Effect of water temperature and size of females on the timing of spawning of perch Perca fluviatilis L. in Lake Geneva from 1984 to 2003

The effects of water temperature on the timing of spawning of perch Perca fluviatilis in Lake Geneva were assessed in a 20 year survey (1984–2003). Spawning started at the end of April or at the beginning of May, when the water temperature rose to >10° C, but most of the spawns were laid when water temperature exceeded 12° C. This led to a positive correlation between the date of the mid‐spawning period and the date when the water temperature rose to >12° C. Large perch had a tendency to spawn later than the small ones, which led to a positive correlation between the date of mid‐spawning period and the yearly mean width of perch egg‐ribbons. The coefficient of multiple determination between the date of mid‐spawning period and the mean width of the ribbon of spawn, combined with the date when the water temperature rose to >12° C was r2= 0·77. The perch generally chose deeper spawning places (12 m) at the end of the spawning period than at the beginning (4 m), which was related to water temperature.