Egg-to-fry survival of brown trout (Salmo trutta) and chemical environmentin the redd.

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.