Effects of downstream migration on myosin heavy chain expression and dihydropyridine receptor density in farmed smolt of Atlantic salmon

The expression of myosin heavy chain (MHC) subunits and dihydropyridine receptors (DHPRs) from red and white tail muscles of cultured smolts of Atlantic salmon Salmo salar was analysed from samples taken: (1) before the fish were transferred to the river and (2) after the migration distance of c . 50 km. The relative work load of migrating fish, estimated on the basis of swimming speed of Atlantic salmon and discharge rate of the River Simojoki, Finland, was maximally 1700 times higher than that of fish in culture. The data show that after five migration days the density of DHPRs in the muscles of the smolts was increased already. Furthermore, the results indicated a transition from the fast‐to‐slow MHC isoform. Transfer of fish to a natural environment and downstream migration thus had a significant effect on the expression of muscle proteins.