Introduction
European seabass industry is primarily supplied with juveniles originating from selective breeding programs. These programs are managed by private hatcheries, which bear the full operational and financial costs of all selection steps. Because sex determination in this species is polygenic and highly sensitive to environmental variations (Vandeputte et al., 2007), the sex ratio among selection candidates varies considerably and is difficult to phenotype the sex of individual fish in vivo before the age of three, as the species exhibits no secondary sex traits. When larvae are reared under standard hatchery conditions, the sex ratio is heavily biased toward males; however, mass selection tends to favor females due to their higher growth rate (Geffroy et al., 2021). This constraint forces hatcheries to rear an excessively large number of prospective breeders for three years just to guarantee a balanced sex ratio among their final breeding stock.
Materials and Methods
This study investigates the conditions under which ultrasonography (echography) can enable early non-invasive sex phenotyping in seabass. To this end, four batches of European seabass were evaluated: aged 10, 22, 34, and 48 months, with mean weights of 118 g, 330 g, 614 g, and 3000 g, respectively. Sex phenotyping was performed using two different ultrasound devices: an ExaPad equipped with an LC1428V probe (IMV Technologies) and a Clarius L7 Vet (Clarius). To mimic commercial hatchery operations, the three younger batches were reared under standard grow-out conditions at a constant temperature of 21°C. Conversely, the oldest batch was maintained for over a year under maturation conditions, where water temperatures followed the natural seasonal variations of the Gulf of Lion in the Mediterranean Sea (13 C in winter to 25°C in summer). Under veterinary supervision, five operators used the devices to determine the sex of 151, 250, 128, and 110 fish (from the youngest to the oldest batch, respectively). Ultrasound examinations were performed on anaesthetised fish. For each fish, the probe was manipulated by one random operator, and the sex prediction was established independently by 2 to 5 observers, resulting in a total of 1,910 predictions. The true sex of the fish was subsequently validated by macroscopic observation of the gonads post-mortem for the three younger batches, and by gamete sampling for the oldest batch.
Results
The proportion of males in the batches was 69.6%, 56.9%, 51.5%, and 28.2% from the youngest to the oldest, respectively. Diagnostic accuracy calculated as the percentage of correctly identified fish out of the total evaluated, yielded the following results: 57%, 81%, 87%, and 96% for the 10-, 22-, 34-, and 48-month-old batches, respectively. The accuracy for the 10-month-old batch was significantly lower than that of the other groups (p= 2.17e-08). Linear model analysis revealed that males were more accurately predicted in the 22- and 34-month-old batches (p= 2e-16 and p= 0.00883, respectively). Furthermore, the operator manipulating the probe did not achieve higher predictive accuracy than the non-manipulating observers.
Discussion
In conclusion, this study demonstrates that it is not possible to reliably predict the sex of 10-month-old fish kept under standard grow-out conditions. However, non-invasive ultrasonography proves to be a highly effective tool for adjusting unbalanced sex ratios in fish aged 2 to 3 years kept in growing condition.
References
Geffroy, B., Gesto, M., Clota, F., Aerts, J., Darias, M.J., Blanc, M.-O., Ruelle, F., Allal, F., Vandeputte, M., 2021. Parental selection for growth and early-life low stocking density increase the female-to-male ratio in European sea bass. Sci Rep 11, 13620. https://doi.org/10.1038/s41598-021-93116-9
Vandeputte, M., Dupont-Nivet, M., Chavanne, H., Chatain, B., 2007. A Polygenic Hypothesis for Sex Determination in the European Sea Bass Dicentrarchus labrax. Genetics 176, 1049–1057. https://doi.org/10.1534/genetics.107.072140