Acclimatisation of rainbow trout to sea water

Abstract

The intention of this work was to investigate the problems concerned with transferring rainbow trout Salmo gairdneri directly from fresh water into sea water with reference to this procedure on a marine trout farm. A laboratory study of some of the physiological changes that occur following direct transfer of rainbow trout to sea water was undertaken. The effect of size and salinity on the osmoregulatory ability of the fish was investigated. Field trials involving sea cages were also performed with fish of different sizes and varying salinities. The results obtained from the field were used to produce a nomogram relating size and salinity with expected levels of mortality; this nomogram is thought to have considerable applications in the marine trout farming industry. The results from the field and the laboratory were compared and they indicated a considerable detrimental effect of prior transportation. Two methods for alleviating the osmotic shock following transfer were investigated in the hope of lowering mortalities. The first method investigated was the feeding of a wet diet containing 50% fresh water following transfer with the intention of reducing the dehydration and high plasma concentrations. This method was shown to be ineffective and possible reasons for this are discussed. The second method examined in an attempt to reduce mortalities was the prior feeding of a high salt diet. It was found that the feeding of a diet containing 10% NaCl for a period prior to seawater transfer significantly reduced mortalities during the first few days in the marine environment. The optimum period of prior feeding of the high salt diet was found to be about 2 weeks as longer periods did not improve the beneficial results and resulted in poorer growth rates. Those fish fed on the high salt diet were found to have significantly lower plasma osmotic concentrations following transfer and this finding is discussed in relation to known mechanics of salt excretion. A study of the “chloride cells” in the gills was undertaken but no attributable effect was found following the feeding of the high salt diet.