The amount of food ingested in a single meal by rainbow trout offered chopped herring, dry and wet diets

Two‐year‐old 1·5‐kg rainbow trout were held in cages and conditioned by feeding either on low‐fat chopped herring (H trout) or dry pellets (P trout) for 15 weeks. Their satiation amounts were then determined under standard conditions. On a wet weight basis H trout ate 2·5‐3·5 times more food than P trout; this was sufficient to compensate for the high water content of herring and thereby maintain the dry matter intake. When P trout were offered herring (PH trout) they consumed more food than when offered dry pellets but not as much as H trout. Stomach capacity restricted the intake and their dry matter intake was reduced by c . 40%. When H trout were offered dry pellets (HP trout) they adjusted their intake immediately close to the level of P trout although their larger stomachs could have accommodated more than twice this volume of dry food. The return of appetite after a satiation meal was almost linear with time. Appetite increased at c . 556 mg g ‐1 body weight h ‐1 for H trout and at 142 mg g ‐1 bw h ‐1 for P trout. The return of appetite in PH trout was significantly slower ( c . 370 mg g ‐1 bw h ‐1 ) than in H trout; the previous dietary history of the PH trout limited their capacity to process larger volumes of wet food in a single meal. Fish offered dry diet (P and HP trout) had similar rates of appetite return despite their previous feeding history suggesting that the property of the dry feed itself might limit meal size. The total gastric emptying time of diets of similar dry matter content (with and without large amounts of water) was similar, but the delay time before gastric emptying starts tended to be longer for dry diets. Dry pellets appear to impose a demand for water that prolongs the gastric delay. This water demand is met partly by drinking since the trout fed on dry pellets drank significantly more (436 ± 189 mg kg ‐1 h ‐1 ) than unfed and herring‐fed trout which drank little or not at all (65 ± 113 and 70 ± 66 mg kg ‐1 h ‐1 respectively). Dietary water facilitated food processing and increased daily dry matter intake of trout when fed four times a day. When only one satiation meal per day was allowed, dietary water had no effect. It is concluded from this work that, in addition to gastric volume, a short‐term limitation on the size of satiation meals in the rainbow trout is the availability of water to moisturize the food and thus to promote gastric digestion and emptying.