Spring flood: a primary cue for hatching of river spawning Coregoninae

Increased water discharge during spring floods gives the primary cue for the start of hatching and drift of European vendace (Coregonus albula) and whitefish (Coregonus lavaretus) in the Norwegian River Lâgen. The drift increased with water flow for discharges between 50 and 850 m 3 ∙s −1 (vendace: R 2 = 0.91; whitefish: R 2 = 0.77). The length of hatching period, as judged from the drift, was negatively correlated with the increase in water discharge (vendace: R 2 = 0.73; whitefish: R 2 = 0.68). The length of the egg incubation period and time of hatching were not significantly correlated with the ambient water temperature. Two artificial floods (220 m 3 ∙s −1 ), released before normal spring flooding, increased the total number of drifting Coregoninae larvae in the river. The first flood increased the number of vendace and whitefish larvae per second from 2.1 and 0.0 to 86.1 and 4.5, respectively, before returning to preflood levels. The second flood gave 18.9 and 1.8 individuals/s.