| Resumo: |
Benthic communities of the Río de la Plata estuary and their trophic link with Micropogonias furnieri, Paralonchurus brasiliensis, Menticirrhus americanus, Cynoscion guatucupa, Macrodon ancylodon and Umbrina canosai, six species of the Sciaenidae Family, were studied. Benthic composition was characterized using 122 sampling sites between 1999 and 2005. Multivariate and univariate statistical tools were used to analyze diversity, abundance and biomass patterns. A total of 370 taxa were found at the study area. Four areas were established using benthic composition: Inner Río de la Plata (RdP), middle RdP, outer RdP and Marine zone. Higher abundance (9,877 ind/m2) and biomass (1,456.8 g/m2) values were found at middle RdP. Nephtys fluviatilis, Heteromastus sp., Alitta succinea, Mactra isabelleana, Nucula puelcha and Onuphis setosa were usually found at the study area. Regarding environmental variables, no clear trend was found between benthic diversity and salinity, which was related to the stratification of the salt wedge and the spatial extension of the estuary. A total of 1493 stomachs contents of Sciaenidae were sampled between 2001 and 2005 to analyze fish feeding strategies. From a total of 123 prey items, 21 were of pelagic or nektonic origin, while 102 were benthic invertebrates. Size classes under 20 cm total length (TL) of M. furnieri, C. guatucupa and M. ancylodon fed on epibenthic and nektonic preys. As soon as they growth they diverge in diet, with the former species feeding on infaunal preys and later species feeding on the same preys than previous size classes. On the other hand, M. americanus, U. canosai and P. brasiliensis began to prey on infaunal invertebrates at 10 cm TL. Smaller fishes (<20 cm TL) presented higher trophic overlap on the mysid Neomysis americana. However, both P. brasiliensis and M. americanus segregated from the rest of the juveniles because of higher infaunal invertebrate ingestion. On the other hand, trophic partitioning among larger fish (>20 cm LT) was observed, with some exceptions. Overlap was detected for M. furnieri-U. canosai and for C. guatucupa-M. ancylodon at middle and outer RdP. U. canosai segregated from the rest of the species because of higher feeding on amphipods and amphioxus. Only M. furnieri displayed a true generalist trophic strategy, feeding on benthic prey items all over the study area. To conclude, the results allowed to state that coexistence in Sciaenidae under winter conditions might be due to differential tolerance to the salinity gradient, then for species size, morphology of feeding structures and finally, closely related with the previous one, search and pursuing behavior. It is suggested that benthic communities should be considered in the management plans of the M. furnieri fishery.
Universidad Nacional del Comahue, Neuquén (Argentina)
|
