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ISRA FACTSHEETS

SOUTH AMERICAN ATLANTIC REGION

ISRA FACTSHEETS

SOUTH AMERICAN ATLANTIC REGION

Guanabara Bay ISRA

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Guanabara Bay ISRA

Guanabara Bay

Summary

Guanabara Bay is located in southeast Brazil, in the state of Rio de Janeiro. This estuary is influenced by variations in salinity from the connection with oceanic waters, the local rainfall regime, and the tides. The area is characterised by the presence of mangroves, rocky shores, small islands, sandy beaches, and muddy and rocky substrates. The area overlaps with two protected areas. Within this area there are: threatened species (e.g., Spiny Butterfly Ray Gymnura altavela); range-restricted species (Shortnose Guitarfish Zapteryx brevirostris); and reproductive areas (e.g., Longnose Stingray Hypanus guttatus).

Guanabara Bay

Guanabara Bay

DESCRIPTION OF HABITAT

Guanabara Bay is located on the southeastern coast of Brazil, in the state of Rio de Janeiro. This semi-enclosed tropical estuary has an average water volume of 1.87 x 109 m3 and a drainage basin of 4,080 km2 with a maximum depth of 50 m in the central channel (Meniconi et al. 2012, Silva-Junior et al. 2016). It is characterised by seasonal variations in salinity, influenced by the connection with oceanic waters, the local rainfall regime and the tides. During the period of low rainfall (June–August), the water column is more homogeneous, with little variation in temperature and salinity. During the rainy season (December–March), the water column becomes vertically stratified, with the appearance of upwelling areas due to the penetration of the South Atlantic Central Water, which enters the estuary through its saline wedge (Silva-Junior et al. 2016).

The area is divided into three sections: (i) the lower estuary, corresponding to the central channel and its banks, comprising the area under the greatest influence of the oceanic waters entering the bay; (ii) the middle estuary, consisting of an intermediate transition area between the more saline waters of the lower estuary and the more brackish waters of the upper estuary; and (iii) the upper estuary, the innermost region of the bay under greater influence of continental waters from the local hydrographic basin. Due to the size and abiotic variability of the estuary, Guanabara Bay contains a wide variety of habitats. Mangroves, rocky shores, coastal islands (e.g., Paquetá Island, Governador Island), sandy beaches, and muddy and rocky substrates can be found in the bay (Silva-Junior et al. 2016, Souza & Vianna 2022, Teixeira-Leite et al. 2023). Many rivers also flow into the bay, providing an important link between the freshwater and estuarine ecosystems.

The area overlaps with two protected areas, the Guanabara Ecological Station and with the Área de Proteção Ambiental De Guapi-Mirim (UNEP-WCMC & IUCN 2024).

This Important Shark and Ray Area is benthic and pelagic and is delineated from inshore and surface waters (0 m) to 50 m based on the bathymetry of the area.

CRITERION A

VULNERABILITY

Two Qualifying Species considered threatened with extinction according to the IUCN Red List of Threatened Species regularly occurs in the area. These are the Endangered Spiny Butterfly Ray (Dulvy et al. 2021) and Shortnose Guitarfish (Pollom et al. 2020).

Two Qualifying Species considered threatened to extinction according to the Red Book of Marine Fishes of Brazil regularly occur in the area. These are the Critically Endangered Spiny Butterfly Ray and the Vulnerable Shortnose Guitarfish (ICMBio 2018).

