Petty officer George Kromah, of the Liberian coastguard, slings his AK47 across his back before disappearing over the side of the Sam Simon, joining his colleagues in the rib below. The boat roars off, quickly followed by a second, speeding through the choppy Atlantic swell in pursuit of a suspected illegal fishing vessel that has crossed into Liberia’s territorial waters from Sierra Leone.

Kromah and his fellow officers are on the frontline of the little nation’s ill-matched crackdown on fisheries crime – which Interpol has linked with the trafficking of drugs and people, as well as fraud and tax evasion.

Petty officer George Kromah issues a warning to the captain of the Oriental Kim, after finding irregularities with its licence. Photograph: Nicole Schafer/Sea Shepherd

The largely ungoverned waters of west Africa are plagued on a daily basis by big industrial vessels from wealthier nations that plunder hundreds of tonnes of fish, at the expense of local fisherman. One 2017 study estimates the cost of illegal, unreported and unregulated fishing (IUU) to just six west African countries at $2.3bn (£1.8bn) a year. It is a cost that Liberia, one of the world’s poorest nations – and heavily dependent on foreign aid after decades of civil war – cannot afford.

But neither has it funds to police its 370km of coastline.

Two years ago, the defence ministry of this tiny country of 4.7 million people took an unusual step to tackle multi-million dollar crimes: partnering with Sea Shepherd, self-styled “eco-vigilantes”, known for controversial tactics against Japanese whalers in the Southern Ocean. The Guardian travelled with the coastguard aboard the Sam Simon, a 56-metre patrol vessel provided to the military, along with its largely volunteer crew.

The pursuit of the suspected illegal tuna boat, the Oriental Kim, ended undramatically with a warning rather than an arrest. It was found to be broadly operating legally, with some discrepancies. But according to Sea Shepherd, news of policing at sea has “spread like wildfire” among vessel owners, creating a deterrent effect.

The Oriental Kim, a 75-metre Senegal-flagged tuna vessel, is seen fishing off Liberia’s coast. Photograph: Nicole Schafer/Sea Shepherd

Onboard, where crew are hosing down the Oriental Kim after hauling their morning catch of 10 tonnes of tuna, Kromah expresses satisfaction. “It was a good operation,” he says. “We did everything we could. The offences did not warrant an arrest but the next time, if they have not corrected the irregularities, they will be arrested.”

A lot has changed since the partnership, known as Operation Sola Stella, began. “A lot of illegal things were going on before in the deep sea, but with our ships we could not get to them. Before, the fishermen didn’t want to get licences and they would just go to the deep sea and take anything they want. Now our water is safe.”

In the capital, Monrovia, Major General Prince Johnson, chief of staff for the commander of the Liberian Army, says: “We are losing a huge amount – millions of dollars – to illegal fishing.

“Since the coming in of Sea Shepherd, Liberia has benefitted a lot. Those who do illegal activities in our waters know they can no longer carry on.”

Data given to the Guardian by the Liberian MoD shows the impact the operation has had in just two years. Arrests have more than quadruped, the range of coastguard patrols have doubled, and the government has recovered at least a million dollars in fines.

In the seven years prior to Sola Stella, the coastguard made only three arrests. One of the vessels, South Korea-flagged Nine Star, remains rusting in Monrovia’s port after its owners abandoned it, leaving $1.5m in unpaid fines, in 2013.

But from February 2017 to January this year, arrests rose to 14. Among them were two internationally blacklisted vessels. The Hai Lung, an Antarctic and Patagonian toothfish vessel, was suspected of absconding from authorities in Nigerian waters, and found to have forged Indonesian documentation, while the Liberia-flagged Labiko 2 was blacklisted by three different organisations under its previous name, the Maine. The latter was using gill nets, instead of the long lines for which it had a licence, and was fishing for deep-water sharks.

