Our understanding of the biology of Atlantic bluefin tuna (Thunnus thynnus) has increased profoundly in the last decade, and the progress is attributed to the development and application of a variety of novel tools. Here we provide a comprehensive examination of available data on the life history and stock structure of T. thynnus by re-examining current databases and literature and highlighting findings from recent studies using approaches such as archival tags and natural markers (e.g., genetics, otolith chemistry). The present review provides a detailed synthesis on the reproductive biology, feeding ecology, growth, mortality, migration, and stock structure of T. thynnus. In addition to characterizing key life history attributes and discussing stock-specific (east versus west) differences, the implication of trans-Atlantic movement and mixing are addressed. We also identify significant data needs that still exist and must be addressed to promote effective management and rapid recovery of T. thynnus populations.

The ovaries of 501 female eastern Atlantic bluefin tuna (Thunnus thynnus Linnaeus, 1758) captured in the Mediterranean Sea from May to September between 1998 and 2004 were analysed histologically. Body size at median sexual maturity (L₅₀) was 103.6 cm fork length (FL), while 100% maturity was reached above 135 cm FL. The age analysis, based on the count of the translucent zones of the first spiniform ray of the first dorsal fin, showed that most of the specimens with FL = L₅₀ were 3 years old while 100% maturity was reached between 4 to 5 years. The reported evidence indicates that for the eastern Atlantic bluefin tuna stock, the size and age of first sexual maturity of females was lower than in the western Atlantic stock.

The greater amberjack Seriola dumerili is a large teleost fish with rapid growth and excellent flesh quality, whose domestication represents an ambitious challenge for aquaculture. The occurrence of reproductive dysfunctions in greater amberjack reared in captivity was investigated by comparing reproductive development of wild and captive-reared individuals. Wild and captive-reared breeders were sampled in the Mediterranean Sea during three different phases of the reproductive cycle: early gametogenesis (EARLY, late April-early May), advanced gametogenesis (ADVANCED, late May-early June) and spawning (SPAWNING, late June-July). Fish reproductive state was evaluated using the gonado-somatic index (GSI), histological analysis of the gonads and determination of sex steroid levels in the plasma, and correlated with leptin expression in the liver and gonad biochemical composition. The GSI and sex steroid levels were lower in captive-reared than in wild fish. During the ADVANCED period, when the wild greater amberjack breeders were already in spawning condition, ovaries of captive-reared breeders showed extensive atresia of late vitellogenic oocytes and spermatogenic activity ceased in the testes of half of the examined males. During the SPAWNING period, all captive-reared fish had regressed gonads, while wild breeders still displayed reproductive activity. Liver leptin expression and gonad proximate composition of wild and captive greater amberjack were similar. However, the gonads of captive-reared fish showed different total polar lipid contents, as well as specific lipid classes and fatty acid profiles with respect to wild individuals. This study underlines the need for an improvement in rearing technology for this species, which should include minimum handling during the reproductive season and the formulation of a specific diet to overcome the observed gonadal decrements of phospholipids, DHA (22:6n-3) and ARA (20:4n-6), compared to wild breeders.

Atresia of ovarian follicles, that is the degenerative process of germ cells and their associated somatic cells, is a complex process involving apoptosis, autophagy and heterophagy. Follicular atresia is a normal component of fish oogenesis and it is observed throughout the ovarian cycle, although it is more frequent in regressing ovaries during the postspawning period. An increased occurrence of follicular atresia above physiological rates reduces fish fecundity and even causes reproductive failure in both wild and captive‐reared fish stocks, and hence, this phenomenon has a wide range of implications in applied sciences such as fisheries and aquaculture. The present article reviews the available literature on both basic and applied traits of oocyte loss by atresia, including its morpho‐physiological aspects and factors that cause a supraphysiological increase of follicular atresia. Finally, the review presents the use of early follicular atresia identification in the selection process of induced spawning in aquaculture and the implications of follicular atresia in fisheries management.

Mature migrating Atlantic bluefin tuna (Thunnus thynnus thynnus) were captured in the Mediterranean Sea with a purse seine and reared in floating cages for 2 to 3 years. During the natural spawning period (June–July) of two consecutive years, fish were randomly implanted underwater with a controlled-release delivery system (implant) loaded with gonadotropin-releasing hormone agonist (GnRHa), in order to induce final oocyte maturation (FOM), ovulation/spermiation, and spawning. At the time of sampling, males were significantly larger than females (ANOVA, P < 0.001), having a mean (± SE) fork length and body weight of 190 ± 3 cm and 122 ± 5 kg, compared to 176 ± 3 cm and 94 ± 4 kg of females, respectively. All fish were reproductively mature, with their age ranging between 5 and 12 years and males being a year older, on average. After GnRHa implantation, fish were monitored for spawning and the release of eggs, and were sacrificed at different times after hormone treatment in order to examine the progressive effect of the treatment on gonad maturation. The in vitro GnRHa release from the produced implants was maximal during the first 2 d, with a mean (± SE) release of 525 ± 166 μ g GnRHa implant⁻¹ day⁻¹. The plasma GnRHa profile in vivo reflected the release in vitro, and statistically significant elevations in plasma GnRHa levels were measured until 7 d after treatment (ANOVA, P < 0.01). The underwater implantation procedure was improved between 2004 and 2005, requiring an average (± SD) of 3.1 ± 1.4 min for each fish, and was 64 and 84% successful in 2004 and 2005, respectively. There were no differences between the histological appearance of the testes of GnRHa-treated and control males, and almost all of them contained intra-testicular spermatozoa. However, the proportion of spermiating control males (n = 17) was only 12% compared to 26% for the GnRHa-implanted males (n = 19). Also, there were no differences between controls and GnRHa-implanted fish in sperm concentration, initial spermatozoa motility, or duration of forward motility, which ranged between 29.02–48.54 × 10¹⁰ spermatozoa ml⁻¹, 58–63% and 8–9 min, respectively. Final oocyte maturation (FOM) and post-ovulatory follicles (POFs) occurred in 63% and 88%, respectively, of the GnRHa implanted females (n = 16), compared to 0% and 21%, respectively, of the control females (n = 14). In addition, two GnRHa-implanted females in 2005 were found to be ovulated at the time of sacrifice, and their eggs were fertilized in vitro with sperm from spermiating males, which resulted in viable embryos and larvae. Finally, although spawning was not observed, fertilized eggs were collected from the cages. Larvae produced from these eggs were identified as Atlantic bluefin tuna, demonstrating that the present GnRHa implantation method can be used to induce FOM, ovulation/spermiation, and spawning in captive-reared Atlantic bluefin tuna.