How to increase knowledge about aquatic mammals in the Amazon and in the largest Delta in the Americas?

Regarding aquatic mammals, the Amazon coastal zone and Northeastern coast are the least surveyed region of Brazil. Stable isotope analysis (SIA) were applied to evidence trophic relationships and resource partitioning among aquatic mammals inhabiting different environments.

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Working in the vastness of the Amazon is already a challenge for any researcher, can you imagine studying aquatic mammals in this region? Until 2005 little was known about the diversity of this group and what were the real threats to the species on the IUCN list classified as Near threatened, including the Guiana dolphin Sotalia guianensis, Vulnerable, the Amazonian manatee Trichechus inunguis and Endangered, the Amazon river dolphin Inia geoffrensis.

Aquatic mammals from region studied in natural environment and rescued manatees. Antillean and Amazonian manatee (Trichechus manatus manatus and Trichechus inunguis), Amazon river dolphin Inia geoffrensis and Guiana dolphin Sotalia guianensis. Photos: Rodrigo Baleia, Alexandra Costa, Catarina Ávila and Instituto Bicho D’água files.

The vast mangrove forests impose enormous challenges in the search for stranded aquatic mammals. The solution often found was to walk the coastline for many hours, monitoring beaches piece by piece in search of stranded dolphin carcasses. Often our team was called in to assist strandings of cetaceans in such remote and inhospitable places, where we had to coordinate the collection of samples with the macro tides of the Amazon coast, which flood extensive coastal areas and could surprise us in the middle of the work.

View of Barra Velha beach, one of the beaches surveyed during monitoring on the east coast of Marajó Island during low tide, at high tide the water floods the mangrove forest transforming completely the landscape. Photo: Alexandra Costa.

Our study also comprised the vast Parnaíba Delta, the only one in the Americas that flows directly into the Atlantic Ocean through five large bays. The labyrinth formed by more than 80 islands is located between two states in the northeast of Brazil, Piauí and Maranhão, in a semi-arid region that comprises a mesotidal coast with large areas of mangrove forests. The monitoring in this area is not easy, it can only be reached by small boats and access to the beaches is done on foot.

Aerial view of the complex of estuaries and mangroves that form the Parnaíba Delta, Caju Bay. Photo: Alexandre Martins (Instituto de Pesquisa e Conservação de Tamanduás no Brasil).

But all this effort was rewarded, we helped to set up one of the largest osteological collections of aquatic mammals in Brazil. The Collection of Mammals of the Museu Paraense Emílio Goeldi has today more than 750 specimens collected and registered. All this material can be used by the scientific community, generating research in different areas of knowledge, such as our work published in Scientific Reports.

The incredible task of dealing with high tides, mangrove forests and large rivers along the Amazon coast: a Risso's dolphin tied to a boat to be conducted to a safer place for necropsy, Guiana dolphin stranded along northern Marajó Island, a False killer whale floating with the tide to be carried more easily and fleeing tide pulling a huge Bottlenose dolphin. Photos: Alexandra Costa, Rodrigo Baleia and Instituto Bicho D’água files.

The trajectory of bones from field collection, cleaning, tipping in the collection and use in environmental education activities. Photos: Salvatore Siciliano and Instituto Bicho D’água files.

After a long period of collection samples we finally started discussions about my doctoral thesis. At that time, my advisor Dr Tommaso Giarrizzo was already involved in projects using stable isotopes analysis, so we followed that path.

Our first ecological questions were to investigate the habitat use and trophic relationships among freshwater and marine aquatic mammal species, evaluate isotopic niche partitioning among the most representative species Guiana dolphin Sotalia guianensis, Amazon and Araguaian river dolphins Inia geoffrensis and I. araguaiaensis, and the Amazonian manatee Trichechus inunguis, within the Amazon estuarine complex and characterize differences in isotopic niches between different populations of Guiana dolphin.

We highlight the main findings of this research: a) Isotopic niches were highly distinct, with no overlap among the assessed species inhabiting the Amazon Estuarine areas; b) The Amazon river dolphin and Amazonian manatee occupied significantly larger isotopic niche space, indicating high habitat plasticity; c) The Guiana dolphin inhabited two coastal regions indicating an ecological distinction, dolphins from Amazon estuarine areas occupy a broader isotopic niche area, indicating higher trophic plasticity; d) Among marine delphinids, the carbon isotopic composition in offshore species mirrored that of nearshore species, contradicting the pattern of decreasing δ13C values characteristic of many areas around the world including areas in Southeastern and Southern Brazil; e) No isotopic niche overlap was observed between S. guianensis and Inia spp.; and results found from Guiana dolphin indicated an ecological distinction between populations.

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Mean (dots) and standard deviation (bars) carbon and nitrogen isotopic values in bone-collagen of aquatic mammals collected from the Amazon Estuary and adjacent coastal zones (AE) and Northeastern coast of Brazil (NC). Stranding events from specimens analyzed in this study, Sperm whale Physeter macrocephalus (Pma), Guiana dolphin Sotalia guianensis (Sgui_AE_NC) and False killer whale Pseudorca crassidens (Pcr).

Our findings provide novel data on the stable isotope composition for these threatened cetaceans and sirenian species in this large section of Brazilian coast.

I would like to greatly thank everyone who helped me over the years in the collection, processing the samples, the analyzes, the review of texts and support in my research.

Also, to thank my academic mentor and my co-author in this publication, Salvatore Siciliano, for his friendship, support and patience during all the years of research.

Alexandra Costa over the years of research in different field activities. Measuring a dolphin in the Marajó Bay, Northern region, cataloging bones on the Caju Island, Parnaíba Delta, Northeastern Brazil, monitoring and photographing Guiana and Amazon river dolphins, Pará state. Photos: Instituto Ilha do Caju e Instituto Bicho D’água files.

Header photo: Rodrigo Baleia (in memoriam)

Our manuscript titled, "Resource partitioning among stranded aquatic mammals from Amazon and Northeastern coast of Brazil revealed through Carbon and Nitrogen Stable Isotopes" is available in:

https://doi.org/10.1038/s41598-020-69516-8

Alexandra Costa
Associate Researcher
Instituto Bicho D’água, Belém, Pará, Brazil

Alexandra Costa

Associate Researcher, Instituto Bicho D'água: Conservação Socioambiental

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