Abstract:The total saline-alkaline land area in China is approximately 99.1 million hectares, distributed throughout northern China, coastal areas, and areas along the bank of the Yellow River. About 45.9 million hectares of saline-alkaline water areas are distributed around these lands, most of which are athalassic saline water characterized by high pH and high carbonate alkalinity concentrations with various types of ion imbalances. The co-effect of high pH and high carbonate alkalinity would directly lead to the respiratory alkalosis of aquatic organisms. High pH affects the excretion of ammonia, resulting in increased blood ammonia and acid-base imbalance. High ionic coefficient affects the osmotic regulation and breaks the ion balance in aquatic organisms. Thus, saline-alkaline water has not been fully used because of its stressful environmental characteristic. Currently, the lack of suitable objects for the saline-alkaline aquaculture restricts the development of the aquaculture industry on saline-alkaline land. Largemouth bass (Micropterus salmoides) is a potential economic target that has been successfully farmed in some saline-alkaline waters. However, largemouth bass's tolerance range and response mechanism to saline-alkaline water are still unclear. This study evaluated the growth performance of juvenile largemouth bass in a saline-alkaline environment to propose excellent farming species for saline-alkaline aquaculture. First, the juvenile largemouth bass response to 48 h carbonate alkalinity and 96 h semi-lethal salinity was determined. Hereafter, the saltwater group [SW, salinity of 7.50±0.07, and carbonate alkalinity of (1.81±0.12) mmol/L], alkaline water group [AW, salinity of 0.35±0.02, and carbonate alkalinity of (9.96±0.03) mmol/L], and freshwater control group [FW, salinity of 0.13±0.01, and carbonate alkalinity of (1.82±0.11) mmol/L] were set to comparatively study the growth parameters, physiology parameters, and muscle texture characteristic indexes of largemouth bass under long-term saline-alkaline stress. For the growth experiment, largemouth basses were acclimated to and reared in FW, SW, and AW conditions for 105 days. Triplicate of 30 individuals each were set for each condition using an experimental plastic tank with 100 L of water. Each fish's body length and weight were measured every 15 days after being anesthetized with MS-222. For the physiology parameters study, five largemouth basses were randomly selected from each group at the end of the growth experiment. The fish were anesthetized with MS-222 to draw 20 μL of blood from the tail vein using a syringe moistened with lithium heparin, which was immediately centrifugated to measure osmolality. Another 30 largemouth basses were taken and subjected to 24 h carbonate alkalinity stress. The experimental conditions were the same as AW group, and the control group was the same as FW group. During the stress period, feeding was stopped, and blood was drawn from five randomly selected fish every 6 h and centrifuged immediately to determine blood ammonia (blood ammonia kit A086-1-1 by Nanjing Jiancheng). Plasma osmolality was measured using an osmometer (Wescor Vapro 5520 Vapor Pressure Osmometer, USA). For the muscle texture characteristic index study, five largemouth basses were randomly selected from each group at the end of the growth experiment. After being anesthetized with MS-222, the muscles on the fish's backside (3.01±0.14) g were taken by using a surgical scalpel and scissors, the muscles' outer skin was cut off, and the sampled muscle sizes were standardized to (2.04±0.12) cm3. After sampling, the TMS-Pro texture analyzer (Food Technology Corporation, USA) was used to measure the muscle texture characteristics, employing the TPA mode, test speed of 30 mm/min, deformation amount of 50%, and return distance of 30 mm. The results showed that in the 48 h carbonate alkalinity group, the semi-lethal concentration was (29.92± 3.90) mmol/L, while the fish could survive safely in water with salinity below 10 mmol/L. After 105 days of farming, there are no significant differences in the survival rate and final weight among different groups, in which the specific growth rate (SGR) showed a regular variation. During 15~45 days and 60~75 days, SGR decreased continuously, while it increased during 45~60 days and 75~90 days. The condition factors of the largemouth bass were less than 3 in all groups, with an increase from FW to AW and from AW to SW groups. In the AW group, the blood ammonia within 24 h showed an increase, then a decreased, and finally stabilized. In the SW group, the osmolality was (319.53±29.51) mOsm/kg, lower than the (300.00±16.44) mOsm/kg observed for the FW group. Largemouth bass raised in saline-alkaline water had better texture characteristics. Largemouth bass raised in SW group had a higher muscle hardness of (34.70±4.86) N, while a higher springiness of (1.06±0.10) mm was observed in the AW group. In summary, the largemouth bass could adapt to the relatively high saline-alkaline environment and be cultured in typical saline-alkaline water with pH from 8.84 to 8.89, carbonate alkalinity from 9.89 to 10.31 mmol/L, salinity from 6.68 to 7.21, showing good muscle quality characteristics with high muscle hardness and springiness. The success of largemouth bass in saline-alkaline water aquaculture has provided an opportunity to promote the aquaculture of this fish in a saline-alkaline stressful environment, providing the theoretical basis for the mechanisms involved in this adaptation process. Our study will broaden the scope of aquaculture in saline-alkaline water, improving the economic benefits and providing the basic parameters for the quality evaluation of fish in saline-alkaline fisheries.
