Influence of basic flow and hydropeaking duration on the drift of post-emergent fry of brown trout: Experiments on a semi-natural stream.

In salmonid streams, hydropeaking management is often considered as respons ible for the decrease in fry density observed downstream of water outlets. The fry carrying, by flow, could be one of the main causes of such a defici t. To identify and quantify factors involved, the old streambed of the Orie ge stream (French Pyrenees) was arranged. In this semi-natural stream, with natural substratum and refuges, it is possible to simulate hydropeaking an d to quantify the associated fry drift. On a section with riffle-glide sequ ence, two hydropeaking duration (10 min and 3 hours), and two basic flow va lues (the flow before the spate) : 150 and 250 l/s were tested. The spate f low was fixed at 1,500 l/s, from 6 up to 10 times the basic flow, as it is frequently recorded on the Oriege river. The results are compared and discussed. We concluded that hydropeakings cau se a forced drift of post-emergent trout fry, which occurs at the beginning of hydropeaking. Moreover, this drift depended on the basic flow value.