ISTANBUL—Russia’s shadow fleet of oil tankers is decking itself out in netting, metal cages and other tools to protect against a wave of Ukrainian drone strikes on the energy exports that are proving such a vital economic lifeline for the Kremlin.

Nearly every day since August, Russian oil tankers have emerged from the Black Sea here in Istanbul, sailing down the Bosporus displaying nets, cages, rubber tires and other antidrone devices.

These protective layers resemble the vast webs of netting draped across buildings and roads that are common in front-line cities in Ukraine, where Russian and Ukrainian forces have turned to drones as the primary weapon in the more than four-year-long war.

At least 60 ships draped in countermeasures to ward off drones have been documented transiting the Bosporus since August, according to images of the vessels reviewed by The Wall Street Journal and provided by Yoruk Isik, a ship spotter and maritime analyst based in Istanbul. Isik, who leads a consulting firm called Bosphorus Observer, photographed the ships, nearly all of which are under U.S. and other Western sanctions for transporting Russian oil.

Metal casings known as “cope cages” initially emerged as a way for Russian and Ukrainian military vehicles to defend against small drones, particularly first person view drones that are piloted remotely and can slip past more expensive and advanced air-defense systems. The Israeli military has also used them in recent years to defend against FPV drones used by Lebanon’s Hezbollah and militants in Gaza.

“It looks like Kharkiv. It looks like the netting they have on the front line,” said Eric Woods, an independent security analyst and longtime Russia observer.

The nets hanging on the Russian oil tankers appear to be a makeshift measure, quickly strung up to protect the most sensitive parts of the ship such as engine components, according to military analysts who reviewed the images for the Journal. Another technique is to hang rubber tires from the deck to protect the engine and rudder, a crude way of stopping a drone from crashing into a ship and disabling it.

“It’s more in-your-face proof that the Russian navy has lost control in the Black Sea,” said Isik. “For Russia, earning foreign currency is under great threat.”

Hunting down the shadow fleet offers Ukraine a number of strategic advantages in its fight against the Russian invasion. Russian oil tankers are likely to be less well defended than Russian military sites that have already been hardened against drone strikes. Hitting the tankers also adds to Ukraine’s broader campaign to pile economic pressure on Russia, in which it has also targeted oil refineries and data centers to hinder its ability to finance the war.

The shadow fleet exposes a specific pressure point on the Russian financial system. The aging tankers operate outside global norms for insuring and regulating shipping, which means they are often insured and underwritten by Russia’s state-backed banks.

“They’re explicitly trying to put pressure on the Russian banking system and their ability to fund insurance,” said Woods.

The commander of Ukraine’s drone forces said in September that they had disabled 300 Russian shadow-fleet vessels since July. The number couldn’t be independently verified.

Ukraine has long targeted Russia’s shadow fleet, but it stepped up attacks in 2025 in an attempt to gain leverage over Russia when the Trump administration was pushing the two countries to negotiate an end to the war. The attacks also turn the tables on Russia after years in which Moscow’s forces have targeted Ukraine’s energy and other vital infrastructure as a part of a campaign to psychologically wear down the Ukrainian civilian population.

In the past, Ukraine has claimed attacks on Russian tankers using uncrewed sea drones that glide along the water’s surface. The shadow fleet’s counterdrone measures are evidence of a shift, showing an effort to defend against medium and long-range aerial FPV drones, military analysts said.

The antidrone nets and cages are a sign of how the Black Sea is becoming an important theater in the war. Russia in recent months has intensified its strikes on Ukraine’s grain-exporting ports and ships, while suspected Russian drone attacks are also increasingly threatening NATO members on the Black Sea. Authorities in Romania and Bulgaria have blamed Russia for repeated drone strikes, including those that have sunk ships near their territorial waters.

Ukraine is fighting back, using drone and missile strikes to push Russia’s Black Sea fleet farther to the sea’s eastern shore. Russian attacks on Ukrainian grain exports have also triggered a spike in the global price of wheat.

The escalation is further squeezing global energy supply chains, and underscores the threat drones now pose in the wars in Ukraine and the Persian Gulf.

Iran has used its arsenal of low-cost, one-way attack drones to overwhelm air defenses used by the U.S. and its allies, and to target ships in the Strait of Hormuz, contributing to this year’s energy supply crisis.

“It’s going to be a global trend,” said Samuel Bendett, an analyst of military robotics with the security policy institute CNA, referring to the nets and cages. “It’s the cheapest and easiest way to protect a ship against UAVs.”

Write to Jared Malsin at jared.malsin@wsj.com