Migratory Atlantic salmon as vectors for the transfer of energy and nutrients between freshwater and marine environments

SUMMARY 1. Annual energy, carbon, nitrogen and phosphorus fluxes across the river mouth by Atlantic salmon were estimated for 18 years (1976–94) in the Norwegian River Imsa. The total energy content of the emigrating smolts in each year varied considerably with a mean value of 237 × 10³ kJ. That of returning adults also varied between years with a mean value of 141 × 10⁴ kJ. One‐sea‐winter salmon (grilse) made up 65% of the total energy content of the spawners in the river. Dead carcasses remaining in the river after spawning were estimated to have a mean annual energy content of 175 × 10³ kJ. 2. The net annual energy flux from the sea to the river varied between 48 × 10³ kJ (1987) and 152 × 10⁴ kJ (1989) with a mean of 616 × 10³ kJ, and a coefficient of variation of 67%. Average net marine import of the returning adults was 83 × 10⁴ kJ year^?1 with a coefficient of variation of 52%. Mean annual export of C, N and P to sea by the smolts was 595, 131 and 22 kg, and by kelts 1535, 352 and 70 kg, respectively, whereas gross import via the adults was 3176 kg C, 735 kg N and 132 kg P. The annual flux across the river mouth was 1046 kg C, 253 kg N and 39 kg P. The net marine import were 1585 kg C, 371 kg N and 60 kg P. The net flux was estimated at 0.2% for nitrogen and 5% for phosphorus of the total river load. 3. The energy flux caused by Atlantic salmon spawning in the River Imsa was relatively high because the general nutrient load in the river is low. Thus, even though most Atlantic salmon survive spawning, their contribution to the nutrient flux in the river is significant.