We review the status of the freshwater fish fauna of Patagonia, an assemblage with 26 native species, comprising fishes of Gondwanan origin, marine dispersants, and oceanic elements of local origin. Several processes, old and new, have shaped the landscape of Patagonia and its fauna: a Gondwanan heritage, the Andes uplifting, Pleistocene ice, volcanic activity, introduction of exotic fishes, mostly Salmonids, and climate change. While there is a significant tradition of taxonomic work on native fish species, research on life history, trophic relationships, and community structure has started to emerge only in the last 15 years. Most studies were conducted in oligotrophic lakes of the Andes; while fauna of streams remains poorly observed. While documentation of impacts by salmonids is scarce, there is some compelling evidence indicating that freshwater communities have been significantly shaped by exotic fish. Impacts by exotic species appear to be dependent on temperature on the east side of the Andes, and land use and watershed perturbation on the west side. In general, freshwater habitat conditions and how they affect fishes are poorly studied. In lakes, habitat complexity and its specialized use by native fishes may have ameliorated the impact by introduced salmonids. Although impacts on rivers abound, led by dam construction, the relationship between stream habitat integrity and native species health is still poorly understood. The future of freshwater resources will largely depend on how able we are to inform managers, the general public and colleagues about their value and the costs of not taking action. But current research capacity is insufficient to deal with most demands because of limitations in people, resources and baseline information. To support our claims, we need to promote regional assessments of freshwater resources and of major threats to their integrity, the building blocks of a regional agenda for their sustainable use.

The examination of population‐specific adaptations of introduced salmonids to the wide range of environments found in Patagonia (southern South America) can help unveil some of the genetic and environmental contributions to life history variation. The rainbow trout Oncorhynchus mykiss introduced into Argentina originated from a few parental stocks. Although some of these stocks were anadromous, all of the dozens of established populations described until now have been freshwater resident. In this paper we provide the first documentation of the presence of an anadromous run of rainbow trout in the Santa Cruz River, the second largest river of Argentinean Patagonia. Microsatellite analysis revealed that anadromous and resident fish from the Santa Cruz River are genetically indistinguishable, probably representing alternative life histories within the same population. Both wild types are very different from the fish of Danish origin that were reared in a local hatchery, suggesting that they are descended more directly from California stocks or that they have been affected by strong drift or selection. Marine growth and freshwater residence are comparable to those of California steelhead. River entrance peaks in early fall. Population age structure and scale pattern analysis indicate that fish enter the river at age 3 but that most do not spawn until their next river entry as 4‐year‐olds. An unusual aspect of Santa Cruz anadromous fish is that they are long‐lived and highly iteroparous. For instance, 20% of the fish analyzed had experienced as many as five spawning events.

We measured the energy density for key species in the freshwater and marine ecosystems of Patagonia, providing the first database of such information for poorly known fauna, including fish, cephalopods, crustaceans, gastropods, annelids, and insects. We then considered an empirical model linking energy density (which is costly to estimate) to water content (which is easy to estimate) and compared the fit of the model with data from different taxonomic groups. Finally, we evaluated the predictive power of models with different levels of taxonomic aggregation to estimate energy density from water content. Fish (7,148‐3,443 J/g of wet weight) had the highest energy density, followed by crustaceans (5,906‐2,507 J/g), insects (5,794‐1,334 J/g), mollusks (5,014‐1,661 J/g), and annelids (4,542‐1,954 J/g). The use of water content as a predictor of energy density provided greatly improved predictions as compared with use of the mean energy density value. As expected, the predictive power improved when more specific levels of taxonomic aggregation were employed. Nevertheless, we were able to identify levels of aggregation that provided high levels of predictive power. We regard the data generated as primary products for future research on energy fluxes in Patagonian aquatic ecosystems.