| Resumo: |
The age-class distribution of the Brazilian flathead Percophis brasiliensis in relation to environmental variables in spring and winter, and the growth of both sexes, were studied in the SW Atlantic Coastal System (SACS)(34°S-31° S). Growth was studied from 1449 individuals (n=853, spring (1998 + 2000), n=596, winter (2004)) to which size (TL), sex, and age were determined. Age was determined considering that an opaque plus a hyaline band (present in the transverse section of the sagitta otolith) represented 1 year of life. The age-class distribution in spring and winter was established through hierarchical clustering and non-metric multi-dimensional scaling (nMDS analysis). This analysis grouped sampling stations (n=109, spring 1998; n=160, winter 2004) with similar sex-age classes structure. The BIO-ENV analysis was used to estimate the association between the distribution of these groups and environmental variables (depth, temperature and salinity of bottom). Although oceanographic regimes differed from their average geographical distribution, P. brasiliensis was present in both seasons in marine coastal waters with lower temperatures and higher salinities and depths; its distribution showed in turn, N-S seasonal changes related to the combined influence of the Río de la Plata (mainly) and El Rincón estuarine regimes. Even though this species is present in the SACS in both seasons P. brasiliensis shows migratory movement, emigrating towards the winter and returning (specially to southern SACS) towards spring to spawn. In this season a marked age-classes ordination associated to salinity is observed whereas in winter this ordination is less evident. In spring, other adults (5-8 year old males, 7,8 year old females) were distributed in higher salinity waters of San Matías Gulf/El Rincón (external coastal zone), younger adults in lower salinity waters of El Rincón (internal coastal zone), and juveniles (0,1 year old males; 0 year old females) in the intermediate coastal zone. In winter, the studied area is more homogeneous with respect to spatial distribution of age-classes mainly characterized by younger adults (2-5 year old males and females). The marked age-classes ordination given in spring would be a mechanism to reduce intraspecific competition for food resources, given the high degree of overlapping between juvenile and adult diets, and the high densities that take place in this season. In both seasons juveniles showed preference for high temperature marine coastal waters, whereas adults tend to occupy colder waters during spring (warmer spawning season); these habitats are suitable for a fast growh of juveniles, and to minimize metabolic costs of adults which may thus allocate more energy to reproductive activity. The degree of overlapping between juvenile and adult habitat is consistent with the 'general concept' of spatial distribution of ontogenetic stages established by Beck et al.(2001).
Universidad de Buenos Aires (UBA). Facultad de Ciencias Exactas y Naturales (Argentina)
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