Rio's Guanabara Bay is infamously filthy.

Key points: Just 34 grey dolphins left in filthy bay where Rio Olympic sailing events to take place

Just 34 grey dolphins left in filthy bay where Rio Olympic sailing events to take place $US1b spent on programs that failed to clean up waterway

$US1b spent on programs that failed to clean up waterway Bay used to be home to thousands of dolphins, several species of whales

Cariocas, as Rio residents are known, have steered clear of its calm waters for decades, preferring to swim in the city's cleaner ocean beaches.

Everything from furniture, dead animals and heavy metals can be found in the bay, as raw domestic and industrial sewage flows freely from rivers and towns on its 143 kilometre perimeter.

It is where sailing competitions will take place during the Olympics in August and it is teeming with so much bacteria and viruses that sports federations wanted to switch locations.

Even Rio's Mayor Eduardo Paes believes Guanabara is moribund after decontamination programs across three decades have failed.

With around 40 days before the Olympics, it seems everyone has given up on the beautiful bay.

But not the grey dolphins.

Thirty-four of the Sotalia Guianensis species live in the bay.

Recently, at least seven females were spotted swimming with their calves in water that many say is too dirty for any traditional aquatic life.

Now scientists hope the survival of the Guanabara Bay dolphins will fuel popular support for efforts to finally clean up the waterway.

"They are the heroes of the resistance," said Jose Lailson Brito, an oceanographer who co-ordinates Maqua, the aquatic mammals research lab at Rio's State University.

"These animals have somehow found a way to survive in this incredibly difficult habitat."

'Most polluted habitat' species found in

Grey dolphins swim in front of the Sugarloaf and Christ Mountains in Rio's Guanabara Bay. ( Supplied: Adriana Brasileiro )

The dolphins survive, but barely.

In 1985, there were about 400 in Guanabara.

In 1995, when Maqua got its first boat and camera to monitor the animals, there were just 70.

Pollution, heavy ship traffic and fishing have taken a toll on the species that live mostly in estuaries and coastal waters in the east and north of South America.

The bay is the most polluted habitat of all the places where the grey dolphin is found, Mr Brito said.

The animals have been contaminated by mercury, lead, zinc, copper, chromium, nickel and much more, according to a Rio de Janeiro's state legislature commission studying the bay's pollution.

And because they are at the top of the food chain, nearly everything the dolphins eat has been contaminated.

Vessel traffic and intense noise levels interfere with their echolocation mechanisms and can disorient them.

Bay once pristine marine paradise

It was not always that difficult to survive in Guanabara Bay.

Historical research and paintings show it was home to thousands of dolphins and several species of whales when the Portuguese founded what is now Rio in 1565.

The animals were such a recognisable presence in the city that in 1896 they were chosen to illustrate Rio de Janeiro's flag.

They are still there — two dolphins framed by laurel and oak branches, protecting an armillary sphere.

Researchers Isabela Lima and Rafael Ramos de Carvalho on Rio's Guanabara Bay. ( Supplied: Adriana Brasileiro )

The remaining Guanabara population uses the entire bay for its activities, but prefers to spend most of their time in the top north-eastern tip, near the Guapimirim nature reserve.

There, a mangrove forest keeps the water cleaner, and the absence of large ships means noise levels are lower.

On a cool sunny morning this month, Maqua biologists Rafael Ramos de Carvalho and Isabela Lima board a small boat for their weekly check on the dolphins.

A group of five swim and leap out of the water near artisanal fishing traps in a beautiful cove framed by the deep-green Orgaos mountain range. They seem agitated.

One turns around and swims with its belly up for a few seconds. Another small group approaches and suddenly a plastic bag flies in the air, with the quick snap of a tail.

"They like to play with the trash,'' Mr Carvalho said, keeping his eye on the viewfinder of his camera.

The biologists identify the animals by photos of their dorsal fins, whose unique scars are the dolphins' fingerprints.

They are shy and normally do not approach the boat.

Some have nicknames: Little Orange, Acerola (a Brazilian fruit), Regina and Auntie.

They have become characters in a campaign to teach children in villages around the bay about protecting their environment.

More than $US1 billion fails to clean up bay

Fishermen who live around Rio's Guanabara Bay use rustic traps made of bamboo for their daily catch. ( Supplied: Adriana Brasileiro )

Ms Lima dips a microphone in the water: lots of clicks and some whistling, with a faint hum that may come from larger boats on the other side of the bay.

In just a few minutes, about 20 dolphins are swimming around the area — a place where Guanabara Bay seems almost pristine, resembling the tropical paradise it once was.

Decades of mismanagement, corruption and neglect have undermined plans for clean water in the bay and over $1.34 billion has been spent on failed decontamination programs.

"The issue isn't just the domestic sewage, it's how the bay is used for different activities such as shipping, the oil industry, large factories and also the residents who depend on the bay for their survival, the local fishermen," Mr Carvalho said.

And pollution is not the only threat.

As Mr Carvalho and Ms Lima returned from their field trip, they received a phone message with a photo of a dead dolphin on a beach nearby.

The animal had been partly carved and it was difficult to even recognise it as a grey dolphin.

"I hope it's not one of ours," Ms Lima said.

Later, an autopsy revealed that sadly, it was.

Acerola, one of the famous dolphins, was probably caught in a fishing net and his flesh removed to be used as bait.

The male was just one year and nine months old.

"He was still a baby," Mr Brito said.