Studies on the effects of environmental factors and selected pathogens on morbidity and mortality of hatchery reared Oreochromis mossambicus (Peters) eggs and fry

Abstract

The dynamics of the physical, chemical and microbiological parameters of hatchery water and their effects on the hatchability and survival of artificially incubated eggs and fry of O. mossambicus in two different recirculatory systems was studied. The tolerance levels of fry stages to important physical and chemical parameters of hatchery water were also estimated. The susceptibility of yolksac fry appeared to increase as they developed and utilised yolk. The egg mortalities were found to be initiated by mechanical injuries to the ohorionic membrane which subsequently attracted bacterial colonisation; the primary cause of death. Fungal infections appeared to have only a secondary role. This mechanism was elucidated using microbiological and SEM techniques. Chemical disinfectants and ultraviolet sterilisation of hatchery water were tested in controlled experiments and an assessment of their relative efficiency in improving hatchability was made. Anti-bacterial activity of the buccal end skin mucus of brooding O. mossambious females was investigated with the view to understanding the protection offered to the progeny whilst brooding. No antibacterial activity could be demonstrated. Experimental infections of Q.mossambicus with I.multifiliis were carried out and lethal and sublethal levels of infections for various age groups of fish were estimated. The sequential pathology of sublethal infections of I.multifiliis on O.mossambicus fry was studied. Adult O. mossambicus females were effectively immunised against I.multifiliis by controlled infections. Antiparasitic factors were demonstrated in both serum and mucus only in immunized fish. Evidence for a specific acquired immune response of humoral origin was found and its implications for aquaculture were discussed. A protection offered to the fry by the immunized females against I.multifiliis infections whilst brooding was demonstrated. This appeared not to operate through a transfer of passive immunity from mother to fry. Alternative mechanisms are suggested