Trophic Impacts of Marine Mammals and Seabirds in the Río de la Plata Estuary and the Nearshore Oceanic Ecosystem

Abstract

As top predators, marine mammals and seabirds play an important role in structuring marine ecosystems, and their trophic role have become a major issue. In this study, we quantify the trophic impact of marine mammals and seabirds in the Río de la Plata estuary based on a previous trophic mass balance model of this estuary. The model comprised 37 functional groups: 3 species of marine mammals (2 cetaceans: Tursiops truncatus, Pontoporia blainvillei and 1 pinniped: Otaria flavescens), 1 coastal bird group comprising 11 species, 17 fish, 12 invertebrates, 2 zooplanktons, 1 phytoplankton, detritus, and 5 fishing fleets. Model results showed that seabirds, O. flavescens and P. blainvillei, are apex predators, with high levels of niche overlap among them, suggesting competition for similar resources such as fish species. Marine mammals and seabirds produce negative effects on commercially important species, while, at the same time, indirect positive effects (increase of the biomass) were also detected in some groups related to trophic cascade effects.

Intensive fishing has not forced dietary change in the South American fur seal Arctophoca(=Arctocephalus)australis off Río de la Plata and adjoining areas

“…Likewise, the interactions between fur seals and other marine mammals in the area remain unknown. These issues can be explored only through ecosystem modelling and the basic parameters of an Ecopath mass balance trophic model for Río de la Plata have been published recently (Bergamino et al, 2012). However, essential information about the South American fur seal population is still missing and for this reason, a precise and updated estimation of population size becomes essential, once its diet has been assessed, to establish effective conservation and management measures for this species.…”

Mouth gape determines the response of marine top predators to long-term fishery-induced changes in food web structure

“…Sciaenids and anchovies are major dietary items for the three marine mammal species considered here, although in varying proportions 20 – 24 . These three top predator are gape-limited 18 , 19 and certainly cannot prey on very large fishes, as shown by available data. The average size of the anchovies consumed by all them is broadly similar and always smaller than 15 cm 22 , this study , which is hardly surprising considering the adult size (average max length: 15.5 cm) of those species 64 .…”

“…Finally, the skull shape of the South American sea lion ( Otaria flavescens ) is indicative of suction feeding 13 , 14 . They all are gape-limited 18 , 19 and available information derived from the analysis of scats, stomach contents and stable isotope indicate that currently have diets primarily based on sciaenids ( Cynoscion guatucupa , Macrodon ancylodon and Micropogonias furnieri ) and anchovies ( Anchoa marinii and Engraulis anchoita ), although in varying proportions 20 – 24 . It should be noted, however, that otariid scats are biased towards the most recent meal and hence are not necessarily good proxies of their global diets.…”

Top–down control by great blue herons Ardea herodias regulates seagrass‐associated epifauna

“…Heron exclusion was associated with increased fish abundance and increased loss of invertebrates to fish predators. Though cascading effects of avian predation are well documented in terrestrial systems for insectivorous birds (Marquis and Whelan , Mooney et al , Barber and Wouk ) and in rocky coastal and pelagic marine habitats (Wootton , Bergamino et al ), the importance of top–down effects by piscivorous birds in vegetated marine habitats is not well documented. Our results provide evidence of cascading trophic effects of piscivorous birds, and support recent findings that top–down control by vertebrate predators is important in seagrass meadows (Moksnes et al , Baden et al , Lewis and Anderson , Burkholder et al , Hughes et al ).…”