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

Abstract

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.