Sado Estuary IMMA

Size in Square Kilometres

104 km2

Qualifying Species and Criteria

Common bottlenose dolphin – Tursiops truncatus

Criterion B (1), D (1)


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The IMMA is located in the Sado Estuary, Portugal, and adjacent waters, including the Natural Reserve of Sado Estuary and part of the Professor Saldanha Marine Park. The IMMA is particularly important for a resident population of bottlenose dolphins (Tursiops truncatus) that is present year-round and which is highly dependent on the region for activities vital to survival, such as feeding, resting, and nursing of young. The individuals of this small population have distinctive genetics and behavioural traits. At the national level, the population is considered threatened by the Red Book of Vertebrates, as it has been declining in the last decades, and is an aging population with high infant/juvenile mortality. Furthermore, the population is impacted by human activities (e.g. waste disposal [resulting in chemical pollution], dredging, boat traffic, and dolphin watching), which have been increasing in the estuary in recent years.

Description of Qualifying Criteria

Criterion B: Distribution and Abundance

Sub-criterion B1: Small and Resident Populations

The first abundance estimate for this population was 40 animals in 1981 (dos Santos & Lacerda, 1987). Except for a small increase in 1997, the population has declined between the period of 1986-2015, with an unsustainable trend of 30% decrease (ICES, 2016). Presently, the Sado population comprises only 25 individuals (Ferreira et al., 2023), of which 10 are males, eight are females and seven are of unknown sex. The long-term steady decline, an ageing population, high calf/juvenile mortality, and low levels of immigration (dos Santos & Lacerda, 1987; Gaspar, 2004; Augusto et al., 2011; Martinho, 2012; Carvalho et al., 2016; Lacey, 2015) collectively contribute to the bottlenose dolphin population inhabiting the Sado Estuary region (Portugal) being one of the smallest resident populations of this species in Europe. Six individuals in the population are more than 40 years old. These dolphins are year-round residents in the estuary and are consistently observed each month (Gaspar, 2003; Martinho, 2012; Lacey, 2015), with regular monthly monitoring conducted by the national conservation institute. The resident dolphins primarily inhabit the shallow waters near the estuary entrance and the south channel (Gaspar, 2003; Martinho, 2012; Harzen 1998; Grilo, 2010), but they also utilize other available habitats, excluding the upper estuary shallows.

Calving and other reproduction-related behaviour is observed mostly during the spring and summer months (Gaspar, 2003; Augusto et al., 2011; Martinho, 2012), coinciding with increased levels of boat traffic in the region (Sequeira et al., 2009; Luís et al., 2014). Behaviour linked to reproduction occurs throughout the estuary and adjacent marine areas (Luís, 2008; Rocha, 2012). Inter-birth intervals range from three to 11 years (Gaspar, 2003).  Feeding has been observed in various parts of the estuary, encompassing both deep (Nunes, 2001) and shallow waters (Gaspar, 2003; Martinho, 2012; Nunes, 2001; dos Santos et al., 2007). In shallower areas, animals can spend a longer time foraging in the substrate for bottom prey (dos Santos et al., 2007). Mostly during springtime, when some of the prey enter the estuary to reproduce, the bottlenose dolphins follow them towards the interior of the estuary (Gaspar, 2003).

Criterion D: Special Attributes

Sub-criterion D1: Distinctiveness

Recent genetic data (based on microsatellite markers, mitochondrial DNA and Single Nucleotide Polymorphisms) have shown that the dolphins of the Sado estuary are genetically differentiated from the surrounding Iberian populations (Carvalho et al., 2016; Afonso et al. 2022). Low levels of genetic diversity, remarkably high incidence of inbreeding and the presence of unique haplotypes suggest an isolation of this population. The long-term site fidelity and the strong residency patterns, indicative of limited movements, in conjunction with low immigration rates and genetic uniqueness of the Sado estuary bottlenose dolphins, serve as clear indicators of limited connectivity and gene flow with neighbouring populations.

