3.3 Gastric Evacuation Time and Related Studies
Many studies have been performed relating to developing an optimum feeding schedule, mostly for salmonids, but also including a number of other cultured fish.
Variables considered with feeding rate and gastric evacuation time included temperature, season, activity, body size, gut capacity, satiety, and metabolic rate. A relatively consistent finding has been that gastric emptying rate declines more or less exponentially (sometimes linearly) with time. Larger meals first are often, but not always, digested at a faster rate than small meals and the amount of pepsin and acid produced was somewhat proportional to the degree of distension of the stomach. Stomach mobility often increases with the degree of stomach distension also. The appetite, digestion rate, and amount of secretions produced all decreased with decreased temperature, but the secretions also decreased if tested at temperatures in excess of the acclimation temperature. Appetite, i.e., the amount of food eaten voluntarily at one time, appears to be the inverse of stomach fullness, although this does not explain the entire appetite phenomenon. Appetite continues to increase for a number of days after the stomach is empty, indicating that additional metabolic or neural mechanisms are operating. Data on gastric emptying time, digestion rate, and temperature for sockeye salmon have been shown to reflect the underlying phenomenon. Direct comparison of data on digestion among different workers is difficult, because of differences in species, food and methods used.