Over the past few decades, aspects relating to reproductive potential of Atlantic bluefin tuna (ABFT) have attracted growing public and scientific interest. This is largely due to the implications of impaired propagation on the species' stock recovery. Nonetheless, determining the indicators of the exact timing of puberty (= first sexual maturity) in ABFT and the capacity of a population to seasonally spawn and produce viable fry, remain elusive.
In this regard, a recent European study that closely followed captive-reared ABFT undergoing first sexual maturity, has successfully established a novel endocrine criteria for differentiating sexual-immature from sexual-mature specimens. The growth parameters recorded for captive ABFT juveniles were consistent with the length-weight relationship established for wild Mediterranean ABFT stocks. In addition, the histological analyses of the gonads indicated advanced sexual maturation in ABFT captive males compared to females. However, no robust conclusion could be drawn addressing whether the observed puberty in 3-year (3Y) ABFT males is a general feature also occurring in wild populations, or a phenomenon induced by the culture conditions.
Measurements of the two gonadotropins, follicle stimulating hormone (FSH) and luteinizing hormone (LH), which are the pivotal regulators of gonadal development and gamete maturation, revealed a clear discrepancy in the pituitary FSH to LH ratios in immature vs. mature ABFT. A 3-fold higher FSH to LH ratio characterized the 2-year (2Y) sexually immature fish, which contrasted to the <1 ratio in the other two groups consisting of (a) 3Y pubertal males and immature females and (b) fully mature adults. Moreover, a similar FSH/LH ratio benchmark has been reported in mammals and humans. In these studies, the intra-pituitary mechanism that prepares the gonadotrope cell population for puberty is represented by an increase in the storage of LH accompanied by a comparable decrease in the storage of FSH and a subsequent decrease in the FSH to LH ratio. Clinical studies that monitored circulating gonadotropin levels in children undergoing pubertal development revealed that the FSH to LH ratios, higher or lower than 1, reliably discriminate between pre-pubertal and early pubertal groups, respectively. Interestingly, both western and eastern ABFT stocks exhibit comparable age and size at sexual maturation, which undermines the previous assumption that western ABFT mature at a much older age than eastern fish.
In summary, the useful FSH/LH criteria for screening sexual maturity among ABFT populations exemplifies the potential of the endocrine approach to identify clinical biomarkers for assessing a vast array of reproductive traits. Following the same rational, future studies combining new advances in genetics and endocrinology may further our understanding of the interplay between energetic balance, growth
and maturation in ABFT. As a result benchmarks could be determined enabling discrimination between ABFT populations that successfully spawn and those who fail to complete their reproductive cycle. Such criteria will, ultimately, help resolve the controversy of "skipped spawning phenomena" among western ABFT stocks.