More than 15,000 species of migratory freshwater fish have been assessed in a new report by the United Nations, revealing a steep decline in their populations. The report, published by the BBC, highlights that these species, which make up half of all fish species, have suffered an estimated 81% drop in numbers since the mid-20th century. This decline is attributed to a range of human-induced threats, including dams, pollution, and overfishing, which prevent the fish from reaching their critical breeding grounds.

Global Priority for Conservation

The report has identified 325 fish species that require urgent attention and have been proposed for inclusion in a global agreement to monitor and support threatened migratory animals. Of these, 30 have been prioritized, including species found in the UK, such as the Allis Shad, River Lamprey, Brook Lamprey, and Atlantic Salmon. These species are not only ecologically important but also hold cultural significance for many communities around the world.

Dr. Zeb Hogan, co-author of the report and a professor of biology at the University of Nevada, Reno, emphasized the importance of migratory fish. ‘Freshwater fish support hundreds of millions of people around the world, so for humans, they’re very important,’ he said. ‘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.’

The report builds on a similar assessment conducted by the UN in 2011, which evaluated only 3,000 species. Since then, the International Union for the Conservation of Nature (IUCN) has collected data on the status of many more species, allowing the current study to assess over 15,000 species for the first time.

Threats to Migration

The main threats to these species include overfishing and habitat loss, which have disrupted the natural migration patterns of fish. According to Michele Thieme, deputy lead for freshwater at the World Wildlife Fund US, the golden mahseer, a species found in the Himalayas, is an example of a fish that faces significant challenges. ‘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,’ she said.

Habitat loss includes things like dams that fragment a river system and change the natural flow regime of a river, making it difficult for fish to reach their spawning habitats. Dr. Janina Gray, head of science at environment charity WildFish, explained that barriers such as culverts and weirs can have a massive impact on migrating fish. ‘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.’

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. These barriers are often small culverts or weirs that could be removed, according to Gray.

International Cooperation Needed

Most species will cross country borders on their migrations, requiring coordinated efforts between governments to tackle these threats and ensure the recovery of national stocks. Prof Hogan said that the Convention on the Conservation of Migratory Species, a global UN treaty signed in 1979, commits countries to work together to protect migrating species of all kinds.

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.

Amy Fraenkel, the UN Executive Secretary for the Convention on Migratory Species (CMS), said that 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. ‘It was now the turn of migratory freshwater fish to benefit from coordinated action,’ she said. The focus of the convention on mammalian species over others like fish and insects has previously been a criticism of the agreement.

With the number of species assessed now reaching over 15,000, the report highlights the urgent need for international cooperation and action to protect these vital fish species. The UN’s proposal to include the 30 priority species in the global treaty is a step forward in addressing the decline in migratory fish populations and ensuring their survival for future generations.