The fish-processing sheds lie between the seemingly endless sandy beaches of Gambia’s Atlantic coast and the lagoons behind them.

For some, the three giant Chineseowned meal mills built on the African nation have brought jobs and prosperity. For others, they mean only higher prices for food – the mills began processing a cheap fish called the bonga locals relied on – and filth.

In the summer of 2018 some Gambians coastal villages Sanyang, Gunjur and Kartong – to the south of Gambia’s main resorts – protested over waste from the plants being pumped back in to the sea.

Activists dug up a pipe from one of the mills to the sea. This made international headlines amid concerns pollution would put off eco-tourism. Gambian government watchdogs asked questions.

Producers, who deny improper dumping, were allowed to re-lay their pipe. Dutch-based Changing Markets chose Gambia as one of three case studies in a major investigation of the fishmeal industry.

It discovered authorities were unaware of even how much meal the mills were producing – with their questions resulting in a brief halt to exports.

It usually takes five kilos of fish to make one kilo of meal, a powder used as feed for farmed fish and prawns and pigs, especially in China.

Changing Markets said just one plant making meal accounted for 40 per cent of Gambia’s annual catches in 2016. Its report, called Fishing For Catastrophe, said: “Neither the FAO [UN Food and Agriculture Organisation] nor the government has records of Gambian fishmeal and fish oil (FMFO) production or exports, even though fishmeal plants have been operating in the country for some years.

“However, our investigation found that all three plants have been exporting FMFO illegally.”

It added: “From May 2017, the three plants had been exporting their FMFO destined for human consumption and feed without securing the required foodsafety certificates, potentially avoiding export fees and falsifying food-safety certificates – meaning the FMFO did not undergo any food-safety controls or inspections. This reveals serious gaps in international oversight on food security and product traceability, putting consumers’ safety at risk.”

Changing Markets said a cloud of secrecy surrounded customers of the Gambian plants. Scottish salmon producers deny they get feed from any of the countries Changing Markets has investigated. Investigators allege loose controls mean farmed fish fed illegally or unsustainably end up in UK supermarkets. This is denied.

Changing Markets has made a series of recommendations including calling for fish farms to switch to species that can have plant-based diet.

Anja Bakken Riise, leader of environmental group Future In Our Hands, said: “The aquaculture industry needs to move away from its reliance on fishmeal and fish oil, an industry that is jeopardising local food security, livelihoods, health and the environment as it stands today. The transition to alternative feed resources is imperative to secure a sustainable future where the aquaculture industry is part of the solution and not the problem.”

Natasha Hurley, of Changing Markets, said: “The economics of the FMFO industry are broken. If aquafeed companies do not move faster to source sustainable alternatives they will face serious economic and reputational consequences that could substantially alter the predicted growth for the industry and those reliant on it. The question is: what comes first – the collapse of natural ecosystems or the collapse of the economic sector that is responsible for the problem?”

Gambians east a lot of fish: an average of 25 kilos each every year compared with the African average of eight. Bonga catches are fluctuating and people are worried they could lose a staple.