The Effects of a Dietary Bacterial Protein on Mineral Balance in Rainbow Trout (S. gairdneri Rich.)

Abstract

A bacterial protein (I.C.I.'s "Pruteen") was tested in diets for rainbow trout, to investigate the effects of the new feedstuff on the mineral balance of the fish as expressed by the carcass and tissue mineral levels. Supplements of the major and trace elements were added to trout diets in various combinations and with Pruteen, casein or fishmeal as protein sources in order to test the effects of these dietary factors on mineral balance. Interactions between dietary factors were investigated and their effects were related to the growth of the fish, the voluntary intake of food by fish, and the mineral balance. Growth improved in fish fed diets containing 65% Pruteen and supplemented with 4% major-mineral mix containing calcium, magnesium, phosphate, iron, sodium, potassium and chloride. Calcium, magnesium and iron are believed to have been the most active minerals in this mix. In order to estimate the abilities of fish to tolerate water of different calcium content, identical diets were fed to groups of fish held in water of two different harnesses (20 and 40ppm.). The growth was suppressed in fish held in water of 40 ppm hardness, which appeared to induce a temporary deficiency of copper. This effect was not detectable after 50 days. Pruteen was found to increase the calcium, phosphorus and magnesium contents of the fish. The palatability of the three protein sources was determined as fishmeal > Pruteen > casein/gelatin mix. Supplementation of fish diets with calcium was found to decrease the calcium content of the carcass, and especially the calcium and magnesium content of the hard tissues, scales and bone. Mineral levels in the water in which fish were held were estimated by a new method which involved the use of ion exchange resin. This had several advantages when compared with conventional water sampling techniques. To facilitate handling the large quantities of data generated by these studies, a number of computer programs were developed, which will be useful in experimental fish culture.