Survival And Growth And Mechanism Of Physiological Digestion In Miiuy Croaker (Miichthys Miiuy) During Early Development

Miiuy croaker Miichthys miiuy is an important commercial species in southern China. High mortality and deformity during early development have hindered the industrialization and commercialization of this species and the protection of natural resources. With an attempt to better understand the early life history of the miiuy croaker and also to provide useful information for improving their artificial reproduction technique, rearing management and recruiting natural stocks, the process of growth and survival, the mechanism of physiological digestion and ecological countermeasures under controlled conditions were studied.The early development of miiuy croaker was divided into two phases: larval and juvenile stages. Morphogenesis and differentiation were more intense during larval stage than juvenile stage. Larval stage was characterized by the replacement of embryonic adaptations and functions by definitive ones, such as respiration, exogenous feeding and active swimming. The results confirmed the hypothesis that growth patterns of fish at early life stages: developed the essential organs for survival in priority, and followed by the lower priority for survival.The larvae started to open mouth and feeding on 3 days after hatching (dah) at 24℃. Yolk absorption was completed on 4 dah and the larvae reached PNR (point of no return) on 6 dah. Larvae first fed on 3 and 4 dah (15%—16%) survived better than those fed on 5 dah (9%) at the end of the experiment (36dah). High larval mortality occurred from first feeding time to 7dah in all treatments, accounting for 40% (first fed prior to 5 dah larvae) to 90% (first fed on 6 dah and no fed larvae) of the total mortality throughout the experiment. No larvae first fed after 5dah could survive up to 8dah. Larvae first fed on 3 and 4dah had better growth performance (specific growth rate, SGR=5%) than those first fed on 5dah (4%). However, neither larval survival nor growth showed noticeable differences between the 3 and 4dah first feeding groups.Starvation had significant effects (p<0.05) on survival of miiuy croaker during early development, no starved larvae could survive up to 8dah; in postlarvae period, a starvation-feeding experiment was done with 4 and 6 days starved larvae, the survival rates were 27% and 15%, respectively; but the survival rate of starved 6 days larvae was 85% for juvenile. Photoperiod and density had no significant effects (p>0.05) on growth at early stages. Significant effects (p<0.05) of photoperiod and density on growth were detected after 20dah. Growth was significantly greater at 18L than at 12L and 24L, and at 10 ind l-1 than at 80 ind l-1. Larvae did not start feeding at 0L, and all larvae died within 7dah. Significant differences (P<0.05) were detected in SGR among all groups with the increasing size of miiuy croaker after 20dah.The digestive tract was undifferentiated at newly hatched larvae, and became functional with the development, gastric glands occurred on 20dah and pyloric caecum was formed on 25dah, which showed the complete function of digestive system. The effects of starvation on histological changes mainly embodied the degeneration of cell in digestive organs, such as cell shrinkage, separation and loss of intercellular substance in liver, pancreas, intestine and stomach, and became more severe with starvation elongated. In addition, the effects of starvation on fish were age-dependent.The specific activities of trypsin, amylase, lipase and alkaline phosphatase showed three distinct phases: a sharp increase in enzyme activity from hatching to the onset of exogenous feeding on 3dah; followed by a fluctuation period from 3dah to 25dah; and then a period of relative constant activity from 25 to 53dah. The total activities of these enzymes showed gradual increase from hatching to 30dah, followed by a sharp increase toward 53dah. In addition, delayed 1d first feeding had no effects on the activities of digestive enzymes, but delayed 2d first feeding larvae had significant effects (P<0.05) in activities of digestive enzymes when compared with those of delayed 1d first feeding larvae and normal fed larvae; the activities of digestive enzymes in delayed 3d first feeding larvae had the same developmental patterns with those in starved larvae, and their activities had a sharp decrease from first feeding to 7dah.As a general trend, enzymes activities were significantly lower (P <0.05) in starved fish. On 4 days starved–fed miiuy croaker postlarvae, digestive enzymes activities took 5 days to reach control group levels. On the other hand, 6 days starved–fed postlarvae showed significant differences (P<0.05) after 2 days feeding again; digestive enzymes activities of 6 days starved–fed juvenile took 5 days to reach control group levels. The results showed that the effects of starvation on digestive enzymes decreased with age. The countermeasures of miiuy croaker to starvation was that they firstly consumed the energy of muscle, then consumed the energy of functional organs; when food was available, they firstly recovered the functional organs, then met with the growth of fish body. Digestive enzymes activities also were affected by photoperiod and density, and the digestive enzymes activities in group with 18L and 10 ind l-1 were higher than those in the other groups. Photoperiod, density, starvation and delayed first feeding greatly affected the activities of digestive enzymes, but no effects on the developmental patterns of digestive enzymes, which might be modulated by internal adaptive mechanism.