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.

The August standing crop of harvestable brown trout (Salmo trutta, 90%), and brook trout (Salvelinus fontinalis, 9%) in the upper Sydenham River, Ont., was estimated at 499 trout/ha (63.2 kg/ha) during 1966 and 1967. Brown trout were of age I (68%), age II (23%), and the remainder of ages III, IV, V, and XIII. Brook trout were ages I (95%) and II only.The average annual harvest by 1501 anglers was 1135 fish (64% brown, 35% brook trout) at a rate of 0.34 fish/hr. Exploitation of the preseason standing populations of brown and brook trout was 23 and 59%, respectively. Catch per unit effort from the downriver section (all brown trout of ages II–IV and mean length of 34.0 cm) averaged 0.10 at an angling pressure of 264 hr/ha. An upriver section occupied predominantly (90%) by brown trout of age I–III and mean length of 26.8 cm yielded 0.67 trout/hr at an angling pressure of 251 hr/ha.The population structure, growth, and exploitation rate of naturalized brown trout favours its use in providing continuous angling of creditable quality in heavily fished headwater streams. Coexistence of self-sustaining populations of brown and brook trout provide a sustained, high-quality mixed fishery.

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