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

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Age-0 brown trout, Salmo trutta, inhabit shallow and slow-flowing habitats where they can easily maintain stationary swimming positions. However, recent results have shown that they use deeper and faster habitats during daylight than at night, suggesting the occurrence of a nocturnal movement toward stream-margin habitats. Experiments were conducted to describe precisely when this diel pattern of habitat use appears during ontogeny. In two indoor channels, free-embryo brown trout were deposited under the gravel. When emerging, 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 catches, fish habitat use (deep habitat or margin), and fish behavior (resting or swimming) were measured by direct observations and trap counts. Three treatments were performed: (1) fish artificially fed on drifting invertebrates, (2) fish exposed to predators (bullhead, Cottus gobio), and (3) control channels (no food, no predator). In control and food channels, a diel pattern of habitat use was observed 1-2 days after the emergence started. Most fish rested in the margin at night, whereas they moved towards the deep habitat during daylight to hold stationary swimming positions. In the presence of bullhead, most trout were cryptic, and visible fish stood in the margin during both daylight and at night. The importance of predation risk and foraging behavior on the ontogeny of the diel pattern of habitat use is discussed. Results support the direct development without larva from free-embryo via alevin in brown trout.