Zooplankton assemblages and hydrography in the spawning area of anchovy (Engraulis anchoita) off Río de la Plata estuary (Argentina - Uruguay)

Abstract

The massive spawning of anchovy (Engraulis anchoita) in Argentine waters takes place in spring in the coastal sector off the Buenos Aires Province, with the highest concentrations of spawners occurring in front of the Río de la Plata mouth. In October 1995, the size structure, composition and abundance of the zooplankton community was analyzed and related to the hydrography and the abundance of anchovy eggs. Oceanographic conditions were different from the recorded means for the season. A higher proportion of the colder and more saline shelf waters were present in the coastal area and strong haloclines were not observed. High values of plankton biomass were observed in the estuarine frontal area and in the weakly stratified waters of the median shelf. The cluster analysis of the stations gave rise to the formation of coastal (CA), transitional (TA) and shelf (SA) faunistic areas. Mesozooplankton smaller than 1 mm total length (TL) dominated the whole area. The copepods, Paracalanus spp. and Oithona similis, and the cladocerans, Evadne nordmanni and Podon spp., constituted the bulk of this mesozooplankton fraction. Oithona similis was the dominant species in the SA. The copepod, Acartia tonsa (1–2 mm TL), was extremely abundant in the estuarine front (more than 17 600 individuals m–3) followed by appendicularians, members of Pseudocalanidae, anchovy eggs and decapod larvae. Species of Calanidae (Calanoides carinatus and Calanus simillimus), the amphipod hyperiid, Themisto gaudichaudii, and the euphausiid, Euphausia lucens, largely dominated the macrozooplankton (>2 mm TL) in the TA and SA. In the CA, significant positive correlations between the abundance of small copepods (size ranges <1 and 1–2 mm) and anchovy eggs were found. Trophic implications of these findings for first-feeding anchovy larvae were discussed. The adequacy of the TA and SA as feeding ground for adults of anchovy was evaluated.