Emigration patterns among trout, Salmo trutta (L.), kelts and smolts through spillways in a hydroelectric dam

The emigration patterns among radio-tagged trout, Salmo trutta L., kelts (n = 41, total length: 60-90 cm) and smolts [n = 27, total body length (BL): 22-30 cm] in the regulated River Gudbrandsdalslågen, south-east Norway, were studied by investigating the influence of sex (kelts) and BL (kelts and smolts) on the timing of emigration. In total, 49% of the kelts emigrated towards the hydroelectric dam shortly after spawning, whilst 51% over-wintered. Female kelts were five times more likely to initiate autumn emigration, and eight times more likely to descend the spillways during the first release of surface water than males. Large individuals of both sexes descended earlier than smaller individuals. Larger smolts were more likely to descend during the first release of surface water than smaller smolts. To safeguard the emigration of iteroparous trout kelts and smolts, the spillways should release surface water both in autumn and spring to avoid selective forces during emigration through spillways.