Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

Summary Smolt traits (length, age) and timing of smolt migration of wild Atlantic salmon, Salmo salar L., were investigated in the Simojoki River, northern Baltic Sea. The aim was to determine whether they responded to changes in parr length, parr density and temperature from 2000 to 2014. Annual electrofishing surveys and smolt numbers determined parr densities by springtime trapping in the river mouth. During the smolt trapping period captured parr and smolts were aged from scales. Water temperature was measured daily. Mean length decreased from 137 mm (TL) to 129 mm among 2-year-old smolts, and from 150 mm to 139 mm among 3-year-olds. Median date of the smolt migration was 10 days earlier, from early June to late May during the study period, linked to the rise in air temperature in May at the nearby Kemi-Tornio airport. However, the median day temperature and the mean daily water temperatures during the second (Q2) and third (Q3) migration quartiles did not change. This implied that migration began when a suitable water temperature was reached, independent of the date.