Dietary transition from pelleted feed to live fry of two brown trout (Salmo trutta) morphs, reared for reintroduction.

The effects of a four-week dietary transition from pelleted feed to live prey fish were assessed for two morphs of brown trout (Salmo trutta) - river trout and sea trout, which were simultaneously divided into four groups. Group C was given the same diet as before; group E was switched onto an experimental pellet; group EF was given both the experimental pellet and prey fish for two weeks, then only prey fish for another 2 weeks; while group F was only given prey fish throughout. Histological measurements were performed using visceral samples obtained after 2 and 4 weeks from ~ 85-110 g trouts (stains: HE, AB-PAS, Perl's Prussian blue). Fish from groups EF and F had shorter intestinal folds, smaller hepatocytes and lesser vacuolization of hepatic parenchyma than fish from C and E. Hepatocyte nuclei and splenic melanomacrophage centers were larger in fish from group F, and the sea trout had larger centers than the river trout. In summary, dietary transition from pellet to prey should last at least 4 weeks prior to fish release, but the (initially) mixed feeding strategy was surprisingly less effective than an abrupt switch onto live prey, especially for the river trout which adjusted quicker to piscivorous diet.