Growth-history perspective on the decreasing age and size at maturation of exploited Atlantic salmon

Decreases in age and size at maturation have been reported for several exploited fish populations, but little is known about possible changes in individual growth trajectories in the course of exploitation. We investigated changes in growth, age and size at maturation in a declining Atlantic salmon Salmo salar population subjected to strong fishing pressure. Based on historical catch records and scale samples collected between years 1948 and 2003, we estimated age and size at maturation and back-calculated individual growth trajectories. The age of the returning salmon declined, smolt size increased and post-smolt growth decreased during the study period. Reductions in age at maturation and shifts in growth trajectories occurred in parallel, suggesting that they were not independent of each other. Although our results cannot distinguish between plastic and genetic effects of exploitation on size and age at maturation, they support the contention that analyses of individual growth trajectories can provide important clues for understanding causes of life-history changes in exploited fish populations.