Assessment of some egg quality parameters and chorion ultrastructure characterization in Atlantic salmon (Salmo salar)

Abstract

Farmed salmonid production requires oocytes and embryos of high quality to supply the increasing market demand; achieving this depends primarily on the management of females and gametes. The chorion of the fertilized egg is a crucial structure for embryo protection; however, it is still poorly studied in some fish species. The object of the present study was to evaluate some parameters of oocyte quality and embryonic development in S. salar, and to characterize the chorion at the ultrastructural level in oocytes and embryos at different stages up to hatching by scanning electron microscopy (SEM). The samples (oocytes and embryos) were obtained from five mature females (4 years old, 9.75 ± 0.86 kg) of S. salar. The results show 32% (20%; 64%) of oocytes with yolk clots, a hydration percentage of 17% (11.1%; 23.3%) and a significant increase in embryo hardness at 12 accumulated thermal units (ATU). A mean fertility rate of 77% (60%; 93%) was recorded. The survival rates were 72% (20%; 100%) and 73% (32%; 92%) for 130 and 318 ATU, respectively. The values of symmetrical blastomeres were high, 65% (45%; 80%); however, the rate of spine deformities was also high at 42.4% (22%; 71%). Several significant correlations were observed between the parameters assessed: there were high-positive correlations between the hydration percentage and blastomere symmetry (r = 0.89), between oocyte hardness and spine deformities (r = 0.88), and between embryo hardness at 130 and 318 ATU (r = 0.97); there were also strong correlations between symmetry and survival rate at 130 ATU (r = 0.95) and between symmetry and survival rate at 318 ATU (r = 0.96). In the ultrastructural analysis, the SEM images revealed that the outer layer of the chorion of both oocytes and fertilized eggs, through development to hatching, exhibits a granular pattern. In contrast, the inner layer is composed of numerous interconnected fibres that form a complex network, absent in the hatching area. Both outer and inner layers show numerous pore canals. The pores in the outer layer are covered by plugs, and are larger than those in the inner layer. The information generated in this study is useful to complement understanding of the biology of S. salar eggs; however, given the ultrastructural variability of the chorion, further studies are needed to expand on these findings.