Egg-to-fry survival of atlantic salmon (Salmo Salar l.) in a river of the southern edge of its distribution area,the nivelle

Survival of Atlantic salmon from egg deposition to fry emergence was estimated in the Nivelle River watershed in the zone colonized by spawners (three stretches) and the upstream uncolonized zone (three stretches). Twelve cm(3) incubation capsules, which contained green eggs, were inserted into artificial and natural redds. Discharge and temperature of surface water, oxygen content of surface and intra-gravel water, and gravel composition of redd substratum were monitored all over the experiment period. Survival, 28.6% overall, was higher in upstream stretches (52.7% in the Very Upper Nivelle and its tributary, the Lapitxuri) than in the others, among which, those of the colonized zone (21.5%). There was no survival difference between artificial and natural redds. Mortality mostly occurred during freshets before hatching. A strong positive correlation was observed between oxygen saturation of interstitial water and survival. For a minimum oxygen saturation < 62%, survival was < 27%. On the other hand, gravel composition did not seem to directly influence survival. This factor was not related to oxygen content, suggesting that, in this river, intra-gravel flow does not exclusively come from surface water, but also from low oxygen content alluvial groundwater.