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Communities of aquatic macroinvertebrates, fish density and biomass, and environmental variables were investigated in three Patagonian mountain rivers affected by urbanization. The rivers Las Minas, Esquel and Carbón that flow through the towns of Cholila, Esquel and Corcovado, respectively (northwest Chubut, Argentina) were selected to assess the degree of impairment. A reference site and an urban site were established on each river. Water quality variables including conductivity, major nutrients, total suspended solids (TSS) and dissolved oxygen, habitat conditions and quality of riparian ecosystems were investigated in autumn, winter, spring and summer [2005][2006]. Macroinvertebrates were sampled concurrently in three riffles and three pools at each site. Invertebrate species richness, EPT richness, the Shannon-Weaver diversity index, % EPT density, and the BMPS index were lower at urban sites, whereas % collectors increased. The most impaired site was below Esquel, the largest town. Senzilloides panguipulli (Plecoptera), Polypedilum and Rheotanytarsus species (Diptera: Chironomidae), Nais communis (Oligochaeta) and Meridialaris chiloeensis (Ephemeroptera) dominated assemblages at reference and moderately impaired sites in summer, whereas the strongly polluted reach below Esquel had low flow in summer and a community dominated by Limnodrilus spp. (Oligochaeta), Helobdella spp. (Hirudinea), and two Hyallela species (Amphipoda). Canonical correspondence analysis indicated that ammonia, conductivity and TSS were important variables structuring invertebrate assemblages. In contrast, fish density and biomass varied in a nonsystematic manner among sites. Overall, urbanization resulted in varying degrees of habitat degradation, sedimentation and nutrient enrichment that were reflected by the macroinvertebrate assemblages, which can be used effectively to monitor the effects of urban communities on Patagonian mountain streams.

Salmon and trout species are not native to the southern hemisphere, however rainbow and brown trout have been established a century in southern South America. Yet most attempts to introduce anadromous salmon failed until the onset of aquaculture by 1980. Escapes of Oncorhynchus tshawytscha (Chinook salmon) from aquaculture after 1990 have apparently produced increasingly important reproductive returns "naturalized", to upper basins in Chile and Argentina south of 39 o S. In this paper we show data on the historic and spatial occurrence of chinook salmon in four Pacific basins during the past decade. Our objective is to establish the progress of the settlement forecasting some ecosystem disruptions in order to project and manage potential impacts. In Chile, sampling took place from 1995 to 2005 including rivers Petrohué, Poicas, and Río Negro-Hornopiren, and Lake Puyehue, in the X Region. In Argentina sampled rivers were Futaleufú, Carrenleufú and Pico. In Chile and Argentina reproductive Chinooks ranged in size between 73 and 130 cm total length, being the smallest sizes those of Lake Puyehue where the population is apparently landlocked. In Río Petrohué, the size of the runs varied from year to year reaching in the peak season of 1996 and 2004 up to 500 kg of fish along 100 m of riverbank. Temporal distribution of juvenile Chinooks suggested mainly a typical ocean type as they are gone to sea within the first year of age. As seen in Petrohue, reproductive populations could import significant quantities of marine derived nutrients as they do in their original habitats thus disturbing natural cycles and balances. Chinook establishment in these pristine watersheds in southern South America poses new challenges for decision makers and fishermen since they may develop a fishery in the Pacific Ocean with consequences to other fishery resources. Additionally they also become a resource for sport fishing. Therefore there is the need of developing management tools and approaches to control the populations avoiding irreversible ecosystem disruptions and social conflicts.

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The aim of this paper is to report the colonization process of three headwater rivers of two large trans-national Pacific river basins shared by Chile and Argentina, by exotic Chinook salmon (Oncorhynchus tshawytscha). To characterize the populations at the sites, fish were analyzed in terms of length, weight, age, and stream residency patterns. Fishermen familiar with the area were interviewed to establish the seasonal occurrence of fish and to reconstruct the history of the colonization process. Records of salmon production in Chile by regions were examined to evaluate the potential origin of fish in coastal net pen aquaculture. The regularity of spawning runs in our study sites indicates that Chinook salmon have established spawning populations in these rivers. While spawning was verified in all three rivers, only one seems to serve as a holding area for pre-spawning salmon early in spring. The correlation in the historical occurrence of Chinook salmon in the three rivers suggests that they have been colonized as part of the same introduction event. Moreover, the temporal correlation with the development of ocean net pen production for this species in the Chiloé region suggests that the fish may originate from escapes from these enterprises. As in all previous reports of exotic Chinook salmon in Patagonia and New Zealand, populations in the three rivers have a combination of stream and ocean ecotypes. We discuss the importance of plasticity in critical life history characteristics for the success of Chinook salmon in Patagonia, compared to other species of anadromous salmonids.