Studies on the acclimation of commercially cultured Sarotherodon species to sea water

Abstract

The initial aims of the present study were to investigate the problems associated with salt water transfer in Sarotherodon spp. with the objective of maintaining high rates of survival and satisfactory growth rates. Members of the euryhaline commercially cultured Sarotherodon species viz. Sarotherodon aureus, S. mossambicus, S. spilurus, S. niloticus and the hybrids of S. aureus/S. niloticus were used. The specific salinity tolerance and the capability of tolerating direct transfers to specified salinities and the comparative abilities of surviving gradual increases of salinity up to full strength sea water were investigated. The involvement of the plasma osmotic concentration in the osmoregulatory process, and the physiological changes following the direct transfer to salt water were examined with special reference to the possibility of using changes of plasma osmotic concentration as an indicator of fish transferability. S. aureus proved to be best able to withstand salinity changes, though in all cases gradual transfer was required to limit mortalities. Feeding of high dietary sodium chloride diet was evaluated for S. mossambicus and S. aureus/S. niloticus hybrids as a method of stimulating osmoregulatory process prior to salt water transfer. This was found to alleviate only slightly the osmotic stress following direct transfer to a known lethal salinity. Water deterioration and temperature fluctuation during fish transportation are inevitable, especially after long journeys. The effects of water quality deterioration, temperature fluctuation and food deprivation, typical of transport conditions, were studied in combination with salinity transfer effects. Effects were significant in all cases though, of the species studied, niloticus appeared to show the highest resistance to the combined effects. Following full acclimation to full strength sea water, the effects of prolonged exposure to sea water on the subsequent survival, growth rates and food conversion were investigated. S. niloticus and the hybrids of S. aureus/S. niloticus were found to be less tolerant to long-term exposure than the other candidates, which were selected for further detailed study. Light and electron microscopic studies of the gills and chloride cells were carried out in S. mossambicus and S. spilurus. This study showed the modified role of the chloride cells in fresh and sea water environments. Increases in number and developments of the ultrastructure of these cells were observed in sea water adapted samples from both of the species. Consequently the significance of the chloride cells in the osmoregulatory process was discussed. The significance of these results was considered in terms of aquaculture practice and suggestions are made for improved transfer methods in this context.