Exotic species introductions in all ecosystems have become a mayor environmental issue, being presently considered as another form of global change, whose impacts affect biodiversity and hence ecosystem structure and functioning. This should be also a central problem to applied ecology as there is the need to find appropriate practical tools to propose sound administrative strategies for species introductions.
The temperate Pacific Coast of the Americas and associated hydrographic basins feature complex hydrological dynamics in freshwater and oceanographic regimes. A very novel approach to comparative studies in these areas among hemispheres is through the Pacific salmon. This group of species have kept one of the most important fisheries in the North Pacific Ocean where they are native, while they have been introduced to freshwater systems of Chile and Argentina and coastal Pacific ocean of southern South America.
Salmon populations are apparently declining in the Pacific Coast of North America due to human directly induced effects such as deforestation, changes in water ways and water chemistry of streams, lakes and coastal oceans, or due to potential indirect or unrelated effects such as global warming and interannual climatic variability. While this is happening, in the southern portion of south America salmon are starting to increase.
The meteoric development of salmon culture in Chile has placed the country as second world producer, this apparently beneficial economic development, however, has implied the massive introduction of exotic salmon species due to accidental escapes during winter storms. Three species are mainly cultivated and are also found free: Oncorhynchus mykiss, Oncorhynchus kisutch and the Atlantic salmon, Salmo salar. Other two species are also present with less frequency, Salmo trutta and O. tschawistcha.
Coho salmon and Chinook present spawning runs of different magnitudes to both Chile and to Argentina through the trans-Andean water ways and successful wild populations are apparently establishing. Runs in some cases are comparable to pre-industrial situation in the NA Pacific Coast and they have a potential large, unprecedented impact due to marine nutrient inputs to river basins where such ecosystemic links did not exist before. In Petrohue river (42 S) the largest Chinook run was registered in April-May 1996, while during early el Niņo in 1997 heavy rains apparently did not allowed spawning while heavy drought apparently associated to late el Niņo did not allow spawing either in 1998. During these same periods, spawning has been observed in the Argentinan side in the largest river basins.
In the inner ocean of southern Chile wild salmon are becoming and important fishing in fisherman nets and coho populations seem to increase in the farther south Aysen Region presenting new challenges both ecological and social since a potential for a new fishery is being considered.
We are reporting here some results of studies in lakes and in the coastal ocean regarding impacts of these introductions and we are showing some potential features for inter-hemispheric comparisons regarding local and global change impacts on salmon populations and the impact of salmon themselves as introduced species.
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