| Summary: | Wild Atlantic salmon (Salmo salar) have declined considerably throughouttheir range, principally through loss and degradation of their freshwaterhabitat, with concomitant overfishing of the remaining stocks(Netboy 1968, 1980; Watt 1989). Marine survival varies, and is hypothesisedto depend on greater mortality and reduced growth of smolts atlow temperatures in some years (Sigholt & Finstad 1990; Usher et al.1991), changes in predators, food and ocean currents (Holtby et al.1990; Reddin & Friedland 1993), size and timing of emigrating smolts(Chadwick 1987), or depensatory survival, with relatively greater mortalityoccurring in smaller schools of emigrating smolts than in larger ones(Hvidsten & Johnsen 1993). Nevertheless, viability of the species,especially to conserve the present diversity of genetic stocks, and tosupport commercial and sport fisheries, depends on sustainability of thefreshwater ecosystems in which Atlantic salmon spawn and are reared.Young Atlantic salmon (henceforth referred to simply as "salmon") spendfrom one to eight years in fresh water before migrating to sea, dependingon climate, photoperiod, and relative productivity of the system (Power1969; Metcalfe & Thorpe 1990). Spawning is in late autumn, dependingon water temperatures (ca. 2 to 6°C), when the adult female fish digs aredd in the coarse substratum of shallow riffle areas, in which are laidseveral pockets of eggs. Survival to the fry stage depends on waterpercolation through the substratum, and its temperature, and theduration of incubation also depends on water temperatures (Crisp 1993).Emergence of the fry is in early summer. In warmer European streams,where growth is faster than in boreal systems, juvenile salmon may becalled "parr" after leaving the redd site, but in Canadian rivers they areusually called "fry" or "underyearlings" for the first year, after which they
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