Abstract
Here, we analyse changes throughout time in the isotopic niche of the Franciscana dolphin (Pontoporia blainvillei), the South American fur seal (Arctocephalus australis) and the South American sea lion (Otaria flavescens) from the Río de la Plata estuary and adjacent Atlantic Ocean to test the hypothesis that fishing may modify the diet of small-gape predators by reducing the average size of prey. The overall evidence, from stable isotope and stomach contents analyses, reveals major changes in resource partitioning between the three predators considered, mainly because of an increased access of Franciscana dolphins to juvenile demersal fishes. These results are consistent with the changes in the length distribution of demersal fish species resulting from fishing and suggest that Franciscana dolphin has been the most benefited species of the three marine mammal species considered because of its intermediate mouth gape. In conclusion, the impact of fishing on marine mammals goes beyond the simple reduction in prey biomass and is highly dependent on the mouth gape of the species involved.
Highlights
Human activities have impacted most coastal ecosystems around the globe, with fishing as a main actor in this process[1,2]
We found that the prey of Franciscana dolphins differed statistically in their δ13C and δ15N values, both between habitats and species
The overall evidence presented here shows that the Franciscana dolphin has undergone a dramatic dietary change in the Río de la Plata estuary and adjacent Atlantic Ocean waters since the 1970s; currently, the Franciscana dolphin forages at the same trophic level than the South American sea lion, despite major differences in mouth gape, feeding mode and skull morphology
Summary
Human activities have impacted most coastal ecosystems around the globe, with fishing as a main actor in this process[1,2]. 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), in varying proportions[20,21,22,23,24]. The impact of demersal fishing has been severe on sciaenid fishes, leading to a reduction of the total biomass[28] and an increase in the relative abundance of the smaller size classes of species such as the stripped weakfish C. guatucupa[31,32] Such a reduction might have increased food availability for small-gape predators, such as South American fur seals and Franciscana dolphins. In turbid water the sweep feeding of Franciscana dolphins is much more efficient than the pierce feeding of fur seals[17] and, the former species is expected to have experienced more dramatic changes in diet than the latter
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