THEIR habitats are among the most threatened ecosystems on the planet.
And despite their critical value as a source of food and employment for hundreds of millions of people worldwide, it is likely that many freshwater fish species have already disappeared without record.
Freshwaters cover just 0.8% of the Earth’s surface and hold 0.01% of its water yet they harbour astonishing biodiversity, with around 16,000 described resident or migratory fishes accounting for roughly half the global total and 25% of all vertebrates.
They provide a third of the world’s annual small-scale fish catch, including 12 million tonnes from rivers, and provide employment for around 60 million people, while in Africa more than a fifth of the population regularly consume fish of which half comes from inland fisheries.
Some have survived for hundreds of millions of years but are now being driven to the brink of extinction or even wiped out by an expanding human population and rampant socioeconomic development.
A recent study investigating the global decline of freshwater megafauna – those species weighing 30kg or more – concluded that populations of the world’s giant fishes, from sturgeons to the Mekong catfish and Chinese paddlefish, have declined by an average of 94% since 1970.
Sturgeons, most famed for the use of their eggs as caviar but also long-exploited for their flesh, are the most threatened animals in the world according to the IUCN Red List, with 17 of 25 species Critically Endangered and four Extinct. The Mekong giant catfish, the world’s heaviest freshwater fish, has declined by at least 90% with only a few hundred individuals thought to remain, while the Chinese paddlefish, which can grow to 20 feet in length, has not been seen for more than a decade.
And it is not only the large species at risk. A comprehensive global overview of extinction risk for freshwater fishes does not yet exist, but of the 8,511 species assessed by the IUCN by the end of 2018, at least 30% are considered to be threatened.
Humanity’s ever-growing thirst for drinking and irrigation water plus dam construction, widespread pollution and overfishing are causing the decline, although introduction of alien species, deforestation and other forms of habitat modification also pose major problems in some regions.
A global analysis published earlier this year showed that two-thirds of the world’s great rivers no longer flow freely due to dams, which block migration routes and access to feeding grounds for countless species, but the hydropower boom shows no sign of slowing. Hundreds of projects are planned in vast, super-diverse basins including the Amazon, Congo and Mekong but also many smaller, still largely pristine rivers in areas such as the Balkan peninsula.
Yet as habitats continue to be lost or degraded at an alarming rate, our understanding of freshwater fishes remains incomplete. From 2002-2012 an average of two new species was described every three days, with more than 100 from the Mediterranean basin alone since the turn of the century increasing regional species richness by a quarter.
At the global scale, 2-3% of described freshwater fish species are estimated to be extinct, a background rate nearly twice that of other vertebrate groups. The real number is likely to be higher, however, with experts often reluctant to declare species extinct even if they have not been seen for years. During a recent Red List reassessment of North African endemic species a total of 15% were found to have vanished within the last 100 years, although none were considered as such when first evaluated in 2006 (Ford et al., unpublished data).
Conservation efforts have often been undermined by a failure to engage public and policy-makers alike, with people tending to have a weak connection to fishes compared to birds and charismatic mammals or undervaluing the considerable ecosystem services provided by inland fisheries, estimated to be worth some $4 trillion USD per year. Public support for conservation of biodiversity is in part shaped by their knowledge and beliefs in terms of the environment, and one study which surveyed 1,000 people in each of four European countries found that they had only limited understanding of freshwater fishes with many failing to recognise even common native species.
Should the global human population reach the predicted 9 billion by 2050, demands for water, energy and food are set to rise by 55%, 80% and 60%, respectively. If freshwater fishes are to survive, new technologies and management approaches will be essential. Dams must allow free passage of fishes both upstream and downstream, river flows must be sufficient to maintain ecological integrity, the spread of alien species must be curbed, protected areas must be designated, established and managed in an effective way, and fisheries must be managed sustainably. A fundamental shift in attitudes will also be required, with freshwater fishes seen as worthy of conservation not simply exploitation.
Precisely how these gaps are closed continues to embody one of the most pressing challenges in biodiversity conservation.
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