A thriving trade in fish maw – made from the swim bladders of fish – could lead to the extinction of the Nile perch fish in east Africa’s Lake Victoria.

Demand for fish maw has spawned such a lucrative business enterprise in the region that it is raising concerns of overfishing.

The high profits involved mean that traders keep a low profile, and are secretive about their haul’s eventual destination, according to the women who gut the perch to extract the precious maw.

“We don’t know where they take it. They come to collect it and we sell it to them,” said Francisca Odhiambo, a mother of five, who sells fish at Dunga beach, on the shores of Lake Victoria.

Nile perch were introduced to Lake Victoria in 1950, and are now an important part of the local economy. Photograph: Yasuyoshi Chiba/AFP/Getty Images

Fish maw has various uses, including the manufacture of surgical sutures, but it is also a delicacy in China, where it is served in soups or stews in addition to being used as a source of collagen. It is also used to make water-resistant glue and in the production of isinglass, a refining agent involved in the manufacture of beer and wine.

Ironically, Nile perch is an invasive species. It was introduced to Lake Victoria in 1950, and has been blamed for the disappearance of the native fish and interfering with the lake’s ecosystem. But it is now an important part of the local economy.

“The fisherfolk [here] came to realise that the fish maw trade is a real business fetching lots of money in China and Hong Kong, and it’s not just a by-product,” said Tom Guda, regional chairman of the Lake Victoria beach management unit.

Guda explained that in the years between 2000 and 2005, Chinese traders came through Uganda looking to buy maw directly from the fishermen. The price of Nile perch shot up from $2 (£1.60) a kilo to between $3-$4.

“That has continued to date so that the Nile perch prices have stabilised because of the fish maw,” said Guda.

According to a report commissioned by the German development agency GIZ in collaboration with the Lake Victoria Fisheries Organisation in August 2018, the Chinese agents supplying maw had better opportunities for business growth compared with others in Uganda.

Fishermen push out their boats at dawn on Lake Victoria. Photograph: Tony Karumba/AFP/Getty Images

For example, Chinese traders provided working capital to agents supplying maw, a facility that was unavailable to local fish sellers.

There is still little knowledge of this trade in the region, and this in itself contributes to unsustainable fishing. For example no guidelines or policy exist to regulate the fish swim bladder trade in Kenya, Uganda and Tanzania.

This means data on the amount of swim fish bladder being exported to China, is hard to come by. The same goes for the information about the population of Nile perch caught solely for the purpose of bladder harvesting.

According to a preliminary study by the Kenya Marine and Fisheries Research Institute (KMFRI) conducted in March 2018, the bladder comprises an average of 2% of processed Nile perch byproduct. It is estimated that up to 290 metric tonnes of the organ is exported from Kenya.

Gladys Okumu holds up a fish swim bladder harvested from the Nile perch fish, at Dunga beach on the shores of Lake Victoria. Photograph: Geoffrey Kamadi/The Guardian

But this figure is only a conservative estimate, because “the value chain of fish bladder is poorly understood beyond the lake region”, explained Chrispine Nyamweya, assistant director of Limnology at KMFRI.

KMFRI said the price in Kenya ranges between 4,000 Kenya shillings (£31) for maw weighing between 100-200g and 16,000 Kenya shillings for a weight of between 601-999g.

On the international market, maw will fetch between $450–$1,000 per kg depending on the market dynamics and quality of product.

Overall, fish stocks have been declining in the lake, due to a number of factors, such as pollution, overfishing and the use of illegal fishing gear. The water hyacinth weed is affecting the perch’s survival chances as well.

“Being a sight predator, clear waters are critical to the survival of the Nile perch,” said Guda.

And since the perch is an apex predator, it requires lots of oxygen – but widespread weed is preventing the fish from getting enough of it.

What this means, explained Guda, is that fishermen are compelled to go deeper into the lake, where there are no water hyacinth, to find the fish.

Different varieties of fish maw in a shop in Hong Kong. Photograph: Anthony Wallace/AFP/Getty Images

And it is not such an easy task. It takes five hours, using a twin-engine speedboat to travel from Mbita (a landing site) to Remba, where the fish are located. Ordinary boats take eight hours.

Venturing deeper into the lake has its own dangers, according to Guda. This is where the Kenyan fishermen and their Ugandan counterparts come into contact. Only one outcome is inevitable when this happens: conflict over fishing rights.

Efforts are in progress to formalise the trade, and introduce taxes, levies and inspection fees.

“Plans are under way to develop guidelines to ensure that the trade is regulated,” said Robert Kyanda, a marine scientist in Uganda, and an official at the Lake Victoria Fisheries Organisation secretariat.

Regulation might be the only answer to ensure sustainable harvesting of the fish.