Abstract
Simple SummaryFish 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.
Highlights
The purpose of this paper is to verify the occurrence of change-points, if any, in three biometric relationships, namely “fork length–body weight”, “fork length–age”, and “surface of first dorsal spine cross section–fork length”, for juvenile and adult Atlantic bluefin tuna (ABFT) caught in the Mediterranean, as the expression of the existence of multiple growth stanzas
The present results clearly show that all of the examined somatic relationships, i.e., fork length–body mass (FLWR) for males and females, fork length–age (VB), and first dorsal spine cross section surface–fork length (SSFLR), are best modelled by multiple-phase regression equations, with the first two being biphasic and the latter being triphasic
As for the correlation between the first dorsal spine section surface and fork length, Section Surface – Fork Length Relationship (SSFLR), the results showed that it displays two change-points, the development of the spine goes through three phases
Summary
As early as the middle of the last century, Vasnetsov [1], referring to early ontogenetic phases, introduced the concept of stanzas of growth, i.e., distinct phases in the life cycle of fish between which rather abrupt changes in physiology induce changes in growth patterns. The passage from one growth stanza to the following one may become evident as a change in body form, which shows up in the length–weight relationship, or as a sudden change in the growth rate [2]. Such a change may affect morphometric correlations among different anatomical structures, as well as to the correlation between body weight and length [3,4]. In addition to the passage from one growth stanza to the following one in fish early ontogenesis [1], another important passage occurs at the onset of sexual maturity [4,5,6,7]
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