From an examination of over 20 yr of data from the Northwest Miramichi River and some additional data from small tributaries to the Nashwaak River, highest densities of 100 underyearling and 80 yearling or older Atlantic salmon (Salmo salar) per 100 m2 were found at sites where water velocities averaged 50–65 cm/s. At sites with lower or higher water velocities maximum observed densities decreased. Experiments in laboratory streams demonstrated that underyearling Atlantic salmon < 7 cm (total length) occurred in shallow (10–15 cm) pebbly (1.6–6.4 cm diam) riffles of natural streams by choice. As they grew they began to prefer deeper (> 30 cm) riffles with boulders (> 25.6 cm diam). Yearlings > 10 cm reduced the numbers of underyearlings < 6 cm in these deeper habitats by chasing them, and occasionally by catching and eating them. Social interactions, such as displays used in territorial defence, did not occur between yearlings and underyearlings until the latter exceeded 6.5 cm, the size at which they began to move to deeper riffles. Planting densities for hatchery-reared salmon recommended in the literature were refined, taking the space and habitat requirements of different-sized juvenile salmon into account. Key words: spatial relations, rearing capacity, stocking density, stream ecology

1. The effect of habitat fragmentation on freshwater species has been addressed using brown trout Salmo trutta L. as a model species with a dendritic population structure. 2. Microsatellite loci were employed as molecular markers. Levels of gene flow and population subdivision were determined in more than 1200 brown trout individuals inhabiting four south European rivers with contrasting patterns of fragmentation, defined by the presence of barriers. 3. The genetic units in the four rivers were restricted by artificial barriers, and gene flow among samples within each river was associated with the level of fragmentation of the river. 4. Loss of genetic diversity and dislocation of the dendritic model have been detected in fragmented rivers. These results emphasise the importance of mitigating the impact of dams by constructing passages to restore gene flow along the river, for fish and other migratory species, as well as the need for caution in relation to stocking in isolated areas to avoid problems of inbreeding.

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Much research has focused on the developmental behaviour of fish and it has been shown that their sensory and physical capabilities evolve very quickly during their early life. Thus, ontogenesis could influence fishes preferences for particular environmental factors. Little is known about the habitat preferences of trout during the post‐emergence phase and it is not known if they correspond to the preference curves established by the PHABSIM method for the‘ alevin phase‘. Here, the downstream movement and habitat preferences of young emerging trout were studied in a flume. In the absence of predators, alevins preferred a water depth of 20 to 30 cm and pebble rather than gravel substratum. When emergence occurred in an area with 1+ trout and sculpin, Cottus gobio, almost all the emergent trout remained cryptic. When visible, most of them were in the shallowest area (10cm depth) where their preference for pebble substratum was less marked. The presence of 1+ trout and sculpin increased the movement downstream of young trout by 20% without changing the general and diel patterns of catches. Their presence also reduced the initial growth of 0+ trout.