Atlantic salmon Salmo salar L. is an anadromous species of high eco-nomic and conservation value that is frequently the focus of man-agement action across Europe and North America. In recent decades, reductions in marine survival (e.g. Chaput, 2012) compounded by habitat loss and degradation have led to widely reported declines in the number of returning adult spawners across much of its geographic range (ICES, 2017; Mills, Pershing, Sheehan & Mountain, 2013). In addition to reductions in overall abundance, some studies have also identified long- term declines in adult body size (e.g. Bal et al., 2017; Bielak & Power, 1986; Jonsson, Jonsson & Albretsen, 2016; Quinn, McGinnity & Cross, 2006), where smaller size at maturity is attributed to lower marine growth rates (Jonsson & Jonsson, 2004). The body size of salmonids is an important characteristic, frequently linked to their reproductive success (Dickerson, Quinn & Willson, 2002).