The Gastrointestinal Tract of Fish - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 10

1.7 LUMINAL PH
Information on the pH of digesta along the GI tract of finfish is not well described in the
scientific literature. In the stomach the pH values can be below 4, while in the pyloric region
and the mid and distal intestine they are above 7 and mostly above 8. The pH of the chyme
seems to be regulated within fairly narrow ranges. In the stomach, pH seem to be higher in
Atlantic salmon (Gislason et al. 1996) compared to mammals (pH 2; Madigan and Martinko
2006). This difference in pH may be of relevance for microbial survival and colonization in
the stomach. No marked decrease in the distal intestine has been observed as might have been
expected if the microbial activity was high.
1.8 PASSAGE RATE AND RESIDENCE TIME
The passage rate and residence time in the various sections along the GI tract influence the
microbial community and subsequently the host and the host-microbial interactions. Stom-
ach evacuation rate and passage time through the intestine have been observed to vary with
temperature, meal size, particle size, feed composition, previous nutritional history, fish size
and stress (Fänge and Grove 1979; Bromley 1994). Diet is also known to affect passage time
(Storebakken etal . 1999) and hence may affect microbial colonization in the gut. To our knowl-
edge, no information is available on the relationship between gut microbiota colonization, gut
passage rate and residence time, and this topic merits further investigations.
1.9 ACKNOWLEDGEMENTS
The authors are grateful to Dr Merrifield, Dr Dimitroglou and Mr Harper for providing their
unpublished micrographs.
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