Growth patterns of Labrador Sea Atlantic salmon postsmolts and the temporal scale of recruitment synchrony for North American salmon stocks

We measured postsmolt growth patterns from scales of Atlantic salmon postsmolts captured in the Labrador Sea and adult returns from stocks at the southern end of the range of salmon in North America. Because Labrador Sea postsmolts are believed to represent the juvenile nursery for the entire stock complex, the growth variation for southern stocks may yield insights on the time scale of stock mixing during the postsmolt year. Circuli spacing patterns were extracted from the scales of 1525 salmon for 3 smolt years. For 2 of the 3 years, growth trajectories for fish from the southern stocks intersected the trajectories for Labrador Sea postsmolts after 4-5 circuli pairs. The time that it takes to deposit a circulus during the spring and summer seasons suggests that distribution patterns for regional groups begin to overlap or stocks begin to experience similar environmental conditions by June or July of the postsmolt year, or 1 to 2 months after their migration to sea. In some years, however, it would appear that this mixing does not occur until fall. These data provide the first indication of the time scale of events during the postsmolt year that regional influences may be acting on stocks to produce synchronous recruitment.