Long Term Fish Assemblages As Units of Management in a Temperate Estuary (Rio De La Plata - SW Atlantic Ocean)

Demersal fish assemblages from trawl surveys in the Rio de la Plata estuary and its inner continental shelf were analyzed from 1975 to 1995. The first two factors of Principal Component Analysis explained 48% of the variance in species distribution, and they are consistent with the results of a cluster analysis. The analysis indicated the existence of three spatially and temporally distinct fish assemblages: internal and external estuarine and inner continental shelf (Uruguayan coastal assemblages). These assemblages were persistent considering the environmental characteristics and their species composition. Despite the changes registered in the species density during the period surveyed, the fish assemblages tend to persist over time. It was demonstrated that the assemblages can be considered as open systems and that there exists a reciprocal flow of organisms between adjacent associations. However, each assemblage showed high spatial and temporal persistence in accordance with the environmental characteristics of the system analyzed. Therefore, and according to the multispecies fisheries operating in the system, each assemblage defined could be considered a unit of management.Original Abstract: Foram analisadas as associacoes dos peixes demersais provenientes de arrastos de prospeccao no estuario do Rio de la Plata e sua plataforma interna no periodo de 1975 a 1995. Os dois primeiros fatores da Analise de Componentes Principals explicaram 48% da variancia na distribuicao das especies sendo consistentes com os resultados da analise de agrupamento. Os resultados mostraram a existencia de tres associacoes de peixes espacial e temporalmente distintas: uruguaia costeira, estuarina externa e estuarina interna. Essas associacoes foram consistentes quanto as caracteristicas ambientais e de composicao de especies. A despeito das mudancas registradas na densidade das especies ao longo do periodo analisado, as associacoes de peixes foram persistentes ao longo do tempo. No presente trabalho foi demonstrado que embora as associacoes sejam consideradas um sistema aberto, um fluxo reciproco de organismos acontece entre associacoes adjacentes. Contudo, cada associacao apresenta diferentes graus de estabilidade, dependentes da estabilidade fisica e temporal do habitat, e das interacoes das especies nas associacoes. No entanto, cada associacao mostrou alta persistencia espacial e temporal, apesar da variabilidade ambiental no sistema analisado. Nesse sentido, e de acordo