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Gastric evacuation rates in Rainbow Trout (Oncorhynchus mykiss) fed different diets McDonald, Shelley Marie


The passage time of ingested food through the digestive tract affects the appropriate timing of meals and the amount of feed that can be consumed. The rate of digestion of dietary components and the efficiency with which nutrients are absorbed may also be affected by passage time. The present study examined the effects of different dietary concentrations of fibre, lipid and gelatin on the rate of food passage in rainbow trout reared in fresh and salt water. Different ingredients were added to diets in two separate experiments. Feed conversion rates for these diets were determined. The effect of these ingredients on the rate of passage was determined through timed dissections of the stomach and intestine of rainbow trout. The contents were weighed and, in experiment 2, the water content determined. In experiment 1, feeding trials involving three experimental diets and one control diet were conducted. The control diet contained 8% lipid and no fibre additions. The experimental diets were: a 20% lipid diet, 12% pectin diet and a 12% cellulose diet. The pectin diet resulted in poor feed consumption and growth, indicating that an attribute of the diet, taste or texture, was disliked by the fish. The high lipid diet had the best feed conversion while the diet containing cellulose resulted in poor feed conversion. In Experiment 2, both a fresh water and a salt water location were used. Four different dietary treatments were imposed on fish in the fresh water location: a basal diet (control); the basal diet containing 10% gelatin; the basal diet containing an additional %10 of oil ; and, a basal diet containing 10% cellulose and an additional 8% of oil . At the salt water location, diets were similar except the omission of the diet containing gelatin. The effects of the dietary additions on the movement of ingesta through the stomach and intestine were determined for each diet at each site . The percentages of water in the ingesta in the stomachs were compared to determine if there were differences when fish are grown in a saltwater environment as compared to when they are grown i n a freshwater environment. In both experiment 1 and experiment 2, the addition of cellulose to the basal diet, (Diet 4) increased the rate of passage during the initial 12 hours after feeding. Consumption of the diet containing cellose was greater than that of the other experimental diets. An increased consumption of diets with a cellulose addition concurs with the findings reported in the literature . The increased evacuation rate, however, is contrary to previous findings. The high lipid diets evacuated slowly for the first 6 hours and then rapidly after that time. There was a significantly higher water concentration in the stomach contents in the fish reared in salt water than in the fish reared in fresh water. The drinking of salt water by fish affects the rate of evacuation from the stomach, decreasing the time the ingesta takes to pass through the gastrointestinal tract.

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