Estuarine fish communities are usually used in an ecological monitoring context to infer the environmental changes ocurring in estuarine environments. In the present paper several analytical tools, namely community analysis indices, species-ranking methods and multivariate ordination techniques, were applied to time series data for the Tagus fish community for 2 periods: 1979 to 1981 and 1995 to 1997. Assessment of the relative importance of anthropogenic versus naturally induced fluctuations is made difficult by the range of natural short-term variations. For the Tagus case study, multivariate ordination techniques, rather than the ranking methods and indices analyzed, provide a good indicator of long-term changes. The major trends in the changes observed in this fish community were a decrease in abundance for several species that use the estuary as a nursery area and an increase in some resident species, namely Gobius spp. and Halobatrachus didactylus. Several species with southern European affinities have increased their importance within the Tagus estuarine fish community. KEY WORDS: Fish community · Environmental changes · Assessment methods · Tagus estuary · PortugalResale or republication not permitted without written consent of the publisher

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SUMMARY: The sand goby Pomatoschistus minutus (Pallas, 1770) and the common goby Pomatoschistus microps (Krøy-er, 1838) are among the most abundant fish species in estuaries, lagoons and along the Atlantic shores of Europe. In the Tagus estuary these species are particularly important in the food web. Fish samples were taken monthly in the upper estuary, and the gut contents of a total of 431 P. minutus and 382 P. microps were analysed in order to study preferences and seasonal changes in their diets and to understand intra-and interspecific relations between these two congeneric gobies. The sand goby had no dominant prey, although mysids were particularly important in the diet of this species. Shrimps and polychaetes were also common prey. P. microps preferentially ingested polychaetes, with isopods, amphipods, bivalves and copepods as secondary prey items. Significant seasonal variations in the ingestion of the main prey groups of both species were obtained, which reflect variations in prey availability. During the reproductive season there were differences in the food consumed by males and females, mainly for P. minutus, reflecting the different activity pattern exhibited by males and females during the breeding season, since parental care is supported by the former. The smallest classes from P. minutus increased their niche widths as they grew. Due to the different length reached by each species, the degree of interspecific overlap decreased with increasing body size. (Krøyer, 1838) están entre las especies más abundantes de peces de estuarios, lagunas y a lo largo de las orillas atlánticas de Europa. En el estuario del Tajo estas especies son particularmente importantes en la cadena alimentaria. Muestras de los pescados fueron tomadas mensualmente en la parte superior del estuario, y el contenido de la tripa de un total de 431 P. minutus y de 382 P. microps fue analizado con la finalidad de estudiar las preferencias y cambios estacionales en sus dietas y comprender las relaciones intra e ínter específicas entre estos dos gobios congéneres. El cabuxino no tenía ninguna presa dominante, aunque los misidáceos eran particularmente importantes en la dieta de esta especie. Los camarones y los poliquetos eran también presa común. Los P. microps injirieron preferencialmente poliquetos, con isópodos, anfípodos, bivalvos y copépodos como presas secundarias. Fueron obtenidas variaciones estacionales significativas en la ingestión de los grupos principales de la presa de ambas especies, lo que refleja variaciones en la disponibilidad de la presa. Durante la estación reproductiva había diferencias en el alimento consumido por los varones y las hembras, principalmente para el P. minutus, reflejando la diferencia en el patrón de la actividad exhibido por los machos y las hembras durante la estación de cría, puesto que el cuidado parental es llevado a cabo por los primeros. Las clases más pequeñas del P. minutus aumentaron el tamaño de sus nichos mientras crecían. Debido a la diversidad de longitud alcanzada por cada e...

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