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.