Effect Of Stocking Density On Adult Turbot Is Growth, Physiology And Water Quality In Recirculating Aquaculture System

Commercial recirculating aquaculture systems (RAS) in intensive culture is one betterstrategy that minimizes waste from culture systems and the risk of disease and is a sustainabledevelopment model. But the higher cost will be one of major obstacle in future development. InRAS, the reasonably intensive density can raise the utilization ratio of water resources andproduction, and decrease the cost of production. The investigation for turbot (Scophthalmusmaximus) growth and change of physiological index in different density would provide areference for high-density culture in RAS, and the observation for water quality change isbeneficial to routine management of RAS. The aim of the present work was to investigate theeffects of turbot stocking density on water quality, growth and physiological index in closed RAS.The fish, with an average initial weight of580.9±44.65g, were random divided three group, i.e.low density (14.30kg/m2), medium density (20.49kg/m2) and high density (31.32kg/m2), and keptin RAS for120days. During experiment, the water quality, growth and physiological index weredetermined in different time. Furthermore, the economic efficiency in three culture density wasanalyzed.The results showed that the survival ratio of turbot was＞95%, and the final average densitywere30.09kg/m2,41.30kg/m2and60.07kg/m2, respectively. In the early stage of the research, theeffects of stocking density on weight gain rate of turbot showed a significant difference (P <0.01),which indicated that high stocking density resulted in crowding stress. The feed coefficients ofthree groups were0.73(low density),0.75(medium density) and0.82(high density), displayingthat there was a positive correlation between feed coefficients and stocking density. Inphysiological index, the level of lysozyme showed a downward trend in high density group, andafter40days, it was lower than the low density group. These presented that high density caused aadverse effect on nonspecific immunity of turbot. Similar results, the cortisol content wasobviously higher in high density group than other at early stage of experiment, indicating the highstocking density was more likely to cause a chronic stress in RAS. However, the level of growthhormone (GH) has no significant variations during reseach and it implied GH of turbot wasinsensitive to stocking density. In water quality analysis, with the increased stocking density, thelevels of total ammonia nitrogen (TAN), nitrite-N (N0—2N) and COD were also elevated, and the elevation in high density group was more significant compared to the other two groups, but thelevels of these index have not exceeded the scope of water quality standard for fisheries. Thenumber of bacteria vibrio in intake kept at low levels, and the average processing rate was morethan99.9%within the research. In contrast, the number of bacteria was strongly increased at laterstage of research in outlet. In the study we also evaluated the profit margin of different densityin RAS, and the data showed that the profit margin in high density group was9%and10%higherthan in low and medium density groups, respectively. In summary, the present study analyzed therelationship between stocking density and water quality, growth, physiological index, profitmargin.