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

Abstract

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.

A multi‐population approach supports common patterns in marine growth and maturation decision in Atlantic salmon (Salmo salar L.) from southern Europe

“…Further temporally resolved monthly or weekly growth patterns could potentially offer a better temporal and spatial resolution and understanding of specific ecological bottlenecks in salmon growth at sea. However, the lack of a consensus on the timing of circuli deposition on scales (Carlson et al, 2021; Peyronnet et al, 2007; Thomas et al, 2019; Todd et al, 2014) would require strong assumptions and potentially higher uncertainty.…”

Temporal shifts in the marine feeding of individual Atlantic salmon inferred from scale isotope ratios

“…For the second feeding season, specific growth rate was computed using length at the end of the first winter and length at capture. For the second summer at sea, the onset of growth was assumed to start at a fixed date set equal to the midpoint (Day 60) of the mid‐February and late March range suggested for annulus formation in marine overwintering salmon (Carlson et al., 2021 ).…”

A semi-automated spectral approach to analyzing cyclical growth patterns using fish scales

“…Some authors proposed or assumed that growth rates are correlated with photoperiod with the midpoint of the zone of closer inter-circuli spacings registered at the winter solstice [ 22 , 23 , 24 ]. More recently, the zone of narrowly spaced circuli was estimated to be deposited during a period of decreasing ocean temperatures in mid-February to late March likely encountered by salmon in the Labrador Sea and the subsequent wider-spaced circuli would be deposited when sea surface temperatures would be increasing in the spring, consistent with ecosystem conditions, bioenergetics, and biological responses of salmon [ 25 ].…”