In nitrate-rich rivers passing through agricultural watersheds, the deposit
ion of fine sediment in the streambed creates anaerobic conditions favourab
le to nitrite and ammonia production, toxic for salmonids. The aim of this
study was to determine if these nitrogen species have an impact on survival
of trout eggs buried into the gravel. The impact of water quality on incub
ating embryos was studied in two tributaries where brown trout reproduction
occurs. Incubators were buried into the gravel at fertilisation and extrac
ted at eyed stage, hatching and emergence of fry. Characteristics of inters
titial water (dissolved oxygen, nitrite, ammonia) were measured each week d
uring incubation. The study demonstrates that: (1) mortalities are high as
soon as egg stage in the two tributaries and more pronounced on schist; (2)
in the streambeds, nitrite concentrations are lethal for embryos, this imp
act being higher when dissolved oxygen concentrations are low. Ammonia conc
entration seems to be insufficient to have an impact on mortalities; (3) em
bryos mortalities have an impact on interstitial water composition by incre
asing nitrite and ammonia concentrations.