Ontogeny of diel pattern of stream-margin habitat use by emerging brown trout, Salmo trutta, in experimental channels: influence of food and predatorpresence

Age-0 brown trout, Salmo trutta, inhabit shallow and slow-flowing habitats where they can easily maintain stationary swimming positions. However, rece nt results have shown that they use deeper and faster habitats during dayli ght than at night, suggesting the occurrence of a nocturnal movement toward stream-margin habitats. Experiments were conducted to describe precisely w hen this diel pattern of habitat use appears during ontogeny. In two indoor channels, free-embryo brown trout were deposited under the gravel. When em erging, alevins were free to choose between margin (2 cm deep, 0-2 cm s(-1) ) or deep habitat (12 cm, 2-4 cm s(-1)), or to leave the channel (upstream or downstream). During the week of emergence, upstream and downstream catch es, fish habitat use (deep habitat or margin), and fish behavior (resting o r swimming) were measured by direct observations and trap counts. Three tre atments were performed: (1) fish artificially fed on drifting invertebrates , (2) fish exposed to predators (bullhead, Cottus gobio), and (3) control c hannels (no food, no predator). In control and food channels, a diel patter n of habitat use was observed 1-2 days after the emergence started. Most fi sh rested in the margin at night, whereas they moved towards the deep habit at during daylight to hold stationary swimming positions. In the presence o f bullhead, most trout were cryptic, and visible fish stood in the margin d uring both daylight and at night. The importance of predation risk and fora ging behavior on the ontogeny of the diel pattern of habitat use is discuss ed. Results support the direct development without larva from free-embryo v ia alevin in brown trout.