The Importance of the Tagus Estuary for Biodiversity

The Tagus Estuary is the biggest wetland in Portugal and ranks among the most important in Europe. Every winter, tens of thousands of waterbirds find shelter here. The estuary also serves as a nursery for fish and crustaceans and is frequently visited by wild dolphins. For these reasons, it is considered a reliable indicator of the health of the Tagus River and the Lisbon coast.

Most people crossing the 25th of April Bridge or walking by the river in Belém have no idea what lies just upstream, past the city. There, the Tagus opens into a wide basin of mud, salt, and brackish water, creating one of Europe’s most productive ecosystems. The Tagus Estuary starts right next to Lisbon, and its value for biodiversity extends well beyond Portugal.

The Tagus estuary covers about 34,000 hectares, making it the largest in Western Europe. Inside it, the Tagus Estuary Natural Reserve (RNET) protects around 14,500 hectares of waters, mudflats, saltmarshes, salt pans, small islands, and farmland across Alcochete, Benavente, and Vila Franca de Xira. The tides shape this landscape, exposing or covering thousands of hectares of mud every six hours. This natural rhythm supports much of the life here.

An estuary is where a river’s freshwater meets the salty sea. This mix creates special conditions: the river brings nutrients, and the tides spread them throughout the area. As a result, the estuary reaches an exceptionally high biological productivity, supporting large numbers of marine worms, molluscs, and crustaceans, in quantities far greater than in the open sea. This food supply attracts fish, birds, and marine mammals.

The importance of the Tagus Estuary is recognized by more than just those who admire its beauty. Since 1976, it has been protected by law as the Tagus Estuary Natural Reserve under Decree-Law no. 565/76. In 1980, it was added to the list of Wetlands of International Importance under the Ramsar Convention, reference number 211 (Ramsar Sites Information Service, n.d.). In 1994, it became a Special Protection Area for Wild Birds under the European Union’s Birds Directive and is also part of the Natura 2000 network as a Site of Community Importance. Few places so close to a European capital have so many layers of protection.

Waterbirds are the main reason the Tagus Estuary is known internationally. According to ICNF, about 100,000 birds spend the winter here each year, and this number can go over 140,000 during migration. In total, around 194 bird species have been recorded, along with 35 mammal species, 9 reptiles, and 11 amphibians in the surrounding area (ICNF, n.d.).

Many of these birds are not born or raised in Portugal. They come from populations that breed in northern Europe and Siberia, traveling thousands of kilometers to Africa each year along the East Atlantic Flyway. On this journey, they need places to stop, feed, and rest before moving on. The Tagus Estuary is one of these key stopover sites between the continents.

Birds use the estuary in different ways. Some, like the pied avocet (Recurvirostra avosetta), which is a symbol of the RNET, spend the winter here in numbers that can make up to 75% of the species’ European wintering population (ICNF, n.d.). Others, such as the purple heron and black-winged stilt, nest in the salt pans and marshes. Some species just stop for a few days to rest before moving on. Flamingos, spoonbills, ospreys, and black-tailed godwits are all part of this ecosystem, each following its own schedule, shaped by the tides, seasons, and the journeys ahead.

Although the estuary is well known for its birds, it is also a vital nursery for marine life. Under the water, the Tagus is a place where young fish like sole and sea bass grow and develop. Studies by the Associação Natureza Portugal (WWF) show that the estuary also sends food to the coast, releasing zooplankton that feeds other fish near Lisbon. In 2009, seagrass meadows were found in the estuary, providing valuable shelter for vulnerable species and helping to capture carbon efficiently (ANP/WWF, 2022).

Along the shores, the salt marsh develops under the direct influence of the tide. It is a zone of extremely high productivity for polychaetes, molluscs and crustaceans and they also serve as nurseries for many fish species. Salt marshes also play a discreet but essential role in environmental regulation: they contribute to carbon dioxide sequestration and to the stabilisation of heavy metals present in the water, providing a service that benefits the entire estuarine system, well beyond the species that live there directly.

The Tagus does not have a resident dolphin population like the Sado Estuary does. Still, since the late 20th century, and especially in recent years, dolphins have been seen in the river more often. The most common species spotted are the common dolphin and the bottlenose dolphin, with some sightings of harbour porpoises (ANP/WWF, 2022).

Dolphins are top predators, so they need a healthy food chain to stay in an area. This means there must be enough fish, clean water, and not too much human disturbance. When a dolphin comes into the estuary to feed, it shows that these conditions are present. That’s why scientists and conservation groups call dolphins a sentinel species: their presence or absence is a sign of the estuary’s environmental health.

Since 2022, the Tagus Dolphin Observatory project – a partnership between the Lisbon Port Authority, Associação Natureza Portugal/WWF, and MARE-ISPA – has been tracking cetaceans in the lower estuary from the VTS Tower in Algés. In the first few months, researchers saw dolphins during more than a third of their observation hours, mostly common dolphins. The project aims to do more than just count animals; it wants to find out which parts of the estuary dolphins use, if the same ones return, and what their presence says about years of work to clean up the river, like treating wastewater and reducing pollution.

This research also reminds us that the Tagus’s biodiversity is still under pressure from humans, and its recovery needs ongoing monitoring.

At SeaEO-Tours, we set out each day not knowing exactly what we’ll find in the estuary or along the Lisbon coast. This unpredictability is what makes every tour special. On board, marine biologists and ornithologists help explain what we see, whether it’s dolphin behaviour, the flight of wading birds, or how to tell a resident bird from a visitor.

We also share our sightings with research projects like the Tagus Dolphin Observatory, adding the information we collect on our tours to the scientific understanding of this ecosystem.

A trip through the Tagus Estuary is, above all, an exercise in attention.

On our birdwatching tours in the Tagus Estuary, we follow the rhythm of the tide in search of waders, herons and, depending on the season, flamingos and ospreys, always guided by a biologist or ornithologist on board.

On our dolphin watching tour in Lisbon, we sail between the river and the Atlantic, where there is a chance of finding common or bottlenose dolphins, always respecting safe distances and the code of conduct for cetacean watching.