Dear Readers, my go-to source for all things scientific is New Scientist. What a treasure trove it is! I love the way that it makes me consider things that had never crossed my mind before. And this week, there is an article about the neglected giant river fish that are quietly disappearing.
A megafish is defined as any freshwater species which weighs more than 30kg (the weight of an adult golden retriever). 206 species fall into this category, and many can reach a much greater size than the criteria stipulated
The beluga sturgeon (Husa husa) is the largest, and it’s critically endangered. The largest specimens can measure 7 metres long and weigh 1.5 tonnes. These huge individuals may be extremely old – sturgeons take 20 years to reach maturity, and it is likely that they can live to be a hundred, if they get the chance. They used to be found in rivers all over Europe, and even today if you should happen to catch a sturgeon in an English river ( a most unlikely event, but it’s as well to be prepared) you will need to hand it over to the Queen as it’s been a Royal Fish since 1362. Sadly, Beluga sturgeon are critically endangered because of the taste for their eggs: a single fish can provide a poacher with 30,000 euros’ worth of caviar. The fish can be farmed, but the product is apparently inferior. Once found throughout the Adriatic, Caspian and Black Seas, the sturgeon is now listed as Critically Endangered.
Still, the beluga sturgeon is better off than the poor old Chinese Paddlefish (Psephurus gladius) which is now thought to be extinct. They have been overfished, and then their migratory route had a hydroelectric dam built across it. Then the river became increasingly polluted, and the volume of shipping was also a challenge. The last live fish – a 3.6 metre-long female – was caught in 2003, but by this stage she was probably ‘dead fish walking’ (or swimming) because a search of the Yangtze basin in 2017 and 2018 didnt turn up a single fish. The scientists who broke the news described the extinction as a ‘reprehensible and irreparable loss’.
The loss of the Chinese paddlefish does seem to have focussed the minds of the Chinese government – they have been working with various conservation bodies to develop ways of protecting their ecosystems, and have introduced a moratorium on fishing in 332 designated areas. However, while there are still the dams and the pollution there’s a long way to go.
Lest we in Europe start feeling too superior, though, let us note that the Danube is the river considered the worst in terms of threats to biodiversity. And a recent report that every single one of England’s rivers was polluted should also give us pause. Fish really do have nowhere else to go if their habitat is damaged, poor things.
Still, as we all have enough to feel sad about at the moment, here are a few happier stories. When I was younger I often used to hang out at the aquarium at London Zoo, where there were some of the biggest fish that I’d ever seen. Pirarucu (otherwise known as arapaima) are huge fish that have to breathe air – they come to the surface to gulp in some oxygen via a special organ called the labyrinth, which enables the animal from the atmosphere. This is probably an adaptation to the sluggish, under-oxygenated waters of the Amazon where the fish lives. It seems to do very well in aquariums, but it is also bouncing back in the wild – on the Juruá river, which feeds into the Amazon basin, sustainable projects have enabled this amazing fish (which can grow to a length of three metres) to start to increase in population.
And although the Chinese paddlefish seems to be lost, the American paddlefish (Polyodon spathula), is now a protected species throughout its last haven, the Mississippi and Missouri basins. This poor fish was not only fished for sport, but also for its caviar. They are nearly blind, because they don’t need to see in the muddy waters, but they do have electroreceptors not only on their sensitive ‘paddles’ but also on most of their body, which enables them to pick up the swarms of tiny zooplankton that they feed upon. They then open their mouths and suck the little animals in.
Although the Mississippi basin is an extremely degraded environment, the American paddlefish seems to be doing ok, and is even increasing in numbers. It would be really something if we were able to save the last species of these giant fish before they’re lost forever.
Photo One By Максим Яковлєв – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=67084661
Photo Two By Zheng Zhong(Life time: 1612-1648 (years active)) – Original publication: Nanjing, China, early-mid 17th centuryImmediate source: Metropolitan Museum of Art, PD-US, https://en.wikipedia.org/w/index.php?curid=62706228
Photo Three By Alneth – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=85605561
Photo Four By George Chernilevsky – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6933126
Photo Five By Хомелка – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25958644