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.

The availability of defatted soybean meal as a substitute for fish meal was evaluated in juvenile (450 g on average) yellowtail fed diets containing several levels of soybean meal (from 20 to 50%). After 153 days, liveweight averages were 980, 925, 795 and 670 g for fish fed diets in which fish meal was partially substituted with 20, 30, 40 and 50% soybean meal, and feed conversion ratios were 2.79, 3.09, 4.57 and 6.52, respectively. Regression analysis showed a negative effect of inclusion of soybean, although fish fed diets containing 20% and 30% of soybean did not present statistical differences and grew significantly more, and had a better feed conversion ratio, than fish fed 40 or 50% diets. Likewise, muscle protein level was lower and lipid content was higher in fish fed 20 or 30% soybean. No differences were obtained for protein digestibility coefficients of experimental diets.Versión de editor

Life-history traits such as spawning migrations and timing of reproduction are adaptations to specific environmental constraints and seasonal cycles in many organisms' annual routines. In this study we analyse how offspring fitness constrains spawning phenology in a large migratory apex predator, the Atlantic bluefin tuna. The reproductive schedule of Atlantic bluefin tuna varies between spawning sites, suggesting plasticity to local environmental conditions. Generally, temperature is considered to be the main constraint on tuna spawning phenology. We combine evidence from long-term field data, temperature-controlled rearing experiments on eggs and larvae, and a model of egg fitness, and show that Atlantic bluefin tuna do not spawn to optimize egg and larval temperature exposure. The timing of spawning leads to temperature exposure considerably lower than optimal at all spawning grounds across the Atlantic Ocean. The early spawning is constrained by thermal inhibition of egg hatching and larval growth rates, but some other factors must prevent later spawning. Matching offspring with ocean productivity and the prey peak might be an important driver for bluefin tuna spawning phenology. This finding is important for predictions of reproductive timing in future climate warming scenarios for bluefin tuna.