Introduction
Arctic charr (Salvelinus alpinus) is a species of high economic value for the Nordic aquaculture. The highly variable reproductive performance that is commonly observed in commercial farms is hindering the industry expansion in Sweden. Semen quality characteristics such as motility-related traits and sperm concentration can influence male fertility in farmed fish species. Change in sperm quality across the spawning period is common in fish and can affect their reproductive success (Bobe & Labbé, 2010; Mylonas et al., 2003). The present study aimed to investigate variation in sperm quality in Arctic charr breeding candidates across the whole spawning season.
Materials and methods
Arctic charr males from the Swedish breeding program (2017-year class) were used in the present study. Semen sampling was performed three times across the natural spawning season (n = 71; early October – November). Additionally, three samplings were performed in males kept under photoperiod conditions resulting in late spawning (n = 77; late October – December). Evaluation of progressive sperm motility (PM, %) and sperm kinematics, including average path velocity (VAP, μm/s), curvilinear velocity (VCL, μm/s), and straight-line velocity (VSL, μm/s), was performed by using a computer-assisted sperm analysis (CASA, Microptic) system and the SCA®Motility imaging software. Furthermore, sperm concentration (SC, ×109cells/mL) was measured using a NucleoCounter® SP-100™ (Chemometec). Descriptive statistics were computed for the CASA-system parameters using statistical R (v.4.0.2). Across-season sperm quality variations were analyzed by non-parametric tests for comparing either two (Wilcoxon test) or multiple groups (Kruskal-Wallis test). Finally, Pearson correlation was used to determine relationships between sperm quality parameters across the spawning season.
Results
Overall, there was a positive trend regarding the mean sperm quality traits in both natural and delayed spawning groups. More specifically, sperm concentration increased by 38% from late October to November in the natural spawning group. In the delayed group, sperm density increased by 39% from October to November and 133% from October to December. Sperm motility parameters increased by 4-13% in October and 8-17% from October to November in the natural group. In the delayed group, the increase was 14-28% from October to November and 38-45% from October to December (Figure 1). In addition, about 30% of the studied males exhibited minor individual variability in sperm quality with the standard deviation for SC < 0.6×109 cells/mL and for PM < 5% during the spawning season.
Moderate to high correlations (0.31-0.68, P<0.01) were estimated for sperm concentration measured at different times across the spawning season. While for motility-related parameters, the Person correlations were between 0.41-0.54 (P<0.001) for recordings measured in late October and November in the natural spawning group and between recordings taken in November and December in the delayed spawning group. In the case of the remaining time points, the recorded motility parameters showed low correlations (–0.01-0.26, P ≥0.05).
Discussion and Conclusion
In the present study, the across-season variation in the recorded sperm quality parameters was evident, showing, in general, a positive trend across the sampling points. These variations can explain to a certain degree the highly variable reproductive performance of Arctic charr in captivity. In addition to that, the individual variability in semen parameters during the spawning period was observed among the studied males. Although the reason for that remains poorly understood, environmental factors and genetic background are considered as crucial contributors to male reproductive performance (Jeuthe et al., 2015; Kurta et al., 2021). Understanding the sperm quality changes throughout the breeding period would aid the management of reproduction and selective breeding schemes in Arctic charr hatcheries.
REFERENCES
Bobe, J., & Labbé, C. (2010). Egg and sperm quality in fish. General and Comparative Endocrinology, 165(3), 535–548. https://doi.org/10.1016/J.YGCEN.2009.02.011
Jeuthe, H., Brännäs, E., & Nilsson, J. (2015). Thermal stress in Arctic charr Salvelinus alpinus broodstock: A 28 year case study. Journal of Fish Biology, 86(3), 1139–1152. https://doi.org/10.1111/jfb.12634
Kurta, K., Jeuthe, H., Pinto, F. L., de Koning, D., & Palaiokostas, C. (2021). A Genetic basis of reproductive performance in selectively bred Arctic Charr (Salvelinus alpinus). Aquaculture Europe 21, 671–672.
Mylonas, C. C., Papadaki, M., & Divanach, P. (2003). Seasonal changes in sperm production and quality in the red porgy Pagrus pagrus (L.). Aquaculture Research, 34, 1161–1170.
ACKNOWLEDGEMENTS
The authors acknowledge support from FORMAS under grant agreement 2018-00869 (NextGenCharr), the Kolarctic funding body under the ARCTAQUA project (grant agreement 4/2018/095/KO4058), and the Royal Swedish Agricultural Academy (KSLA, project no. VAT2021-0002).