SEASONAL-VARIATION IN TRYPSIN ACTIVITY IN JUVENILE ATLANTIC SALMON UPPER AND LOWER MODAL GROUPS

Juvenile salmon in their first year of growth showed a bimodal distrib ution of body lengths by December. For experimental purposes samples o f fish from the upper 2% of body lengths were taken as representing th e upper modal group (UMG), whilst fish from the bottom 5% of body leng ths were taken to represent the lower modal group (LMG). The populatio n of fish from which the samples were taken were fed nd libitum From D ecember to July. During the winter months to March. neither group incr eased in weight. Growth resumed between March and July. The LMG fish h ad a very low food intake, as indicated by the relative weight of dige sta in the stomach, in the winter months. However, following resumptio n of feeding, the relative weight of stomach digesta of the LMG fish e xceeded that of the UMG fish between May and July. The activity of try psin in the intestinal digesta followed a similar pattern, the LMG fis h showing a higher trypsin activity in the spring months. Starvation o f UMG fish for 5 days in winter resulted in accumulation of trypsin in the pancreatic tissues, whilst injection of the trypsin releasing hor mone cholecystokinin (CCK) into starving UMG fish resulted in reductio n of trypsin in the secretory tissues. CCK also caused reduction of tr ypsin in the pancreatic tissues of LMG fish, suggesting that the pancr eas of this group is potentially fully functional during the winter pe riod. Ultrastructure studies of the pancreatic acinar cells showed evi dence for lower secretory activity in the LMG fish, as indicated by sm aller numbers of zymogen granules, less well developed Golgi systems a nd a smaller number of active secreting cells. It appears that trypsin secretion by the pancreas in LMG fish is at a low level during the wi nter, in response to the reduced amounts of food passing through the g ut, which is ultimately controlled by changes in food intake, lowered metabolic level and lowered appetite levels. (C) 1997 The Fisheries So ciety of the British Isles.