New Scientist - USA (2020-10-03)

42 | New Scientist | 3 October 2020

“ The river megafauna are hidden below

the surface of human perception”

in conservation biology circles, but this

group is declining faster than any other.

They are collectively known as “freshwater

megafauna” – monster fish such as

sturgeons, giant catfish, river sharks and

rays, along with river dolphins, porpoises,

seals, manatees, crocodiles, alligators,

snakes, turtles and salamanders.

All told, there are more than 200 species

of freshwater megafauna; most are in deep

water and some are probably already doomed

to extinction. Yet they are largely overlooked

by efforts to save the world’s biodiversity. “It

really is a neglected area,” says Sonja Jähnig

a the Leibniz-Institute of Freshwater Ecology

and Inland Fisheries in Berlin, Germany.

The Chinese giant paddlefish (Psephurus

gladius) would never have made it onto a

list of the world’s most beautiful

endangered species. But its demise has

turned it into a poster child for the dire

conservation status of the world’s last

remaining pool of megafauna.

We are used to thinking of megafaunal

extinction as something that happened many

thousands of years ago, as humans spread

around the world and, probably not

coincidentally, ran into the last mammoths,

ground sloths, giant flightless birds and many

more. Some terrestrial megabeasts survived,

but in the past 50,000 years about two-thirds

of these species have vanished. One place

where they survived was in freshwater,

probably for the same reason that we ignore

them today. “They are hidden below the

surface of human perception,” says Jähnig.

But if we looked, we would be amazed.

There are 206 living freshwater megafaunal

species – defined as those that can exceed

30 kilograms, about the size of an adult

golden retriever. Three-quarters of the

world’s major river basins are home to

at least one, with the Amazon, Congo in

Africa, Orinoco in South America, Mekong

in South-East Asia and Ganges-Brahmaputra

basins especially rich in them.

Many can grow much bigger than the 30 kg

lower limit for this oversized club. In 1931, a

Chinese biologist claimed that paddlefish in

the Yangtze near Nanjing, China, could reach

7 metres in length, although the largest

recorded in more recent decades before they

vanished are only about half that. That is still

a very big animal (see “The river giants”, left).

The largest freshwater fish in the world is the

beluga sturgeon (Huso huso), which is can

reach 7 metres and 1.5 tonnes. A small adult

whale shark is about the same size.

Final swimmer

The Chinese paddlefish was one of two

remaining species of an ancient lineage

of fish (Polyodontidae) that evolved in

the Jurassic and survived the extinction

of the dinosaurs. The last one swimming

is now the American paddlefish (Polyodon

spathula), which is found in small numbers

in the Mississippi river basin.

Its Chinese cousin had been in trouble

for years before finally gasping its last. They

were once common, with around 25 tonnes

caught every year for food. But a survey of

the Yangtze river in Sichuan province in the

summers of 1974 and 1975 found that large

specimens were already rare, a fact that was

attributed to overfishing.

In 1981, the huge Gezhouba hydroelectric

dam was built across the Yangtze, preventing

paddlefish from migrating upstream to their

ancestral spawning grounds. This seems to

have been the last straw for a species already

under pressure from overfishing, habitat

destruction, shipping and pollution. The last

live specimen – a 3.6-metre-long female – was

unexpectedly caught in 2003 near Yibin in

The river giants

Many rivers around the world were once home to megafish longer than

2 metres, but they are now a rarity. Here are some of the biggest:

Beluga

Status: Critically endangered

This sturgeon is the world’s largest freshwater fish, found in the

Black, Azov, Caspian, and Adriatic seas and their tributaries. The

largest was over 7 metres, but the average is around 2 metres

Giant Chinese paddlefish

Status: Extinct

Once common in the Yangtze river, China, this

fish grows up to 3.6 metres and possibly even

double that length

Pirarucu

Status: Recovering from overexploitation

This air-breathing fish from the Amazon river

can grow larger than 3 metres