CRITERION B

RANGE RESTRICTED

This area holds the regular presence of the Shortnose Guitarfish as a resident range-restricted species.  This species has been regularly recorded in the area in research surveys, by divers, and on social media (Goncalves-Silva et al. 2018b; Teixeira-Leite et al. 2023; iNaturalist 2025). Between July 2005 and June 2007 (bi-weekly), and between November 2012 and March 2015 (monthly), trawl surveys (tows of 7 m length) were conducted in the area (Goncalves-Silva et al. 2018b). Sampling was conducted in five locations across the area in the first period and two locations in the second period. Between 2005–2007, 17 Shortnose Guitarfishes were recorded in the lower part of the area. Individuals measured between 14–48 cm total length (TL). The species was not observed between 2012–2015 as sampling was only conducted in the middle and upper part of the area and the species is mostly found in the lower parts as it is the most influenced by marine waters. However, since then, catches have been reported by fishers and in scientific surveys (Teixeira-Leite et al. 2023; CV Texeira-Leite et al. unpubl. data 2025). The species was reported by recreational divers and by citizen scientists in 2023, confirming their contemporary presence in the area (Instituto Mar Urbano 2023; iNaturalist 2025). Sampling of fishing activities operating outside the area along the coast of Rio de Janeiro (across 130 km of coastline) recorded five Shortnose Guitarfishes between 2016–2019 (Araujo et al. 2020) which is a lower number of individuals than recorded for Guanabara Bay, highlighting the importance of this area for the species. This species occurs primarily in the South Brazil Shelf Large Marine Ecosystem (LME) and marginally in the East Brazil Shelf LME and the Patagonian Shelf LME.

CRITERION C

SUB-CRITERION C1 – REPRODUCTIVE AREAS

Guanabara Bay is an important reproductive area for two ray species.

Between July 2005 and June 2007 (bi-weekly), and between November 2012 and March 2015 (monthly), trawl surveys (tows of 7 m length) were conducted in the area (Goncalves-Silva &Vianna 2018b). Sampling was conducted in five locations across the area in the first period and in two locations for the second period.

From these surveys, 76 Spiny Butterfly Rays were recorded across all years in the middle and upper part of the area (Goncalves-Silva &Vianna 2018a, b). Forty-nine of the individuals were measured and of these 28 (57.1%) ranged in size from 27–49 cm disc width (DW). These individuals were classified as neonates/young-of-the-year (YOY) as the reported size-at-birth for the species is 38–44 cm DW (Last et al. 2016). In some locations inside the area (e.g., Paquetá Island, Governador Island, Jurujuba), >90% of individuals sampled were neonates/YOY (Goncalves-Silva &Vianna 2018a). These life stages were recorded in larger numbers between November–February. Spiny Butterfly Rays were also sampled from fishing activities operating outside the area, along the coast of Rio de Janeiro between 2012–2013 (Goncalves-Silva & Vianna 2018a). Individuals (n = 48) were mostly juveniles and adults, highlighting the importance of Guanabara Bay for early-life stages of this species: this is the only location in the southwest Atlantic where neonates/YOY are regularly found (Goncalves-Silva & Vianna 2018a). Sexual segregation due to reproductive diadromous movements has been observed, with females spending the gestation period in the estuary and adjacent areas and giving birth inside the bay. Neonates/YOY remain inside the estuary until they reach sexual maturity (CV Teixeira-Leite unpubl. data 2025).

Pregnant female and neonate/YOY Longnose Stingrays have been recorded regularly in the area by fishers and divers, and during scientific surveys (Goncalves-Silva &Vianna 2018b; Saüt et al. 2024; Laboratorio de Biologia e Tecnologia Pesqueira unpubl. data 2025; J Campos de Amorim unpubl. data 2025). From the trawl surveys, three Longnose Stingrays (2006 = 1, 2013 = 2) measuring 18.5–20.8 cm DW were recorded (Goncalves-Silva &Vianna 2018b). Size-at-birth for this species is ~15 cm DW (Last et al. 2016), confirming that these individuals were either neonates or YOY. Between 2018–2024, 26 Longnose Stingray were recorded from opportunistic observations made by divers and scientists, and from incidental catches in fishing nets mostly around Paquetá Island (Saüt et al. 2024; J Campos de Amorim unpubl. data 2025). During this period, the majority of individuals were estimated to measure 25–35 cm DW, with only two adults being observed. Based on their size, these individuals were classified as YOY and juveniles. Neonate/YOY Longnose Stingray are regularly seen in shallow areas (<5 m deep), mainly between July and September each year while adults are observed between October–February (Saüt et al. 2024; J Campos de Amorim unpubl. data 2025). There have been reports by fishers of females aborting pups when caught in fishing nets, however, the identification of the species has not been confirmed.

Guanabara Bay

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