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Ontogenetic changes in digestive tract histology and digestive enzyme histochemistry were investigated 11 to 36 days post-hatch in white sturgeon Acipenser transmontanus larvae. From initiation of exogenous feeding (12 days post-hatch), larvae were fed a commercial salmonid diet for the ensuing 24 days. The digestive system of white sturgeon displayed a high degree of morphologic organization and functionality at the onset of exogenous feeding. An enhancement of digestive capacities occurred with transition to active feeding. On day 2 of feeding, there was a clear increase of alkaline phosphatase, aminopeptidase M, dipeptidyl peptidase IV, and γ-glutamyl transpeptidase activity in the brush border of the spiral intestine. This strong activity is an apparent confirmation of the importance of this segment of the intestine for protein digestion and nutrient absorption. The functional development of the pyloric intestine occurred on day 4 and was concomitant with an increase in the activity of brush border and cytoplasmic enzymes such as acetylcholinesterase, dipeptidyl peptidase II, α- and β-galactosidases. The absence of acetylcholinesterase, lactase, nonspecific esterase, and weak activity of exopeptidases and alkaline phosphatase in the anterior intestine suggests that this segment of the intestine may be less important in nutrient absorption than the pyloric and spiral intestines. The observed quantitative and qualitative differences in enzyme activity along the intestine indicate a high degree of specialization of each segment for specific digestive and absorptive processes.

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Experiments were undertaken to investigate the apparent digestibility coecient (ADC) of a range of macro-and micronutrients in rainbow trout using three external digestibility markers (chromic oxide, acid insoluble ash and titanium dioxide, alone or in combination), and three methods of faeces collection (modi®ed Guelph system, St-Pee system and abdominal massage; hereafter referred to as column', 'collect' and`strip', respectively). For each method of faeces collection, three replicate tanks of ®sh were acclimated for 5 days to a practical diet containing the external marker followed by 5 days of faeces collection. Both marker type and collection method signi®cantly in¯uenced ADC estimate in a similar manner for dry matter, protein, nitrogen-free extract, ash and energy. The ADC was consistently highest for the column and lowest for the strip methods of collection, and titanium dioxide gave higher ADCs vs. the other markers. Lipid ADC did not dier between the column and collect methods, but collection of faeces by the strip method gave lower ADCs, irrespective of marker type. In general, nutrient ADC was higher with the combined markers than when single markers were employed. The in¯uence of marker type and collection method was less clear for the individual minerals studied. KEY WORDS