The nutritional value of dietary fibre for rainbow trout (Salmo gairdneiri)

Abstract

The nutritional value of dietary fibre for rainbow trout, Salmo gairdneri was investigated using juvenile fish (lO-30g) maintained in freshwater at ambient temperatures under a natural photo period. A preliminary experiment was conducted using five purified dietary fibre sources, namely, a~cellulose, lignin, lignosulphonate, galactomannan and chitin ·which varied in physical, chemical and textural characteristics. A commercially available, powdered polyethylene was also used as an inert control ingredient and all sources of fibre were included at a, realistically low level of 5'% in separate semi-purified diets. Although there were no significant differences in the growth of fish at the end of the 10-week trial, several nutritional parameters were affected for rainbow trout fed the different experimental treatments. Mean daily food intake was re"d uced for trout receiving the lignin, chitin and galactomannan diets compared to the polyethylene control. Similarly the food conversion ratios (FeR) and protein efficiency ratios (PER) were relatively inferior for diets containing chitin and galactomannan compared to the lignosulphonate treatment. Maximum net dietary nitrogen utilization was obtained for the polyethylene control t' ... ra ~on whilst lower values were again observed with chitin and galactomannan. Apparent dry matter (DM) and nitrogen digestibility coefficients however were in close agreement for each of the dietary treatments except for the lignin diet which was poorly digested. Generally the results implied that the properties of chitin and galactomannan were worthy of further study at higher inclusion levels and in different physical states. A specific·investigation in which 0, 5, 10, 15 and 20% additions of a purified a-cellulose replaced dietary starch in separate experimental di e t s f a~'1 e d to produce any significant changes in the growth performance of trout and only slightly influenced nutrient utilization at the higher 15 and 20% inclusion levels. Negative digestibility coefficients for the 'unavailable' carbohydrate fraction of diets calculated from the 'total' and 'available' carbohydrate contents of diet and faecal samples was considered to be evidence of the non-nutritive and bulking qualities of a-cellulose. Growth and digestibility trials were then undertaken to examine the effects of including different levels (10 and 20%) and particle size ranges (45-500, 500-1000 microns) of chitin (poly-Nacetyl- D glucosamine) as a natural source of dietary fibre for trout. In a similar experiment, graded amounts of galactomannan polysaccharide (0, 10 and 20%) were added to moist pelleted diets to examine the long term effects of feeding a gel-type fibre characteristic' of many commercially available binding agents. Negative digestibility coefficients for both chitin and galactomannan based on specific biochemical measurements together with the failure to detect any chitinase activity in stomach and intestinal tissue was confirmation of the inability of rainbow trout to degrade and utilize these materials. Coarse grades of chitin at the 10 and 20% levels impaired food intake, growth performance and nutrient utilization as shown by the Poorer FCR, PER, net nitrogen utilization and digestibility coefficients compared to the diets containing finely ground chitin or the a-cellulose control treatment. Similar findings were obtained with increasing additions of galactomannan and there were associated reductions in the serum glucose and protein concentrations with each increment of dietary gala t .. ' . c omannan. The final carcass compositions of fish were also affected by the gel fibre which caused a significant reduction in the tissue lipid content and an inverse trend in moisture content compared to trout receiving an a-cellulose control diet. Further investigations using a sacrificial method to follow and quantify the passage of food through the gastrointestinal tract revealed that the physical properties of fibre such as particle size composition, water retaining capacity and viscosity were among several factors which modified gastric evacuation and digestion rates in rainbow trout. From the predicted gastric emptying times (GET), it was apparent that coarsely graded chitin (20%) and both 10 and 20% inclusions of galactomannan considerably increased the residence time of the gastrointestinal contents compared to finely ground chitin and a control diet without added fibre. Although an exponential relationship was found to best describe the stomach emptying profiles obtained, linearization of the data was achieved by applying surface area and volume dependent mathematical models which emphasized the importance of these physical factors. The combined nutrition and physiological studies supported the contention that fibre is the non-nutritive part of the diet, but it Was concluded that the level and nature of the fibrous material has· important consequences on the processes controlling food intake and the efficiency of digestion, which in turn may affect the assimilation of nutrients and the performance of growing rainbow trout.