The South American sea lion (Otaria flavescens) is an apical marine predator with a wide geographical extension.
The Rio de la Plata estuary is one of the richest and most productive ecosystems worldwide.
The 75 % of the world population of the South American fur seal (Arctocephalus australis) is in Rio de la Plata.
Consequences of industrial fishing regarding marine mammals also depend on the morphological features -body size and mouth gape- of each species.
With the analysis of stable isotopes, experts compared the current diet of these marine species to the food they ate in the past.
Fishing activity in the Rio de la Plata estuary, in the Atlantic coast in Southern America, has reduced the average size of the fish in this ecosystem, which made prey capture easier for marine mammals with smaller mouth gapes. The Franciscana dolphin, a small endemic cetacean in Rio de la Plata and which is an endangered species, is the one benefitting the most from this collateral effect caused by southern fisheries. This is stated in an article led by the experts from the Research Group on Large Marine Vertebrates of the Faculty of Biology and the Biodiversity Research Institute (IRBio) of the University of Barcelona.
The study, published in the journal Scientific Reports, analyses the impact causes by industrial fishing in Uruguay and Argentina on the eating habits of the Franciscana dolphin (Pontoporia blainvillei), the South American fur seal (Arctocephalus australis) and the South American sea lion (Otaria flavescens).
Conclusions show that consequences of industrial fishing regarding marine mammals go beyond a simple reduction in the abundance of prey biomass, and that these depend on the morphological features –body size and mouth gape- of each species.
The new study is signed by the researchers Massimiliano Drago, Àlex Aguilar and Lluís Cardona, from the Research Group on Large Marine Mammals (UB-IRBio); Valentina Franco Trecu (University of the Republic, Uruguay), and Ángel M. Segura, Meica Valdivia and Enrique M. González (National Museum of Natural History, Uruguay).
A wide estuary as shelter for endangered marine mammals
The Rio de la Plata estuary, in the western coast of the Southwestern Atlantic Ocean, is one of the richest and most productive ecosystems worldwide. The merging of the cold current from the Falklands and the warm one from Brazil, which join the freshwater masses in Rio de la Plata, creates a great productivity of commercial interest for the fisheries in the area.
After 1970, Argentina and Uruguay started practising industrial fishing in this area. Since then, this activity has reduced the possibilities of prey species and made it harder for the population of the marine mammals to recover, which have been commercially exploited from the 18th century up to the second half of the 20th century.
At the moment, the Franciscana dolphin (P. blainvillei) is regarded as the most threatened cetacean of the southwestern Atlantic Ocean. With a limited territory extension, this is the only species of river dolphin living in coastal marine waters and is listed as a vulnerable species by the International Union for Conservation of Nature (IUCN).
The South American sea lion (O. flavescens), another apical marine predator with a wide geographical extension, concentrates the 6 % of its worldwide population in the area of Rio de la Plata and is listed in the red list of threatened species by the IUCN in the least concern rang. Regarding the South American fur seal (A. australis), the 75 % of the world population is in Rio de la Plata, an area of great ecological value for the conservation of the species and the maintenance of the genic flow between the populations.
Franciscana dolphin: the most threatened cetacean in the southern Atlantic
"Populations of marine mammals in the area of Rio de la Plata are a great study model to understand changes that occur in trophic relations between the species before and after the development of industrial fishing", notes Massimiliano Drago (UB-IRBio), first author of the study and member of the Research Group of Large Marine Vertebrates led by Professor Àlex Aguilar.
With the analysis of stable carbon and nitrogen isotopes in bone remains, experts compared the current diet of these marine species to the food they ate in the past.
According to the study, Franciscana dolphins are the ones whose resource pattern has changed the most compared to other marine mammals. At the moment, young demersal fish –those living near the seafloor- are their main prey.
"In Franciscana dolphins, the palate breath is intermedium between the sea lions' (robust constitution and a wider palate) and seals (smaller and with a tighter palate). Since intensive fishing reduced the average size of preys, this effect was beneficial to marine mammals with a smaller mouth gate, that is, Franciscana dolphins, and fur seals too", notes Drago, researcher at the Department of Evolutionary Biology, Ecology and Environmental Sciences of the UB.
Therefore, Franciscana dolphins would be listed in the same trophic level as fur seals despite the big difference in their mouth dimensions. Regarding the Franciscana dolphins, these big changes in trophic patterns can be influenced by factors such as a reduction of the population of prey species –midshipman fish, Porichthys poosissimus- which were abundant in the past and have declined now due bycatch.
Fur seals are the least benefited species from the fishing intensification in this area, according to the study. However, this does not mean that the O. flavescens is the most vulnerable species. "The condition of vulnerability depends on a series of factors (reduction of resources, trophic plasticity, commercial exploitation, degradation of habitat, etc.).
The Franciscana dolphin is the most vulnerable species in this ecosystem due the high mortality caused by bycatch and the degradation of the natural habitat", concludes Drago.
Images: Massimiliano Drago (UB-IRBio)
