The relative role of density-dependent and density-independent survival in the life cycle of Atlantic salmon Salmo salar



1. Density-dependent factors appeared important for the survival of juvenile Atlantic salmon in the River Imsa whilst density-independent factors were more important for the older fish at sea. In fresh water, density dependence was indicated by a stock−recruitment relationship with increasing loss-rates from eggs to smolts and from eggs to adults as egg density increased. 73% of the loss-rates were explained by variation in egg density. At sea, density independence was indicated by the lack of a significant relationship between loss-rates and smolt densities.

2. The relationship between smolt density and initial egg density was best described by an asymptotic ‘Cushing’ type relationship with a plateau at densities higher than approximately 60 000 eggs for the total river areas of 10 000 m2. The number of smolts developed from the eggs spawned varied between 350 and 2400.

3. The relationship between smolt biomass in wet mass (kg 10 000 m−2) or energy (kJ 10 000 m−2) and the amount of salmon eggs in the River Imsa increased asymptotically. Annual smolt biomass ranged from 13 to 88 kg 10 000 m−2, or 66 000 and 431 000 kJ 10 000 m−2. Variation in egg density accounted for approximately 45% of the variation in smolt biomass (mass or energy).

4. Total wet mass and energy of adults (kg 10 000 m−2 and kJ 10 000 m−2) produced in relation to the amount of eggs at the start of the year-class, were not significantly correlated, due to a high variation among years. The biomass of adults ranged from 73 kg 10 000 m−2 to 655 kg 10 000 m−2 and in energy from 370 000 kJ 10 000 m−2 to 3 270 000 kJ 10 000 m−2.

5. Total adult biomass (adults caught at sea and in rivers) and the returning adults to the River Imsa in mass or energy were correlated with the size of the smolt cohort from which they originated. Yearly total adult biomass ranged between 240 and 3711 kg 10 000 m−2, when the number of smolts ranged from 397 to 2751, respectively. The biomass of adults returning to the River Imsa was between 59 and 614 kg, produced from between 672 and 1621 smolts.