Difficulties in Aging Atlantic Salmon (Salmo salar) and Brown Trout (Salmo trutta) from Cold Rivers Due to Lack of Scales as Yearlings

Scales in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) from two cold Norwegian rivers first appeared when the salmon were 33–34 mm long (total length) and the trout were 35–38 mm long. A considerable proportion of the yearlings did not form scales in years with a low water temperature (less than 1000–1100 degree-days from May to October); these individuals lacked the first annulus in the scales. Scales with five circuli or more seldom occurred. The number of days per year with a water temperature above 6.5 °C was better correlated to the formation of scales in Atlantic salmon than the degree-days. In years when the water temperature exceeded 6.5 °C for more than 70–80 days, only minor errors arose in age determination of the salmon; when the temperature exceeded 6.5 °C for less than 70–80 days, some individuals lacked the first annulus in their scales.Key words: Atlantic salmon, brown trout, age determination, temperature, scale formation