Fish Distribution and Movements in a Deep Ultraoligotrophic Lake in Northern Patagonia, Argentina

The past 15 years have seen a substantial increase in the number of studies in relation to ultraoligotrophic Andean Patagonian lakes fish communities. However, these studies based on gill nets, tow nets and traps rarely exceed sampling depths below 50 m in environments that commonly average 100 m depths. This is due not only to the difficulties of setting nets at higher depths, but also to low fish densities which require high fishing efforts. Thus, understanding of fish community composition, structure and distribution in these lakes is still partial. In order to shed light on the subject a hydroacoustic study was conducted on Moreno Lake. This typical Patagonian Andean lake of glacial origin is ultraoligotrophic, with a maximum depth of 106 m and thermally stratifying in summer. With only 12 km² it has two basins with equivalent areas but different morphometric characteristics. Vertical and horizontal distribution shifts were revealed by gill nets, fish traps and horizontal and vertical hydroacoustic surveys stratified by depth and by mixis-stratification periods. Horizontal studies used a complex transect design taking into account areal differences between coastal and deep open water areas. Vertical displacement studies were performed over a fix transect for 24 hour periods, taking into account full and new moon influence. Data were acquired using a Biosonics DE 4000 echosounder, with a 120 kHz, 3.25º half angle split beam transducer, and analyzed using Visual Acquisition, Visual Analyzer and Echoview software´s. Results allowed for hydroacoustic identification of two main size fish groups: “big fish” (Oncorhynchus mykiss, Salmo trutta, Salvelinus fontinalis, Percichthys trucha and Galaxias platei) larger than 12 cm total length and “small fish” less than 12 cm TL (G. maculatus adults and galaxiid fish larvae). Horizontal distribution showed a differential use of coastal areas defined as down to 60 m depths, which are mostly used by the bigger sized fish (both native and salmonids) and adult G. maculatus. Whereas the deeper parts of the lake are mostly used by native fishes (smaller sized G. platei and galaxiid fish larvae). Galaxiid larvae aggregates showed an extensive Regular Diel Vertical Migration from 100 m depths to surface waters related to antipredation behavior and influenced by moon light. Thus distribution and movements of fish and the ensuing energy habitat coupling between habitats in deep ultraoligotrophic lakes of the Patagonian Andean Range has shown to be a far more important and complex process than previously known.