摘 要：
Feeding quality and feeding strategies can directly affect the survival, growth, feed utilization efficiency and environment of farmed fish. The factors affecting feeding rate and feeding frequency of farmed fish are fish species, body sizes and types of feed. Therefore, determining the appropriate feed, feeding rates and feeding method can improve fish farming production and feed utilization efficiency, reduce the pollution on aquaculture environment. Russian sturgeon (Acipenser gueldenstaedtii) grows fast and has high - quality eggs, but there is no optimum feeding frequency reported in details. Therefore, this study determined the effects of feeding frequency on the growth of juvenile Russian sturgeon, digestive enzymes and ammonia excretion rate, in order to provide guidance of production. Juvenile Russian sturgeon were obtained from a farm in Quzhou, Zhejiang, in July 2012, and acclimated to the experimental conditions for 14 day after being soaked in the povidone - iodine ( 10 ppm) for 5 min prior to the feeding experiments. During the experiment, a commercial diet was used to feed the fish. Experimental fishes were grouped as follows : F2 group : feeding two times per day ( 7 : 00 and 16 : 00) ; F3 group : three times per day (7:00, 11:30 and 16:00) ; F4 group:four times per day (7:00, 11:30, 16:00 and 22:00) ; F6 group:six times per day (7:00, 10:00, 13:00, 16:00 19:00 and 22:00). The total amount of each group was kept as the same as the feeding level of F2 which was fed to satiation. 600 healthy Russian sturgeon juveniles (4.42 + 0.06) g were randomly selected and placed in 12 enamel tanks (140 ~ 60 ~ 60 cm). And then they were randomly divided into four groups, each group contained three repeats of 50 fish. The survival rate was counted after experiments. The fishes were anesthetized at a concentration of 120 mg ~ L-1 of tricaine methanesulfonate (MS - 222) after a starvation of 24 h, and weighed. Three fishes from each tank were sampled randomly, crushed and mixed, and stored at -20 ~C for chemical analysis. And then the other three fishes were randomly selected from each tank, measured length, weighed liver, viscera and body weight. The liver was reserved for the determination of digestive enzyme activity. In the experiment of 35 - 37 day, ammonia excretion rate and oxygen consumption rate experiment were performed in triplicate. Two hours after uniform feeding at 7:00, a fish from each tank was randomly selected, weighed and put in metabolism bag with 20 L water, drained the air in the bag and tightly enclosed the bag. Water temperature, dissolved oxygen, ammonia and other indicators were measured before and after the excretion experiment. The feed and whole fish samples were used to analyze the approximate biochemical composition. Enzyme activities were assayed with commercial kits. Experimental data were expressed as mean + standard error, and were analyzed by one - way analysis differences among mean values, of variance. Duncan' s multiple if there were significant differences. -range tests were used to compare the All statistical analysis were assessed at a significance level of P 〈 0.05. The fishes in F3, F4 and F6 exhibited higher weight gain (WG) than those in F2 ( P 〈 0.05 ). F6 recorded the highest values for WG (461.33%), which did not differ significantly (P 〉0.05) from F3 (409.67%), F4 (443.33%). The feed conversion rate (FCR) was higher in F2 followed by F3, F4 and F6, whereas, F3, F4 and F6 were not different (P 〉 0.05) for this index. When feeding rate was constant, appropriate increase of feeding frequency could improve WG of Russian sturgeon juvenile, and lower FCR significantly. The reason was that reasonable feeding frequency can avoid excessive ingestion in one meal, improving feed digestion and utilization, thus improving the growth and utilization efficiency of feed of juvenile. The condition factor (CF) in F2 and F6 were significantly lower than those of F3 and F4 (P 〈 0.05). In addition, the hepatopancreas somatic indices (HSI) and viscera somatic index (VSI) of experimental fish slightly decreased as the feeding frequency increasing, with no significant difference (P 〉 0.05 ). The moisture, crude protein ( CP), ether extract (EE) and ash were not significantly affected by the feeding frequency ( P 〉 0.05 ). Feeding frequency did not show a significant effect on the liver trypsin activity. There were no differences in amylase activity between F3 and F4, F4 and F6; and the amylase activity of three groups above was significantly higher than F2 ( P 〈 0.05 ). The highest lipase activity was recorded in F6 [27.13 U ~ (g prot) -1~, followed by F3 and F4, F2 recorded the lowest values[ 12.21 U· (mg prot) ^-1- Ammonia excretion rate of fish showed a downward trend with the increasing of feeding frequency. Fish in F2 attained significantly higher ammonia excretion rate ( P 〈 0.05 ) than those of F4 and F6. The ammonia excretion rate in F3 was significantly higher ( P 〈 0.05 ) than the values obtained for F6, but was not different ( P 〉 0.05 ) from that of F4. The oxygen consumption rate obtained from F6 was significantly lower (P 〈 0.05 ) than other groups. However, oxygen consumption rate in F2, F3 and T4 were not different ( P 〉 0. 05 ). The best results were recorded at high feeding frequency ( 6 meals·d-1 ). Ttherefore, combined with the practice of production, feeding frequencies equal to or more than 4 meal·d-1 are recommended to farm Russian sturgeon juveniles.