Fishermen aboard the Oriental Kim haul in the morning’s catch. Photograph: Nicole Schafer/Sea Shepherd

Five of the 14 vessels were found guilty of illegally fishing, the rest were found guilty of related crimes, including fraud, tax evasion and under-reporting catches. Two of the five, the Bonheur and the Starshrimper 25 trawlers, were found fishing inside the six-mile inshore exclusion zone, from which industrial ships are banned by the Liberian government so that the waters can be reserved for the country’s 33,000 small scale fishermen.

Not everyone is happy with the arrangement, however.

“The Europeans don’t like the fact the Liberian coastguard are inspecting their ships,” says Johnson. “They feel they are reporting their catch and are working with the fisheries authority. Sometimes, they even accuse us of acting illegally, even though we have a Liberian flag.”

He has had pushback from many quarters, particularly about firearms. “They want us not to have weapons – they want us to go in with our hands shaking. I would never send my men to sea without a weapon.

“The majority of those found fishing illegally, who we have arrested in our waters, are Chinese. We have also arrested one or two European vessels.”

Often, the real ownership of vessels acting illegally remains a mystery, hidden by layers of shell companies.

Ernest Vaffe, deputy minister for operations, coastguard, says the truth frequently remains undiscovered. “You will get a local lawyer, who says: This is my client. They pay the fine.’ We have never had an international lawyer say: ‘This is our vessel.’”

But none of the vessel owners have ever argued they are not guilty.

Vaffe argues that even legal fishing by foreign vessels in Liberia’s water is unfair, due to what he describes as a “flawed” system of cheap licences issued by the National Fisheries and Aquaculture Authority.

“These vessels don’t take the fish to bring them back to African ports, they take them to Europe and elsewhere,” says Vaffe. “They benefit a lot, influencing their economy. To the detriment of our economy. They are also creating unemployment among the local fishermen. The cost is huge.”

Fishing families in Robertsport, a town on the Cape Mount peninsula, near the Sierra Leone border. Photograph: Nicole Schafer/Sea Shepherd

In Robertsport, one of Liberia’s main fishing communities, near the Sierra Leone border, narrow wooden canoes line the beach. The fishermen there have another word for what foreign vessels are guilty of.

“They are stealing what is ours,” says town governor Kofu Weah. “They come into Liberia, take the fish and then they leave.”

Robertsport’s fishermen, who go out alone in small boats, or in groups with larger, motorised canoes, say the industrial foreign trawlers often cut through their nets, costing them between $150 and $1,500, depending on the size, and often capsizing their boats.

Emmanuel Frances, 36, says: “I’ve been fishing my whole life. We sometimes leave the nets and go elsewhere. But when we return we could find the net gone. Trawlers cut through them.”

A good catch, for 25 men in a motorised canoe, would make just over a US dollar each for a night’s work – not enough to pay for new nets. They welcome the joint patrols, which they sometimes see from shore, and say the number of incursions into their waters from foreign trawlers has fallen.

Ibrahim Turey, 43, a newscaster at the local Radio Pisco station says: “Illegal fishing was rampant, in the war and afterwards. But they are no longer coming here with their boats. We see Sea Shepherd out there on patrol.”

Captain Alistair Allan looks on as the Oriental Kim’s net is winched aboard. Photograph: Nicole Schafer/Sea Shepherd

On the bridge of the Sam Simon, captain Alistair Allan, 27, a tattooed Australian who has worked with Sea Shepherd since he was 19, says he believes the campaign against illegal and unregulated fishing is vital for the planet.

“Humankind has thought of the ocean as a never-ending resource. It’s so vast, how can we make a dent in it? But we’re not just making a dent in it, we are wrecking the whole thing. It is happening over the horizon, out of sight.”

He gestures at the vast expanse of ocean around us.

“Out there, you can see the over-reaching arm of the industrial fishing industry. The ocean cannot sustain that level of fishing. The scale is enormous. And then, there is a social justice element. There are 33,000 people in Liberia dependent on fish. And the Chinese and the EU steal it from under their noses. It is important to understand the battle that developing nations face.”