Characteristics of digestive processes in Atlantic salmon ( Salmo salar ). Enzyme pH optima, chyme pH, and enzyme activities

Symptoms indicating digestive malfunction have been observed in cultivated salmon over the last 10–15years, at least partially as a consequence of use of new feed ingredients. The present knowledge on many aspects of digestive functions in fish is limited, in particular for Atlantic salmon. Strengthening is needed to understand the mechanisms underlying challenges to gastrointestinal health and to find means to prevent their development. The present paper supplies information regarding digestive enzyme optima and stability, chyme pH and enzyme activities along the digestive tract of Atlantic salmon. The data presented includes new data from four studies on Atlantic salmon as well as a review of comparative literature from various other fish studies. The four experiments comprised salmon of different sizes, kept in salt or fresh water, and fed high fish meal diets with different protein to energy ratios (P/E). These studies showed an average pH in the stomach chyme, i.e. across diets and experiments, of 4.8 (SD=0.7), whereas the observed pH optimum for pepsin was 3.0. In the intestinal chyme, pH increased throughout the intestine from a mean of 8.1 (SD=0.23) in the proximal intestine to 8.4 in the mid intestine (SD=0.27) and distal intestine (SD=0.29). Total proteolytic activity of chyme in the proximal intestine, measured as the sum of the work of all proteolytic enzymes on casein as substrate, showed its maximum at pH8.8, i.e. clearly above prevailing chyme pH. For the individual proteolytic enzymes, pH optimum varied from 7.10 for elastase 1 to 8.98 for trypsin. Chyme pH did not appear to be influenced by fish size, but was significantly higher in the fish raised in fresh water than in those raised in salt water. In the intestinal chyme pH showed a significant negative regression on diet P/E. For the investigated pancreatic enzymes in the intestinal chyme, the activity in the chyme decreased from proximal to distal regions and faecal activities were very low compared to the activities in the proximal intestine. The activity profile for leucine aminopeptidase (LAP), however, trended to increase from mid to distal intestine. Chyme enzyme activities along the intestinal tract did not differ clearly between the experiments and no clear relationship between P/E on enzyme activities along the intestinal tract was observed. Remarkable in vitro stability was observed for the proteolytic enzyme activities.