As the first dolphin fin surfaced from the Bosporus, a ripple of excitement went around passengers on a free boat ride to see one of Istanbul's most graceful sights.

Whether they live there or are just passing through, dolphins and porpoises feel very much at home in the busy strait that bisects the Turkish megapolis of some 16 million people.

Istanbul city council organizes free summer dolphin spotting trips with the World Wildlife Fund (WWF) to raise awareness of the dangers facing sea mammals.

The wildlife project's head Ahmet Yaşar Yıldız said the city council was doing "everything they can" to keep them in the strait linking the Black Sea to the Mediterranean.

"This is their home and dolphins must continue to live here," the 59-year-old said.

Their presence in the Bosporus was a sign of a "perfect ecosystem," he added. "The strait is clean, cleaner than most bays, and we want to keep it that way."

Microphone in hand on the boat's top deck, WWF Türkiye's marine mammal program leader Cansu Ilkılınç explained to the crowd below that the strait is home to two species of dolphin and one species of porpoise – an impressive number for an exceptionally busy urban waterway.

Climate change, habitat loss

As a key corridor for international maritime navigation, 39,000 ships passed through the Bosporus last year, according to Türkiye's Ministry of Transport – not including pleasure boats and the ferries that constantly ply from one side to the other.

Despite the volume of shipping and straddling Türkiye's largest city, dolphins seem to prefer the strait's waters to the Black Sea and Sea of Marmara on either side.

This is because fish are abundant and the strong currents spirit away pollution, Yıldız said.

Yet, all is not pristine for the dolphins in the strait, the wildlife project lead warned, with threats from climate change, pollution, overfishing and habitat loss.

"Uncontrolled construction and industrialization are a problem everywhere, and it's a serious problem in Istanbul," he added.

Yet, since the first free educational outings began in 2022, their success has been remarkable.

"Only once have we not seen a dolphin," Yıldız told Agence France-Presse (AFP), adding that the hundred or so spaces on the trips are booked up "within three minutes" when they go online every fortnight.

Following seagulls

During these outings, the WWF attempts to keep a record of the strait's resident dolphins, hoping to identify them by a distinctive feature such as a mark or a scar.

"One of them has been living here since 2012, you can recognize it from its dorsal fin," smiled Ayşe Oruç, head of WWF Türkiye's Marine Biodiversity program, marveling at the variety of life in the heart of "one of the biggest cities in the world."

Besides the harbor porpoise, the Bosporus is home to both the common and bottlenose dolphin, Ilkılınç said.

The 31-year-old offered up a few tips for spotting them: "When the seagulls dive and disturb the water, it means they've seen fish underneath, which the dolphins feed on."

She also advised those hoping to glimpse a dolphin pod to follow in the wake of cargo ships, tankers and – above all – fishing boats.

Twenty-four-year-old student Deniz Dinçergök came back to shore delighted.

"At one point, a baby dolphin came out of the water and turned around, showing its belly. It was a magnificent moment," he said.