1998 and 2010: "fork length-weight" regression, the von Bertalanffy growth equation, and "first dorsal spine cross section surface-fork length" regression. All of the examined relationships were found to be best modelled by multiple-phase regression equations, and all of them showed a change-point within the range of 101-110 cm fork length, which corresponds to 3-4 years of age. The present results, based on reproductive state-independent analyses, corroborate the disputed hypothesis that Atlantic bluefin tuna from the eastern stock in fact reproduce for the first time at this age.