Monthly indices of the post-smolt growth of Atlantic salmon from the Drammen River, Norway

Indices of the monthly post-smolt growth of three maturity groups of Atlantic salmon Salmo salar from the Drammen River in Norway were examined. Scale samples were collected from 1983 to 2003, providing a retrospective growth history for 2854 individuals. Overall post-smolt growth of all three maturity groups decreased throughout the study period. When the post-smolt year was segmented into 8 monthly indices, there was a significant decline in growth during the fourth and fifth month, typically months of maximum growth. The pre-fishery abundance of one and two sea-winter fish from Norway also showed a decline throughout the study period and was positively correlated to the post-smolt growth of fish returning to the Drammen River. Freshwater growth was negatively correlated to the pre-fishery abundance, thus indicating that factors affecting post-smolt growth during the fourth and fifth months at sea may be responsible for declining return numbers in Norway.