Ecological effects of tilapia stocked in seawater experimental enclosures

The results of enclosure experiment and other in situ ecological experiments,which were carried out between 20 August and 16 September 1996,show that food selection of hybrid tilapia,Oreochromis mossambicus×O.niloticus,and its ecological effects are dependent on food resources available in enclosures.Biomass and production of phytoplankton in the enclosures that stocked tilapia are usually higher than the control enclosure.However,flagellate algae such as Prorocentrum micans,is difficult to be dominant species when more than 8 000 tilapia per hm² is stocked.Tilapia do not exhibit significant impacts on copepods when their preferable food is abundant,but selectively capture copepods of large body size when other food resources are scarce.Concentrations of nitrite,nitrate,ammonia and total nitrogen,and chemical oxygen demand are not significant different between the enclosures stocked tilapia and the control enclosure.Concentration of reactive phosphate in the enclosures that stocked tilapia with densities of more than 8 000 fish per hm²,is relatively high because of improved phosphorus cycle.