Early life development of brown trout (Salmo trutta) eggs under temporary or continuous hypoxial stress : Effects on the gills, yolk sac resorption and morphometric parameters.

Ecological studies on oxygenation in redds in salmonid streams suggest that the egg-to-fry survival strongly depends on oxygen supply at key stages of development. This probably results in an increased eggs mortality. Thus tw o experiments have been carried out to test the effects of temporary or con tinuous hypoxial stress on early life development of brown trout (Salmo tru tta) eggs. Survival, growth, hatching and yolk-sac resorption dynamics were analysed as well as branchial development after hatching in oxygenated and deoxygenated (3 mg/l at a 300 ml/min water flow) conditions. The results show that this oxygen concentration is not lethal for the embry os at hatching. However sublethal effects are observed when hypoxial stress is applied three weeks after fertilisation : hatching time is delayed, ale vins are shorter, water content and yolk-sac conversion rate are lower than the ones from the control. More pronounced effects are observed under cont inuous hypoxic condition and higher mortalities are counted after hatching and until yolk sac resorption. This is probably due to limited respiratory exchanges linked to a reduced development of the gills.