Supporting Information

Afonso, R.O*, Carvalho, I*., Sgarlata G., Borges, F., Martinho, F., Silva, M.A., Prieto, R., Cascão I., Freitas L., Esteban R., Verborgh P., Sequeira, M., Ferreira, M., Marçalo, A., Lopez A., de Stephanis R., Giménez J., Gauffier, P., Pierce, G.J., Chikhi, C. 2022. ‘Fine scale population genomics of the bottlenose dolphin off Western Iberia (northeast Atlantic) ‘. Abstract 34th European Cetacean Conference, Galicia, O Grove, Spain 2023, pp. 50.

Augusto, J., Rachinas-Lopes, P. and dos Santos, M.E. 2011. ‘Social structure of the declining resident community of common bottlenose dolphins in the Sado Estuary, Portugal’. Journal of the Marine Biological Association of the United Kingdom, 92(8):1-10. DOI:

Caeiro, S., Goovaerts, P., Painho, M. and Costa, M. H. 2003. ‘Delineation of estuarine management areas using multivariate geostatistics: The case of Sado Estuary’. Environmental Science and Technology, 37 (18):4052-4059. DOI:

Carvalho, I., Martinho, F., Brito, C., Pierce, G. and Chikhi, L. 2016. ‘Implementação de um programa de caracterização genética, fisiológica e toxicológica da população de roazes’. Technical Report, ICNF, Lisboa, Portugal, 31 pp. Available at: (in PT)

Gaspar, R. 2003. ‘Status of the resident bottlenose dolphin population in the Sado estuary: past, present and future.’ Doctoral Thesis, University of St. Andrews.

Grilo, S. 2010. ‘Estudo da utilização do território pela população de roazes (Tursiops truncatus) do estuário do Sado a partir de dados obtidos em embarcações de Dolphin Watch.’ Master thesis, Lisbon University.

Harzen, S. 1998. ‘Habitat use by the bottlenose dolphin (Tursiops truncatus) in the Sado estuary, Portugal.’ Aquatic Mammals, 24(3),117-128.

ICES. 2016. ‘ OSPAR request on indicator assessment of coastal bottlenose dolphins.’ ICES Special Request Advice. Northeast Atlantic Ecoregion. 14pp.

Lacey, C. 2015. ‘Current status of the resident bottlenose dolphin population in the Sado estuary, Portugal.’ Master thesis, University of Edinburgh.

Luís, A.R.F. 2008. ‘Avaliação do impacto de construções portuárias no comportamento e no ambiente acústico da população de golfinhos-roazes (Tursiops truncatus) do estuário do Sado.’ Master Thesis, University of Lisbon.

Luís, A.R., Couchinho, M.N., dos Santos, M.E. 2014. ‘Changes in the acoustic behavior of resident bottlenose dolphins near operating vessels.’ Marine Mammal Science, 30(4), 1417-1426. DOI:

Martinho, F. 2012. ‘Residency and behavioural patterns of coastal bottlenose dolphins (Tursiops truncatus) in the Arrabida and Troia shores (Portugal).’ Master thesis, University of Lisbon.

Nunes, S. 2001. ‘Estudo da utilização do habitat pela população sedentária de golfinhos-roazes Tursiops truncatus (Montagu, 1821), no estuário do Sado, Portugal.’ Graduation report, University of Algarve.

Rocha, A. 2012. ‘Vocalizações dos golfinhos-roazes (Tursiops truncatus) residentes na região do estuário do Sado e estudo da influência do tráfego marítimo nos padrões de emissão dos assobios.’ Master Thesis, University of Porto.

dos Santos, M.E. and Lacerda, M. 1987. ‘Preliminary observations of the bottlenose dolphin (Tursiops truncatus) in the Sado estuary (Portugal).’ Aquatic Mammals, 13, 65-80.

dos Santos, M.E.D., Coniglione, C. and Louro, S. 2007. ‘Feeding behaviour of the bottlenose dolphin, Tursiops truncatus (Montagu, 1821) in the Sado estuary, Portugal, and a review of its prey species.’ Revista Brasileira de Zoociências, 31-40.

Sequeira, M., Matias, S., Farinha, J.C., Gaspar, R., Silva, C., Augusto, J., et al. 2009. ‘Bases para o plano de acção para a salvaguarda e monitorização da população de roazes do estuário do Sado.’ Instituto da Conservação da Natureza e da Biodiversidade.


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