Observations sur le comportement d'éclosion et de post-éclosion chez l'ombre commun, Thymallus thymallus(L.)

The present investigation was carried out in May-June 1975 in a small experimental fish hatchery supplied with water from a tributary of the River Ourthe (River Meuse basin, Belgium). A first set of observations deals with the duration of the incubation period (177 degree-days or 22 days at 8,05°C), the diurnal rythm of hatching, the mortality rate at hatching time (10%) and the relationship between the size of eggs and newly hatched embryos. At 13°C, the complete resorption of the yolk sac is achieved after 12 days (156 degrees-days). Exogenous feeding starts 4-5 days after hatching. Unfed embryos begin to die at a high rate the day following the complete resorption of the yolk sac (hatching + 13 days); 100 % mortality is observed ten days later (hatching + 23 days). By simulating the gravel environnement in which incubation naturally takes place, we demonstrated in Th. thymallus a post-hatching sub-gravel behavior : the newly hatched embryos spend 5-10 days burrowed in gravel and then emerge into open water. This emergence behavior is connected with the onset of exogenous feeding. Survival rate at emergence is 45-58 % depending on gravel size.