| Yellowtail kingfish, Seriola aureovittata, is a long-distance migratory oceanic species belonging to the Carangidae family of Perciformes, which has a global distribution and inhabits temperate and subtropical marine waters. S. aureovittata is large in size, has a fast growth rate, and is highly favored by international consumers owing to its excellent flesh taste, nutritional quality, and economic value. Furthermore, it is a promising candidate for the global farming industry and is particularly suitable for rapidly developed open ocean aquaculture in China.
Currently, yellowtail kingfish aquaculture occurs in over 10 countries including Japan, Australia, New Zealand, South Africa, Chile, Greece, Holland, USA, Mexico, and China. In 2017, a great breakthrough in seedling production of S. aureovittata was achieved, and currently juveniles are mass-produced in China by combining the “engineering pond” and “land based indoor tanks” modes, which led to the rapid development of the Seriola fish farming industry in China. Nowadays, Seriola species are farmed in Liaoning Province, Fujian Province, and Shandong Province of China, and the combined annual farming yield is approximately 500 tons. However, we found that during seedling production of S. aureovittata, especially at the early larval growth stage, the hatching rate of eggs was variable among different spawning batches, and the survival of early larvae was low especially when the larvae reached 8~10 d post hatching. Occasionally, the high total death rate was attributed to the sudden “sinking death” of larvae, which may have been caused by stress as a result of changes in environmental factors. Therefore, it is necessary to determine the ecological and physiological effects of environmental factors, especially temperature and salinity fluctuations, on the early life stages of S. aureovittata under artificial breeding conditions.
In the present study, the effects of two key environmental factors, temperature and salinity, on embryonic development and early larval growth of S. aureovittata were investigated using experimental ecology, morphological measurements, and molecular methods under laboratory conditions. The indexes including hatching rate of eggs, deformation rate of newly hatched larvae, absorption of yolk sac, IGF-1 gene expression, survival index (SAI), and point of no return (PNR) were determined. Moreover, the vitality of newly hatched larvae was tested and evaluated. The results showed that the highest hatching rates of 75%~81% were obtained under temperatures of 20~22 ℃, and the deformation rates of newly hatched larvae were lower than 6.7%. In addition, according to the Q10 calculation, the most appropriate water temperature range for embryonic development of S. aureovittata was confirmed to be 20~22 ℃. Meanwhile, the total length and yolk sac volume of newly hatched larvae of yellowtail kingfish hatched from the 20℃ and 22 ℃ groups were larger than those in the other temperature groups. Regarding salinity, the fertilized eggs floated on the water surface when salinity was over 30 ‰, were suspended in the water when salinity was between 20~25, and sank to the bottom of the container when salinity was lower than 15. The optimum salinity range for embryonic development of S. aureovittata was therefore determined to be 30~35, when hatching rates were between 79%~80%, and the deformation rate of newly hatched larvae was 6%. The yolk sac absorption by newly hatched larvae was measured under four temperatures (18 ℃, 20 ℃, 22 ℃, and 24 ℃). It was found that the absorption and exhaustion speed of the yolk sac increased with temperature, and the yolk sac was exhausted at 7 d post hatching at 18 ℃, whereas the time decreased to 6 d, 5 d, and 4 d at 20 ℃, 22 ℃, and 24 ℃, respectively. The highest SAI value for newly hatched larvae was observed at a salinity of 30, whereas the lowest was observed at a salinity of 10, which was consistent with the hatching results of embryos under different salinities. The highest first feeding rate of newly hatched larvae was observed at 6 d post hatching, and the PNR appeared between 7 d and 8 d post hatching at culture temperatures ranging from 20~22 ℃. IGF-1 mRNA levels in newly hatched larvae from different temperatures and salinities were detected, and significantly higher expression levels were found at temperatures of 20~24 ℃ and salinities of 30~35. Under continuous starvation conditions, the IGF-1 mRNA in larvae significantly increased at 2 d post mouth open and decreased at 3 d and 4 d, although expression levels remained relatively high, and then continually decreased to a significantly lower level as starvation continued. Results from the present study provide basic knowledge and useful tools for the construction of standardized technological methods for optimal embryonic hatching and seedling production of S. aureovittata. |
