High synchrony in size at maturity of Atlantic salmon over a large geographical scale

Size at maturity is an important demographic characteristic due to its strong effect on fecundity. Synchrony in size at maturity across populations suggests similar environmental conditions, partly through shared feeding grounds. We study synchrony in size at maturity over 30 years across 187 populations using data of about 200,000 individual Atlantic salmon belonging to four geographic regions in Norway. Synchrony was very high among the three southern regions, while the northernmost region had more independent fluctuations. Synchrony among closely separated populations was high, declined with distance, but was still significant 1000 km apart. Fulton’s condition factor was consistently larger in the North region, further emphasizing the difference between this region and the three southern regions. The high synchrony reflects that fish within each population have a wide oceanic distribution and therefore, on average, experience a similar environment across populations. Despite that, the northernmost populations forage more north and east of the more southern populations. The high synchrony in size at maturity will generate synchrony in egg deposition, with demographic consequences on a broad geographic scale.