Why whale sharks gather in a war-torn oil corridor

The whale shark is so rare that estimates suggest less than 0.1% of people will ever see one in the wild. But near the Strait of Hormuz, and along the Persian Gulf, where tensions have flared, hundreds of them gather every summer to feed on fish spawn.

This annual congregation had initially surprised scientists. The waters of the Persian Gulf were too shallow and the region too hot for whale sharks. Summer water surface temperatures can hit 35°C; the giant fish prefer 21-30°C.
Scientists would later discover it was an interesting sea effect that was drawing the fish to the region, one triggered by how the Gulf exchanged its water with the Indian Ocean.

During high evaporation in the summer, the Gulf’s waters turn extremely salty and start to move along the seabed, before emptying out into the ocean through the Strait of Hormuz. On top of this undercurrent enters the lighter, less saline water from the Indian Ocean, forming a double-decker current — high salinity at the bottom, but less salty water on top.

At the same time, the currents maintain cooler waters at depth in the central part of the Gulf, creating what is called a hydrographic front. This leads to a marine ecosystem where mixing of nutrients from both seas provides an abundance of food for the species in the area, and perfect spawning conditions for tuna. It’s the tuna that are ultimately responsible for what scientists have described as one of the most spectacular events on Earth — the gathering of as many as 400 whale sharks.

“Millions of these tuna fish storm into the Gulf and the sharks eat their eggs,” says Steffen Bach, a senior researcher at the Qatar Whale Shark Research project in Doha and at the Department for Environmental Impact Assessment at Rambøll, Copenhagen.

Bach adds the whale sharks of the Gulf show up mainly near the Al Shaheen Oil Field, one of the world’s largest offshore oil fields.

“They use the Strait of Hormuz as a transit corridor, just like ships,” he says.

But the entire region is under threat as missiles and drones fly at oil tankers.

“Imagine trying to breathe through a straw that has been covered in a thick layer of sticky glue. That’s what oil does to the gills of a fish,” says Seithikurippu PandiPerumal, a distinguished professor of research at Chandigarh University.

Prof Pandi-Perumal says an oil spill in the Strait would also spread very quickly.

“The reason is its narrowness, only 33km at its narrowest point. Within hours, the oil would be driven toward the shores of Oman and Iran due to strong currents and heavy ship traffic.”

The result, he adds, could be catastrophic for the entire region’s ecosystem.

“The Strait is both a highway and a nursery. In fact, some of the most productive seagrass beds in the area thrive in those waters. Massive amounts of microscopic, photosynthetic phytoplanktons (‘grasses of the sea’ or ‘plant drifters’), the foundation of the food web, could perish,” he says.

Ironically, the blockade on the surface brings some good news to the whale sharks below. Traffic is down — 120 vessels used to pass through the Strait, which is down to 20, according to reports.

“Many whale sharks I saw bore scars from propeller strikes. So, in a way, this blockade has made the Strait safer for them,” says Bach.

“Still, an oil spill where they feed would spell disaster for the tuna and whale sharks. The tuna eggs will most certainly die and the sharks may be exposed to toxic effects and clogging of their gills.”