The gilthead seabream (Sparus aurata), a cornerstone of Mediterranean aquaculture, has captivated fish farmers with its impressive egg production. But how does captivity influence its spawning behavior over time? A new study sheds light on this fascinating topic and offers valuable insights for broodstock management.
Scientists from the Institute of Marine Biology, Biotechnology and Aquaculture (Greece) and the University of Crete (Greece) studied the spawning performance (relative fecundity and fertilization success) in two broodstocks of gilthead seabream (Sparus aurata) raised in hatcheries under relatively constant well water temperatures (18-20°C) and a simulated natural photoperiod over five consecutive spawning seasons.
The Reproductive Success of the Gilthead Seabream
In their natural habitat, gilthead seabream reproduce seasonally, responding to temperature fluctuations. However, years of dedicated research have transformed them into aquaculture superstars. Today, they spawn easily in captivity, producing more than 2 million eggs per kilogram of female body weight over extended periods.
While temperature plays a role, various studies have confirmed that the photoperiod, the light-dark cycle, is the main environmental cue triggering reproduction in captive gilthead seabream. By mimicking seasonal light changes, fish farmers can induce spawning for most of the year.
Studies have also delved into the social dynamics of spawning. Typically, a female joins forces with one to three males for a synchronized release of eggs and sperm. Interestingly, even during periods of food scarcity, gilthead seabream prioritize reproduction, maintaining high fecundity, fertilization success, and egg quality.
Researchers have explored several factors influencing spawning success. Light manipulation is crucial, of course. But diet, female age, and the presence of environmental contaminants can also have an impact. Notably, studies emphasize the importance of understanding sex ratio changes in captivity.
Protandry: A Twist in Spawning Behavior
Gilthead seabream exhibit protandry, meaning they start life as males and can potentially change sex to become females later. This natural phenomenon raises concerns for broodstock management. Over time, uncontrolled populations could end up with too many females, leading to sperm shortages and reduced fertilization success.
To counteract this potential imbalance, fish farms often remove older fish, usually females, and replace them with younger males. This study aimed to monitor the effectiveness of this practice over multiple spawning seasons.
The Study’s Approach: A Multi-Year Look at Spawning Performance
Researchers observed two populations of gilthead seabream over seven years, across five breeding seasons. They tracked egg production, fertilization rates, and changes in the sex ratio. Additionally, they tested the impact of removing older females and introducing younger males on subsequent spawning performance.
Key Findings
Extended Spawning Season and High Fecundity
The study, conducted over five consecutive years, revealed an extended spawning season for gilthead seabream kept under controlled conditions with a constant water temperature (18-20°C) and a simulated natural day/night cycle. Notably, females produced a significant number of eggs, ranging from 1.48 million to 3.1 million eggs per kilogram of body weight per year. Interestingly, the highest egg production occurred during the first two spawning seasons.
Fertilization Success and Sex Ratio Fluctuations
While monthly egg production remained consistent over the years, the study observed variations in fertilization success. The highest rates were achieved during the second and third spawning seasons, with a significant decline in the last year. This suggests that factors beyond egg quantity may influence fertilization success, warranting further research.
The sex ratio within the broodstocks also showed fluctuations. Initially, males constituted 35% of the population. This percentage declined in subsequent years, stabilizing around 15-20%. An attempt to increase the male ratio to 50% by introducing smaller males failed. This triggered sex changes in larger males, transforming them into females, ultimately leading to a lower male percentage (18%) in the following season.
Implications for Gilthead Seabream Aquaculture
These findings hold significant value for the gilthead seabream aquaculture industry:
- Reduced Management Costs: The stable sex ratio eliminates the need for constant male introductions into the broodstocks, simplifying management practices and potentially reducing costs.
- Consistent Egg Production: The high and consistent egg production throughout the spawning season ensures a reliable supply of eggs for aquaculture operations.
- Efficient Broodstock Management: Understanding the extended spawning season and optimal timing for fertilization success allows for more efficient broodstock management strategies.
Additionally, the study suggests that maintaining a high male-to-female ratio may not be crucial in the long term. This is because the sex ratio stabilizes naturally after the first spawning season, and egg production and fertilization success remain high even with a lower percentage of males. This simplifies broodstock management and reduces costs associated with maintaining a large number of males.
Conclusion
“Study demonstrated the long spawning season of gilthead seabream, especially under constant water temperature, high fecundity and fertilization success, and stabilization of the male percentage at ~20% after the first spawning season or when modifying a broodstock to increase the male percentage,” the researchers conclude.
Overall, the research provides valuable insights for optimizing broodstock management in gilthead seabream aquaculture. By understanding the natural reproductive patterns of the species and the dynamics of the sex ratio, aquaculture professionals can achieve efficient and sustainable production of this commercially important fish.
The study was funded by the Institute of Marine Biology, Biotechnology and Aquaculture of the Hellenic Center for Marine Research.
Contact
Constantinos C. Mylonas
Institute of Marine Biology, Biotechnology and Aquaculture, Hellenic Center for Marine Research
P.O. Box 2214, Iraklion, Crete, 71003, Greece
Email: mylonas@hcmr.gr
Reference (open access)
Papadaki, M., Karamanlidis, D., Sigelaki, E., Fakriadis, I., & Mylonas, C. C. (2024). Evolution of sex ratio and egg production of gilthead seabream (Sparus aurata) over the course of five reproductive seasons. Aquaculture and Fisheries, 9(4), 534-542. https://doi.org/10.1016/j.aaf.2022.10.006
