Exotic rainbow trout Oncorhynchus mykiss support an economically valuable recreational fishery in Patagonia but also create concern for impacts on native organisms. These concerns are intensified by the possibility of hatchery release programs in this region. We estimated losses of different prey from predation by rainbow trout in Lake Moreno, Río Negro Province, Argentina, using a bioenergetics model combined with input data from directed sampling on growth, seasonal diet, distribution, and thermal experience. The fish community was sampled seasonally using gill nets, hydroacoustics, and ichthyoplankton [Article] nets. Pelagic galaxiid larvae and benthic juvenile and adult small puyen Galaxias maculatus were the most important components of the diet. Bioenergetics simulations showed that over a 6-year life span in the lake (ages 1-7), rainbow trout attained a body mass of 2.3 kg and consumed 74.7 kg of food, of which 20% consisted of galaxiid larvae and 16% consisted of adult small puyen. Based on an estimated abundance of 29,000 rainbow trout of ages 1-7, this predator exerted significant but sustainable mortality on the native prey populations, consuming 44 metric tons or an estimated 23% of the annual larval galaxiid production and 35 metric tons of adult small puyen, which represented an unknown fraction of the postlarval population. Galaxiids supported the estimated predation demand under current conditions. However, simulations of stocking strategies normally proposed for this region showed that consumption demands on prey would increase to unsustainable levels, reducing native fish populations and likely reducing growth of rainbow trout. It is also probable that the fish community composition would shift further in response to the increased demand for prey by stocked predators. This implies that in some cases, stocking could jeopardize sport fisheries; stocking strategies should be evaluated on a case-by-case basis to be consistent with specific objectives for native fish conservation and sustainable food web interactions. 1406 VIGLIANO ET AL. 1408 VIGLIANO ET AL. 1416 VIGLIANO ET AL.

Andean Patagonian lakes are ultraoligotrophic, highly transparent water bodies with very low densities of organisms at the different levels of their trophic food webs. Little is known about spatial distribution and habitat use by fish in these lakes. Hydroacoustic and active sampling techniques were used to study the distribution, composition, and displacement of organisms in an Andean lake throughout the diel cycle on two different moon phase days. Sound scattering layers (SSLs) were found both in the near-shore and pelagic habitats at different times of the day. These SSLs, formed by galaxiid larvae and adults, underwent displacement during the time periods of dawn and dusk, giving rise to the redistribution of organisms in the different habitats. The organisms show high sensitivity to light intensity, displaying different behaviors depending on moon phases. Extensive diel vertical migrations show that the deep pelagic habitats provide diurnal refuge for native galaxiids in Andean Patagonian lakes. Due to the high densities of galaxiid larvae and adults, it is likely that diel migrations generate an active flow of energy and matter between habitats, which could have a profound influence on whole lake dynamics.

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