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The age-class distribution of the whitemouth croaker Micropogonias furnieri and its relationship to environmental factors was evaluated from 278 stations sampled between 1993 and 1999 during early winter and spring within the Rio de la Plata estuary (36°S, 56°W). A detrended canonical correspondence analysis (DCCA) was used to define sampling station groups based on age structure of whitemouth croaker, and to estimate their association with depth, type of substrate, temperature and salinity of surface and bottom waters. Complementary information for surface temperature and location of the turbidity fronts was derived from NOAA-14-AVHRR imagery obtained in 1999. Results indicated that bottom salinity has a major influence on the spatial distribution pattern of M. furnieri, and that distinct preferences for different salinities by the various age-classes of this species result in a differential distribution pattern along the main axis of the estuary. In both seasons, turbidity fronts were located at the inner central area of the estuary and showed a close fit with the distribution of the youngest age-classes. KEY WORDS: Micropogonias furnieri · Age-classes · Environmental factors · Río de la Plata · EstuaryResale or republication not permitted without written consent of the publisher

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Seedling browsing by livestock has been indicated as major threat for forest sustainability use. Nothofagus pumilio forests are part of the livestock raising system in Patagonia, but because of the sparse understory cover, livestock graze mainly on flood meadows within the forest matrix. The complexity of the environment under study (forests and flood meadows) means that an adaptive predictor is needed to evaluate the intensity of resource use by livestock in order to assess its effect on the forest. Distance to flood meadows was evaluated as a predictor of the use intensity of the forest by livestock and its effect on the understory. The study was conducted at three sites in Chubut Province, Patagonia-Argentina. We established transects in the forest 320 m long, starting at the edge flood meadow-forest. In these transects, the livestock presence indicators (soil compaction, density of feces and trails) and composition of the understory were evaluated. Generalized Linear Model for repeated measures for longitudinal data were used. The indicators showed that distance is efficient for estimating forest use intensity by livestock. The understory varied with distance, the cover of exotic herbaceous was higher near the flood meadow. Far from the flood meadow, the cover was entirely composited of native species. The results support the use of distance to flood meadows as a tool for decision making in livestock and forest management in N. pumilio forests, and for further research on livestock effect on the forest.