THE EFFECT OF MALE CHANGE ON PARENTAGE CONTRIBUTION, EGG PRODUCTION AND QUALITY IN GILTHEAD SEABREAM Sparus aurata

Introduction

Gilthead seabream (Sparus aurata) is the most important fin fish species in Mediterranean aquaculture, with global production of 341,000 tons in 2023. Gilthead seabream broodstocks are well adapted in aquaculture and spawn volitionally for a prolonged period of time (December to May), with very high fecundity (2 million eggs kg^-1 body weight) and fertilization success (Papadaki et al., 2024). Selective breeding programs have been implemented in many European countries, using family or mass selection (Janssen et al., 2017). However, the effectiveness of mass selection programs has been limiting, due to the poor male parental contribution to each day's spawning event. The objective of the present research was to examine the potential of a management method to increase male parentage contribution in gilthead seabream spawning, thus resulting in larger numbers of families and more even progeny distribution. Therefore, we abruptly replaced the males with new individuals in replicate populations and examined the effect i) on egg production and quality and later ii) on male parental contribution in the mass-spawning events immediately after new male introduction.

Materials and methods

Two broodstocks (1:4 male:female, 20 individuals), were monitored during the spawning season of 2025-2026. On 27 January 2026 (day 0), all males were replaced with new, 2-year-old spermiating males. Daily relative fecundity and fertilization success were recorded. Egg quality was evaluated at key developmental stages (24-h embryo survival, hatching, and 5-day larval survival) using 96-well plate assays. Sperm quality, including total, progressive, and rapid motility and velocity parameters, were assessed using computer assisted sperm analysis (CASA) in old and new males. Statistical comparisons between periods (before vs after replacement) were conducted using Welch's t-tests. DNA was also extracted from the breeders and from 100 eggs per spawn, 3 days before and 3 days after male exchange, from each broodstock. All samples were genotyped at 12 microsatellite loci using multiplex PCR. Fragment analysis was performed on an automated capillary sequencer, and allele scoring enabled assignment of offspring to parental genotypes. Pre- and post-replacement periods were compared using Welch's t-tests.

Results and discussion

Following male replacement, daily relative fecundity showed no significant change (p = 0.637). Fertilization exhibited a decreasing trend (p = 0.091), while the other quality parameters remained high, and no consistent differences between the periods. Sperm velocity parameters differed between the old and new males and significant reductions were observed in motility-related parameters. The percentage of motile, progressive and rapid sperm cells was markedly lower in the new males (p < 0.001 for all), while straightness (STR) of the trajectory remained unchanged (p = 0.933).

Figure 1. Mean ± SEM daily relative fecundity (bars, eggs kg^-1) and fertilization success (triangles, %) in two replicate populations of gilthead seabream, 3 days before and after male replacement

Figure 2. Mean ± SEM curvilinear (VCL, μm sec^{− 1}), straight line (VSL, μm sec^{− 1}) and average path velocity (VAP, μm sec^{− 1}) for gilthead seabream already in a broodstock vs the new younger males used to replace them (left). Percentage (%) of motile, progressive and rapid szoa, and straightness of the trajectory (STR) (right). Letter superscripts indicate differences between groups in each parameter (Welch's t-Test, P < 0,005).

The data indicate that abrupt exchange of males during the reproductive season does not affect egg production and quality, even if the sperm motility characteristics of the new males added indicated a lower quality. Our analyses are ongoing and the final results on male parental contribution in the mass-spawning events immediately after new male introduction will be presented in the meeting.

Acknowledgements

Funded by the European Union project SELECT ORGANIC (HEurope GA101188188) to CCM. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union.

References

Janssen, K., Chavanne, H., Berentsen, P., & Komen, H. (2017). Impact of selective breeding on European aquaculture. Aquaculture, 472, 8–16. https://doi.org/10.1016/j.aquaculture.2016.03.012

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