Fish species that undertake mammoth migrations through rivers, lakes and ponds are facing a combined threat from dams, pollution and overfishing preventing them from reaching their breeding grounds.

The golden mahseer and goonch catfish are some of the thirty flagship fish species which have now been prioritised for global conservation by the UN in a new report.

A team of international scientists assessed more than 15,000 species of freshwater migratory fish - which make up half of all fish species - and found their numbers had dropped by an estimated 81% in the last half century.

The authors said for too long the group has been ignored but "are an essential part of people's livelihoods".

"Freshwater fish support hundreds of millions of people around the world, so for humans, they're very important," said Dr. Zeb Hogan, co-author of the report and professor of biology at University of Nevada, Reno.

As well as providing enough food for 200 million people globally, he said migratory freshwater fish have immense diversity which needs protection.

"You have fish that can get over 650 lb (295kg) that can migrate over 7,000 miles. Every shape and size of animal you can imagine, these beautiful golden dorado that make long-distance migrations in South America, tropical eels that are moving thousands of miles in Oceania," he said.

The UN first looked at the status of this group back in 2011, but only 3,000 species were assessed. Since then the International Union for the Conservation of Nature (IUCN) has been collecting data on how the populations of many more species are faring.

This time around, the UN was able to look at the condition of more than 15,000 species.

They identified 325 fish that need to be added to a global agreement for monitoring and supporting threatened migratory animals in order to recover. Of these there are 30 priority fish, some of which are found in the UK including Allis Shad, River Lamprey, Brook Lamprey and Atlantic Salmon.

The shorter list is made up of fish that are both threatened and are also culturally significant for global communities.

"For example, the golden mahseer is [an] iconic fish of the Himalayas. It's called the tiger of the river, and many of the people in that region value that fish for cultural and spiritual practices that go far beyond the economic values," explained Michelle Thiem, deputy lead for freshwater at World Wildlife Fund US, who also worked on the report.

Like many fish on the list it is facing two major threats from overfishing and habitat loss she said.

"Habitat loss includes things like dams that fragment a river system, and can change the natural flow regime of a river, which results in fish not being able to connect to their spawning habitats," Thiem said.

Disruption to migration is an issue that is particularly prevalent in Europe, where fish encounter a barrier to their routes every kilometre (0.6 miles) on average.

Often the barriers are small culverts or weirs that are redundant and could be removed, said Dr Janina Gray, head of science at environment charity WildFish.

But she said the cumulative effect on migrating fish is enormous: "It can have a massive impact in terms of the energy reserves fish are using to get over the [barriers]."

That makes them more susceptible to disease. They're more likely to get predated. They will spawn in suboptimal grounds."

Most species will cross country borders on their mammoth migrations. This requires coordinated efforts between governments to tackle these threats and ensure the recovery of national stocks, said Prof Hogan, co-author of the study.

The Convention on the Conservation of Migratory Species is a global UN treaty signed in 1979 which commits countries to work together to protect migrating species of all kind.

The UN is now proposing that the 30 fish species be added to the treaty at its annual meeting in three years' time.

The Saiga antelope is an example of a species that has benefited from being listed under the convention.

In 2015, the antelope suffered a mass die-off and it was feared that the Ice Age survivors would become extinct. But an international anti-poaching team was established and since then population levels have rebounded to more than a million.

"The focus has clearly been on some of the more well known, more visible species, such as the big cats, on antelope, on whales, on dolphins," said Amy Fraenkel, the UN Executive Secretary for the Convention on Migratory Species (CMS).

She said it was now the turn of migratory freshwater fish to benefit from coordinated action.

The focus of the convention on mammalian species over others like fish and insects has previously been a criticism of the agreement.

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