Aquatic species that embark on mammoth journeys through inland waterways are facing a combined risk from human-made barriers, contaminants, and excessive harvesting preventing them from reaching their breeding areas.
Examples including the golden mahseer and giant devil catfish are among 30 key species which have now been prioritised for worldwide conservation by the United Nations in a recent assessment.
"These aquatic species sustain many millions of people globally, so for people, they're very important," said a co-author researcher on the study.
A group of international experts evaluated over 15,000 types of freshwater migratory fish – which represent 50% of all known fish – and discovered their numbers had fallen by an estimated 81% in the past 50 years.
The researchers said that for too long, this group of species has been overlooked but "represent an vital part of people's survival."
Overfishing and Habitat Loss
The focused list is consists of fish that are both threatened and also culturally significant for worldwide populations.
"For example, the golden mahseer is an revered fish of the Himalayan region. Numerous people in that region value that fish for traditional and sacred purposes that go far beyond economic values," noted a freshwater advocate who contributed to the study.
Like many fish on the list, it is confronting two key dangers: excessive fishing and habitat loss.
"Habitat loss includes things like obstructions that fragment a waterway, and can disrupt the seasonal patterns of a river, which causes fish not being able to reach their spawning areas," the scientist said.
Disruption to migration is an problem that is especially widespread in some continents, where fish face a obstacle to their routes every kilometre.
Frequently the barriers are unnoticed structures that are no longer needed and could be eliminated, said an environmental charity lead.
But the collective consequence on migrating fish is massive: "It can have a significant impact in terms of the energy reserves fish are using to pass the [barriers]."
That makes them more vulnerable to illness. They're more likely to get eaten. They will spawn in poor grounds."
Back from the Brink
The majority of species will traverse national borders on their lengthy migrations. This necessitates coordinated actions between nations to combat these issues and secure the rebounding of national populations, argued a study contributor.
The global treaty on migratory species is a international United Nations agreement created in 1979 which obligates countries to cooperate to protect migratory species of every type.
The United Nations is now recommending that the 30 fish species be listed to the treaty at its yearly conference in the near future.
The distinctive antelope is an illustration of a species that has seen improvement from being listed under the treaty.
Following a population crash, an international conservation effort was established and since then population levels have recovered substantially.
"The focus has traditionally been on some of the more visible species, such as large mammals," commented a high-ranking UN official.
She noted it was now the turn of river fish to benefit from coordinated protection.
