Nahuel Huapi, Lake Fish Community Evaluation Project: an Ongoing Case History Study for the Patagonian Andean Range of Argentina

Lake Nahuel Huapi is the biggest (554 km²) and deepest ultraoligotrophic lake in Northern Patagonia, centered within Nahuel Huapi National Park. In 2005, we started an ongoing project to provide National Park and Municipal and regional authorities scientifically based information to aid in management. Short and long term objectives were establishment of the lake´s: fish community composition, population structures, distribution, and modeling of trophic food web, energy flow processes, and links with tributaries and sport fisheries. The lake fish community was sampled using depth and area stratified gill netting, icthyoplankton net tows, traps and hydroacoustics. Results to date indicate a fish community with five native fishes and three introduced salmonid species. Galaxias maculatus a small landlocked galaxiid is the most abundant, followed in importance by Oncorhynchus mykiss, Percichthys trucha, Salmo trutta, Galaxias platei, Olivaichthys viedmensis, Salvelinus fontinalis and Odontesthes hatcheri. This latter species was apparently the most impacted by salmonid introductions; its current distribution has been reduced to a few highly vegetated littoral refuge areas. All other species seem to coexist through food resource partitioning and some such as G. platei through habitat segregation. Bioenergetic modeling has shown that G. maculatus and a native crayfish Samastacus sp. are key links in energy transfer. Hydroacoustics have shown the formation of extensive galaxiid larvae aggregations, which through diel vertical migrations could have a profound influence in habitat coupling between the hypolimnion and epilimnion. The lake is a common rearing area for native species and non native adfluvial salmonids. Despite the fact that the lake represents a common pool for the three salmonid species, the assemblages of tributary streams and rivers present varying species compositions which could be related to varying tributary habitat templates. The lake´s salmonid sport fishery, which offers Memorable Stock Density Index specimens for O.mykiss and S. fontinalis and Trophy Stock Density Index specimens for S. trutta, generates locally important economic influx (> 8,000,000 U$A year^-1), attracting fishers from all over Argentina and beyond. Ensuing research includes stable isotope analysis of the lake´s food web structure, evaluation of the fish assemblages of lake tributaries including population and bioenergetic modeling, assemblage composition relation to tributary habitat templates and habitat coupling between tributary and lake fish populations