Since 1874, the endemic range of the rainbow trout, Salmo gairdneri Richardson, has been extended through introduction to include eastern North America and the continents of Africa, Asia, Australasia, Europe, and South America. The present range of the species extends into low latitudes by the use of waters at high elevations to over 4500 m above sea level.Environmental factors considered to be of primary importance in the survival of introduced populations are water temperature and precipitation. The presence of suitable spawning grounds, coupled with seasonal water temperatures below 13 C, are essential for the establishment of self-sustaining populations.A further extension of the world range of rainbow trout seems unlikely except perhaps in northeastern Asia. However, distribution within the present range is likely to be increased through the use of new ponds and reservoirs as they are constructed for water supply and flood control by many countries throughout the world. The farming of rainbow trout, now of local significance in parts of eastern Asia, western North America, and central and western Europe, offers an unrealized potential on all continents if warranted by a protein and market demand.

Redd‐substrate composition, water velocity, depth, and other environmental variables associated with redd‐site selection and spawning by brook trout Salvelinus fontinalis and brown trout Salmo trutta in southwestern Ontario streams were examined. Sympatric and allopatric populations spawned in similar ranges of specific conductance (225–810 μmhos/cm), pH (7.0–8.2), dissolved oxygen (>83% saturation), and stream gradient (0.2–2.3%). Brook trout spawned exclusively in areas of groundwater seepage, typically near headwaters where streamflow did not exceed 177 litres/second. Brown trout spawned in a wider range of flows (21–600 liters/second), and utilized locations with and without groundwater seepage. Spawning by brook trout usually began by the second week of October, by brown trout a week later. Brook trout spawning periods lasted 3–5 weeks, those of brown trout, 2–4 weeks. In sympatric populations, an overlap in spawning time occurred for up to 3 weeks. Reuse of redds was mostly intraspecific, although interspecific reuse of brook trout redds by smaller brown trout did occur, particularly below barriers to upstream movement. Mean water depth over redds selected by brook (24.0 cm) and brown trout (25.5 cm) were similar (P > 0.05). However, mean stream velocities were significantly (P < 0.001) slower at brook trout (17.6 cm/second) than at brown trout redds (46.7 cm/second). Average geometric mean sediment size of brook trout redds was significantly smaller than that of brown trout redds (5.7 mm versus 6.9 mm; P < 0.02), but less well sorted. Redd‐site preference by brook trout for areas of groundwater seepage and by brown trout for faster water velocities and coarser substrates minimized species interactions during spawning. Larger body size of mature brown trout (18.0–54.5 cm fork length) than of mature brook trout (8.4–29.0 cm) was probably a factor in the brown troutˈs ability to utilize faster currents where coarser gravels were found. Received December 3, 1982 Accepted August 5, 1983

During the past century, the endemic range of the brook trout, Salvelinus fontinalis (Mitchill), has been extended to include western North America and the continents of Europe, Asia, Africa, and South America. Water temperature appears to be the most important single factor limiting the geographic range, but adequate precipitation and suitable spawning areas are necessary also for the establishment of self-sustaining populations. It is improbable that, with the possible exception of Asia, the present range of the brook trout will be greatly extended through further attempts at introduction.

Since the mid-19th century, numerous attempts have been made to restore or enhance populations of the Atlantic salmon, Salmo salar, within the endemic range, and to establish the species in other parts of the world. Despite all efforts, salmon have become naturalized locally only in eastern North America, Argentina, the Faeroe Islands, and New Zealand. Principal factors affecting the disappearance, restoration, or naturalization of populations are water temperature and availability of suitable spawning and nursery sites. Commercial exploitation of the species in coastal and offshore waters is viewed internationally as a factor of increased importance in the numerical regulation of river spawning runs. A further expansion of the world distribution of Atlantic salmon by naturalization seems unlikely except in areas where preliminary plantings have been made or brood stocks are presently held. Atlantic salmon produced by commercial sea farming operations in Norway, Scotland, Spain, and France are considered to be of comparable quality to fish harvested from the wild. Also, there is presently interest in the potential of sea ranching of Atlantic salmon. Key words: zoogeography, endemic range, naturalized range, artificial propagation, environmental impact, exploitation, aquaculture

During the past century the Eurasian and North African range of the brown trout, Salnz.o truttoL., has been extended to include discontinuous populations on all continents except Antarctica. Primary factors affecting the establishment of naturalized populations are water temperature, precipitation, and suitable spawning grounds. Any future major expansion in the world distribution of the brown trout, with the possible exception of Asia, is unlikely.