Acclimation of commercially cultured Oreochromis species to sea water — an experimental study

Abstract

An experimental study was conducted on the acclimation of fresh water Oreochromis species to sea water in order to evaluate the possible marine culture of the species in the subtropical Red Sea waters of Saudi Arabia. Although O. aureus, O. mossambicus, O. spilurus, O. niloticus and O. aureus/O. niloticus hybrids withstand direct transfer to 18‰ salinity, the species exhibit different mortality rates when transferred directly to higher salinities (>21.6‰). The salinity range of 23.4‰–30.6‰ is the critical tolerance level for all the fish. Pre-acclimation to salt water and gradual transfer to higher salinities produced better survival rates. The species O. aureus, O. mossambicus and O. spilurus required a shorter acclimation time (4 days) than O. niloticus and O. aureus/O. niloticus hybrids (8 days) for a transfer to full-strength sea water. A biphasic osmolality response with a sharp increase in the plasma osmotic concentration (within 6–12 h) and a gradual return to equilibrium were recorded in the fish directly transferred to 21.6‰ salinity sea water. The body water content of the hybrids O. aureus/O. niloticus dropped to 72.2% from about 78% within 12 h after the transfer to sea water. The dehydration coincides with the increase in the plasma osmotic concentration of the fish. Among the species tested, O. aureus, O. mossabicus and O. spilurus prove to be the more suitable for marine culture upon gradual transfer.