The growth of rainbow trout ( Salmo gairdneri) given diets containing chitin and its relationship to chitinolytic enzymes and chitin digestibility

Abstract

The growth of rainbow trout fed diets containing 4, 10 and 25% chitin, over a 12-week period, was significantly depressed ( P < 0.001) when compared with controls fed diets containing 25% starch. There was no difference in growth rate between control fish and those fed diets containing 10% N-acetyl glucosamine (GlcNAc). Isotopically labelled amino sugars were shown to be oxidized when injected intraperitoneally into rainbow trout. Relatively high levels were found of chitinase activity in the stomachs and of chitobiase in the intestines. These enzyme activities were similar in all the trout, irrespective of the amount of chitin in their diets, except that chitobiase in the intestines of fish fed diets containing GlcNAc showed higher levels of activity than the controls ( P < 0.05). Chitin was not significantly digested when fed at 10 and 30% of the diet but the apparent digestibility of pre-cooked starch was 50% when fed at either 15 or 25%, on the basis of the inert indicator (Cr 2O 3) method. Chitinase and chitobiase activities were not reduced in fish fed diets containing 10% chitin and an antibiotic (Tribrissen ®) although the alimentary microflora were completely eliminated. Enzyme activities were not enhanced when live chitinolytic bacteria ( Vibrio alginolyticus) were incorporated into diets with 10% chitin; these bacteria were only recovered from the intestine. The evidence indicates an endogenous origin of chitinolytic enzymes in the trout gastro-intestinal tract. The presence of either antibiotic or bacteria in the diet had no effect on the digestibility of chitin.