| The total saline-alkaline land area in China is about 99.13 million hectares distributed throughout northern China, coastal areas and areas along the bank of the Huanghe River. About 46 million hectares of saline-alkaline water areas are distributed around these saline-alkaline lands, most of which are thalassic and characterized by a high pH value in excess of 8.8 associated with high carbonate alkalinity concentrations, and various types of ion imbalances. Saline-alkaline waters are stressful environments in which only relatively few organisms are able to survive. In consequence, most of the saline-alkaline water resources have been desolated for a long time. The effective utilization of saline-alkaline water resources will benefit restoration of saline-alkaline habitats and the expansion of aquaculture space. Naked carp (Gymnocypris przewalskii) is endemic to the austere saline-alkaline environment of Qinghai Lake. Due to overfishing in the 1960s and environmental changes in the lake area, the resources of naked carp in Qinghai Lake declined significantly. At present, the main measures to protect the naked carp and maintain the ecological balance of Qinghai Lake are through the fishing forbidden and artificial stocking and releasing. The feeding behavior of fish under natural conditions has obvious rhythm characteristics, which is an important research direction for healthy aquaculture. In order to explore the characteristics of self feeding rhythm and growth performance of fish in saline-alkaline environment and provide basic data for the protection of saline-alkaline native fish, naked carp was taken as the representative in this study. First, the freshwater and lake water group with natural photoperiod(14L:10D) and the lake water group with darkness(24D) were set. The artificial lake water was prepared according to the ionic composition of Qinghai Lake, with the content of Na+ 23.05%, K+ 1.34%, Ca2+ 0.11%, Mg2+ 6.88%, HCO3- 7.09%, CO32- 5.07%, Cl- 40.39% and SO42- 16.07%. The measured salinity of the artificial lake water was 15.08, and the carbonate alkalinity was 27.53 mmol/L. According to the local photoperiod of Qinghai Province, the whole day was divided into five period as 5:00-8:00, 8:00-11:00, 11:00-15:00, 15:00-19:00, 19:00-5:00. The feeding rhythm experiment lasted for 5 days, and the average food intake of each period was calculated. The results showed that naked carp had obvious daily feeding rhythm during natural photoperiod. In the natural photoperiod, the feeding peak was from 8:00 to 11:00, and the low feeding period was from 5:00 to 8:00 in freshwater. In the lake water, naked carp showed high and continuous feeding during 8:00 to 19:00, and its average hourly feed intake was significantly higher than that during 5:00 to 8:00 and 19:00 to 5:00. So naked carp was determined as the type of daytime feeding. In addition, the higher proportion and the continuous feeding in daytime in lake water indicated that the osmotic and acid-base regulation of naked carp in saline-alkaline water may enhance its diurnal feeding rhythm. Whereas, in the continuous dark environment, the feeding rhythm of naked carp was weakened, and the average hourly food intake of each period was similar. In order to explore the growth performance of naked carp under self feeding rhythm, the lake water group [L15, salinity of 15.18 ± 0.04, and carbonate alkalinity of (27.94 ± 0.24) mmol/L] and the freshwater control group [FW, salinity of 0.47 ± 0.02, and carbonate alkalinity of (2.17 ± 0.07) mmol/L] with natural photoperiod were set up. After 63 days of self feeding culture, the individual body length and weight of the naked carp were measured after being anesthetized with MS-222. The growth rate of body length (1.19%) and body weight (10.66%) and the specific growth rate (0.16%/d) of naked carp in the lake water group were significantly lower than those[length growth rate (18.66%), mass growth rate (67.32%), and specific growth rate (0.82%/d)] in the freshwater group, indicating that the growth of naked carp was inhibited by high salinity and carbonate alkalinity environment. Naked carp used more energy for osmotic regulation and acid-base regulation than growth. The parameter b of body length-weight relationship curve of the naked carp in both the lake water group and the freshwater group was less than 3, which showed that the naked carp was a negative allometric growth fish. The b value of the lake water group was lower than that of the freshwater group, and the body length of naked carp grew faster than body weight in the lake water. The growth characteristics of naked carp was affected by the high saline-alkaline environment. The self feeding rhythm and growth performance of naked carp provided a basic knowledge for making feeding strategy for fish cultured in saline-alkaline environment, and recovering endangered native saline-alkaline fish. |
