Date of marine annulus formation in Atlantic salmon (Salmo salar) and implications for retrospective growth analyses using scales.

Fish scales have increasingly been used to quantify annual and seasonal growth trends and in efforts to relate growth to environmental conditions. Understanding the timing of formation of an annulus (a group of narrowly spaced circuli) is critical when assessing the influence of marine ecosystem conditions on seasonal growth patterns of Atlantic salmon, yet the literature does not provide consistent answers regarding the timing or drivers of marine annulus formation. This study demonstrates a novel method for estimating stock-specific annulus formation timing based on marked individuals with known emigration and return/recovery dates. An equation was applied to estimate the date of annulus completion for Atlantic salmon (Salmo salar) using known dates, number of circuli after the most recent annulus and marine circulus deposition rate. Five marine circulus deposition rate scenarios were tested, some of which accounted for individual, seasonal and age-related variability and others which use previously published marine circulus deposition rates. Based on these results, an argument is presented to reconsider the practice of assigning annulus formation dates to winter solstice in favour of dates estimated by a scenario that accounts for individual, seasonal and age-related variation in circulus deposition. This scenario suggests that annulus formation occurs between mid-February and late March. In this case, the annulus would be formed during the coldest part of the year in the primary overwintering area for North American Atlantic salmon.

Demographic and genetic description of Greenland's only indigenous Atlantic salmon Salmo salar population.

A survey of the Kapisillit River system was conducted in 2005 and 2012 to study the only indigenous Atlantic salmon Salmo salar population in Greenland. Little is known about its characteristics or its relationship with other S. salar populations across the species range. Juvenile S. salar were captured in all stations surveyed within the lower river with the highest densities lower in the river and decreasing densities with increasing distance from the river mouth. Captured juveniles ranged from 0+ to 7+ years old and the predominant smolt age was between 4 and 6 years. Median length of 0+ and 1+ juveniles in August-September was 38.8 and 70.4 mm, respectively. The proportion of mature male parr increased from 4% for 1+ year old fish to 95% for fish greater than 2 years old. Genetic analysis using 96 single nucleotide polymorphisms (SNP) revealed a high degree of genetic similarity between collections, extremely low genetic diversity and low estimates of effective population size (Ne = 28.7; 95% CI = 19.7-42.4). Genetic comparison to range-wide S. salar populations demonstrated that the Kapisillit River S. salar is an outgroup of the eastern Atlantic stock complex, which is consistent with the hypothesised colonisation from the east. River morphology and the absence of glacier runoff are hypothesised to be the main reasons for the relatively high river temperatures supporting this self-sustaining population of S. salar. Given its uniqueness and persistence, this population represents an important part of range-wide biodiversity of S. salar.

Climate and ecosystem linkages explain widespread declines in North American Atlantic salmon populations.

North American Atlantic salmon (Salmo salar) populations experienced substantial declines in the early 1990s, and many populations have persisted at low abundances in recent years. Abundance and productivity declined in a coherent manner across major regions of North America, and this coherence points toward a potential shift in marine survivorship, rather than local, river-specific factors. The major declines in Atlantic salmon populations occurred against a backdrop of physical and biological shifts in Northwest Atlantic ecosystems. Analyses of changes in climate, physical, and lower trophic level biological factors provide substantial evidence that climate conditions directly and indirectly influence the abundance and productivity of North American Atlantic salmon populations. A major decline in salmon abundance after 1990 was preceded by a series of changes across multiple levels of the ecosystem, and a subsequent population change in 1997, primarily related to salmon productivity, followed an unusually low NAO event. Pairwise correlations further demonstrate that climate and physical conditions are associated with changes in plankton communities and prey availability, which are ultimately linked to Atlantic salmon populations. Results suggest that poor trophic conditions, likely due to climate-driven environmental factors, and warmer ocean temperatures throughout their marine habitat area are constraining the productivity and recovery of North American Atlantic salmon populations.