The marker technique has been used widely in trout digestibility studies due to practical difficulties in obtaining “true” faeces samples directly from fish. However this technique is also relatively difficult and there are few good systems for faeces collection. Reliable data on raw material digestibilities are therefore scarce. Traditionally, chromic oxide has been used as a marker. However this substance can give rise to skin allergies in humans and has been implicated as a carcinogen. Jagger et al., (1992) suggested that titanium dioxide could be used successfully as a digestibility marker in pig diets. The present study was designed to determine the digestibility of four feed ingredients for trout (prairie meal, maize distillers, soya 50 and extruded wheat) and to evaluate the use of chromic oxide or titanium dioxide as a digestibility marker in fish diets.

Each test ingredient was included in a basal diet at 0, 100, 200, 300 and 400 g/kg. Titanium dioxide and chromic oxide were included in the basal diet at 1 and 3.7 g/kg respectively.