Abstract
Atlantic salmon (Salmo salar) aquaculture is of great importance to Norway. The egg and hatchery sector, is important both finacially and biologically to salmon production, since it is essential to provide a broodstock with good egg quality. Although the Atlantic salmon industry is well-established, little is known about the early stages of development in relation to time and sensitivity to mechanical handling during the embryonic period. Facing these research gaps, this study aimed to provide a detailed description of the S. Salar embryonic development in relation to time in degree-days (dd) and to identify the sensitive stages from fertilisation to hatching.
Approximately 15000 eggs, pooled from three females, were fertilised with milt from one male and incubated at 8℃. From fertilisation to hatching the embryonic development was recorded through pictures. To test the sensitivity, a mechanical shock device was used where the eggs were dropped from 4 known heights: 25 cm, 50 cm, 100 cm and 125 cm, and also a control where eggs were handled but not dropped. The eggs were kept for 7 days after they were stressed and mortality was then recorded. LD10 and LD50 were estimated, which is the estimated height causing 10 and 50 % mortality, respectively. The embryonic development and sensitivity tests were conducted daily until no mortality was obvserved in heighest height tested, and after that 4 times per week.
The embryonic development timing found in this study was, to some extent, similar to what was previously described. Discrepancies might be due to genetic variation and adaptation. However, this study provided a more practical embryonic development in relation to time in degree-days, which is the main unit used by commercial hatcheries to follow development.
For the sensitive stages, the embryos presented no senstivity (LD50 > 100 cm) to mechanical shock during the first 32 dd after fertilisation. After that, a steep increase was observed during the gastrulation phase, where maximum sensitivity was reached and the embryos were extremely sensitive (LD50 < 10 cm), followed by a decrease after blastopore closure. After 184 dd, the eggs were classified as not sensitive to mechanical shock (LD50 > 100 cm). The window where the embryos were not sensitve was wider than what was antecipated by commercial hatcheries, since they usually do not handle eggs from 24 dd after fertilisation to the eyed stage, approximately 275-300 dd.