Introduction
Sex control and thus the understanding of sex determination and sex differentiation in fishes is crucial in aquaculture due, for example, to the need for controlling reproduction, producing monosex populations. The production of such populations is of particular importance owing to differences in growth rates or economical values of sexes in some species (Yamazaki, 1983). Various methods, including hormonal treatments, chromosomal manipulation, hybridization, and selective breeding, have been employed to achieve sex control in fishes (Wang et al., 2018). Besides these techniques, some studies reported that the use of electric shock can also influence sex ratios. For instance, Okomoda et al. (2020a) showed that applying electric shock to red hybrid tilapia ( Oreochromis mossambicus × O. niloticus) resulted in a 70% male population. Electric shock has been widely used for triploid production in various species, including mussels (Cadorel , 1992), African catfish ( Clarias gariepinus) (Okomoda et al., 2020b ), tilapia (Hassan et al., 2018; Okomoda et al., 2020a), and coho salmon (Oncorhynchus kisutch) (Teskeredžić et al., 1993). The main objective of our study was to investigate the effects of an electric shock on the development of embryos of rainbow trout gonads varying the timing of the shocks after fertilization while avoiding the production of triploid populations. To do so we measured sex ratio and quantified, growth performance, and fatty acid composition that ultimately will affect the nutritional values for consumers.
Materials and methods
Three male and five female rainbow trout (Oncorhynchus mykiss ) were used for gametes collection. Fertilized eggs of rainbow trout were exposed to a direct electrical current of 12V for 10 minutes at different times post-fertilization (20, 30, and 40 minutes), using well water with different salinity levels (5ppt and 0ppt). The electric field was generated using a rechargeable battery connected to rectangular aluminium electric probes, placed 30 cm apart to match the container’s cross-sectional area within a plastic container of 35x25x15cm . Nitrate, chloride, and carbonate/bicarbonate were also tested using standard methods. Analysis for potassium, magnesium, sodium, cobalt, barium, beryllium, bismuth, selenium, chromium, iron, nickel, copper, and calcium was performed following Method 200.7 . Erythrocyte measurements were analysed to assess ploidy levels . G rowth performance, live weight gain, survival rate, total feed intake, feed conversion ratio, and specific growth rate were calculated . Histological procedures were performed t o identify the cell types involved in spermatogenesis or oogenesis; samples were stained with Hematoxylin-Eosin . The sex ratio was determined by histological examination of the gonads, with groups categorized based on electrical exposure post-fertilization. Fatty acids analysis was conducted in muscle and liver tissue using Gas Chromatography.
A two-sample t-test was used to compare fertilization rates between treatments. Fertilization and hatching rates, water parameters, growth performance, and fatty acid profiles were analysed using one-way ANOVA, followed by Tukey’s post hoc test (P < 0.05). Ploidy determination was conducted using one-way ANOVA with Fisher’s post hoc test (P ≤ 0.05). All statistical analyses were performed using Minitab 18. Principal component analysis of erythrocytes was carried out in RStudio.
Results
The results of the application of an electric shock at different post-fertilization times (20, 30, and 40 minutes) and salinity levels (0 and 5 ppt) did not significantly affect the fertility or hatching rates. Ploidy determination showed significant differences among treatments (P ≤ 0.05) ; however, the values are consistent within the diploid range, reinforcing the conclusion that the observed characteristics are typical of diploid organisms. Groups shocked at 0 ppt salinity after 20-, 30- and 40-minutes fertilization showed greater values of cell-nuclear area ratio and cell-nuclear volume ratio to P rincipal C omponent 2, where the nuclear structure may be more affected by electric shock. Triploidy was not observed, as triploid individuals exhibited significantly larger cell and nuclear dimensions . Initial weights did not significantly differ among treatments (P > 0.05). However, significant differences were observed in final weight and total weight gain (P < 0.05). The highest final weight was recorded in the at 0 ppt salinity after 30 minutes fertilization (39.07 ± 0.08g), which was significantly higher than most other treatments. The sex ratio results showed no significant differences among the experimental groups or the control group (P > 0.05). Histological examination of gonads from the experimental groups and the control group showed no differences between the experimental and control groups regarding gonadal structure, both in males and females. No significant differences were observed in the overall proportions of saturated, monounsaturated, polyunsaturated, or total trans fatty acids in both muscle and liver, indicating that the electrical shock treatments did not alter the global balance of fatty acid groups. However, linoleic acid (C18:2) was significantly higher in the 20 minutes post-fertilization group compared to the control. Similarly, linolenic acid (C18:3) was significantly elevated in the 20- and 30-minutes post-fertilization groups relative to the control. Eicosapentaenoic acid (EPA, C20:5) was also significantly higher in the 30 minutes post-fertilization group compared to the control.
Discussion and conclusions
This study demonstrates that exposure to a 12V ( voltage gradients 0.4 V/cm) for 10 minutes at different times (20, 30, and 40 minutes) after fertilization have no negative effects on fertility and hatching rates in rainbow trout embryos. Despite the relatively low voltage gradient, 12V electrical shock still induced significant developmental impacts, reinforcing the sensitivity of embryos during early developmental stages. No triploidy induction was confirmed, as erythrocyte characteristics remained consistent with diploid individuals. However, 12V electrical shock influenced growth performance and showed differences in specifics fatty acid profiles, possible due to metabolic stress rather than ploidy-related effects. Future research should investigate the combined effects of a 12V electrical shock with other environmental factors, such as temperature, oxygen levels, or salinity, to better understand their interactions and potential applications in aquaculture.
Acknowledgments
This master thesis was supported by Istanbul University Scientific Research Projects Executive Secretariat, Project ID: 40303.
References
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