GASTROINTESTINAL RESPONSES OF RAINBOW-TROUT TO DRY PELLET AND LOW-FATHERRING DIETS

Long-term voluntary-feeding experiments were carried out on farmed, 2- year-old rainbow trout offered a commercial dry feed, or chopped low-f at Baltic herring. Despite large differences in dietary water, protein and lipid content, the trout adjusted their intake to consume similar amounts of dry matter. After an Ii-week trial, the stomach volumes of the herring-fed trout were significantly larger (30-35%) than those f ed on the dry diet. Greatest differences were observed when fish were fed one meal per day; increasing the number of daily feeding opportuni ties reduced these expected stomach volumes on each diet by 15-20%. Th e relative increase in stomach volume was shown to be due to growth of the cardiac stomach region (corpus) rather than the pyloric region, a nd not to muscle relaxation; the change was completed within 10 weeks. Data were collected in a separate study to investigate stomach size i n fish (age 0+, 1+, 2+) of similar genetic backgrounds which had been grown using dry pelleted diets. Despite considerable variation between populations, stomach volume to body weight relationship was allometri c (S=a W-b) with the exponent in the range of 0.3-0.4. (C) 1996 The Fi sheries Society of the British Isles