Changes in scale circulus spacings of an endangered Atlantic salmon Salmo salar population: evidence of a shift in marine migration?

Abstract

Post‐smolt scale circulus spacing patterns for two Atlantic salmon Salmo salar populations from the Southern Upland (SU) of Nova Scotia, Canada, were compared with spacings from two endangered populations from the inner Bay of Fundy (iBoF) Nova Scotia and New Brunswick, to determine if growth patterns differed among these populations, and if growth patterns had changed as the abundance of these populations declined. An analysis of numbers of marine circuli from scales of post‐smolts and one‐sea‐winter adults of known age indicated that circuli were deposited at a rate of about one circulus per week in summer and slowed to one every 2 weeks in winter. During the summer and the autumn, mean circulus spacing in the iBoF populations, known to have occupied the outer Bay of Fundy during these seasons, was lower than in the SU populations, which are known to migrate to the North Atlantic. Similar circulus spacing patterns within SU populations is suggestive of a common marine distribution for these populations. In contrast, a cluster analysis revealed that within the geographically intermediate Big Salmon River (iBoF), some individuals exhibited wider spacing patterns that resemble the distant migrating SU populations, while others exhibited narrower spacing similar to other iBoF S. salar. Within the Big Salmon River, the relative abundance of the wider and the narrower spacing patterns varied in the earlier years, but all fish sampled since 1999, exhibited wider spacings similar to distant migrating SU S. salar. The apparent disappearance of the narrower pattern, characteristic of localized migration and indicative of historical iBoF populations, suggests that local migration may not presently be a successful strategy for these populations.