Giant Fishes of the Rivers

How much longer will we have them?

The vertebrate class of Pisces contains members of two major groups of fishes: the cartilaginous species and the bony fishes. The most massive (if not the longest as well) of all fish are the Whale Shark (Rhincodon typus) and the Basking Shark (Cetorhinus maximus). These two species are peaceful giants, feeding on plankton that they filter out of the ocean water with their comb-like gill rakers. However, in this blog I want to write about giant freshwater and not marine fish. Yes, river fish, for several of them are bigger than the marine bony species (with one exception: the oarfish Regalecus glesne, of which during the 1967 “Walther Herwig” South Atlantic Expedition I caught a glimpse of one 7.30 m long specimen. This midwater species is said to reach a maximal length of 13 m).

What prompted me to write this blog was the sad news that the giant Yangtze River paddlefish Psephurus gladius was officially declared extinct. It thus follows the Yangtze freshwater dolphin Lipotes vexillifer, affectionately called ‘the baiji’ in Chinese but declared extinct in 2006. In 1970 there were still 25 tons of Yangtze paddlefish caught, but the last individual was probably killed in 2007. Specimens of at least 5 m in length existed. Another Chinese giant with a length of approx. 4 m is the Chinese sturgeon Acipenser sinensis, but it may also soon be gone forever for its population decreased by 98% from 1973-2010. As of this year (2021) all commercial fishing in the Yangtze will be banned for 10 years, but as welcome a measure as this is, it cannot bring back what is lost forever.

Probably the largest with a maximum length of around 7 m of all freshwater giants is the Beluga sturgeon Huso huso of the Caspian Sea and the Sea of Azov. Because of the value of its roe (the famous “Caspian caviar”) it is farmed, but the eggs of its also farmed but only 5.5 m long relative, the Huso dauricus, are considered even tastier. In northern Europe adults of the 3 – 4 m long sturgeon Acipenser sturio used to enter rivers to reach their breeding grounds upstream, but one of the few streams where these sturgeons now still breed is the Gironde in France. Recolonisation efforts are in progress elsewhere, but so far with minimal success. A huge surprise for our professor and us students of the Zoology Class at the University of Kiel in 1968 was when an almost 2 m long catfish of the species Silurus glanis was delivered. It had been caught the day before in the small North German freshwater stream known as the Trave. The East Asian Mekong River Pangasianodon gigas catfish and South American species of river catfish are said to reach maximum lengths of 4 m and can become a danger to bathers. Of the other South American freshwater giants the Arapaima gigas with a reported maximum length of 3.4 m and the electric eel Electrophorus electricus with a length of 2.5 m need to be mentioned.

Africa’s largest freshwater fish is the Nile perch, but it hardly reaches 2 m. On the other hand there are some quite big bony fishes in the North American Mississippi-Missouri river system. There is the North American paddlefish Polyodon spathula and the fierce-looking alligator gar that can both reach a length of 3 m. Although not in imminent danger of becoming extinct, these two species are considered vulnerable and an analysis of what led to the extinction of the Yangtze River paddlefish may help save the North American species and other freshwater fish giants too. It is believed that the biggest threats to the vulnerable freshwater fish giants (but even smaller species as well) are dams that block the fish’s access to upstream spawning grounds, and furthermore overfishing, the use of illegal methods (like electro-fishing, nets with unacceptably small mesh widths, and explosives). Pollution of the river water by chemicals like insecticides, by effluents and other wastes from urban areas, and by fertilizers used in agriculture must not be forgotten. But what is certainly interesting is that rivers compared with the oceanic habitat contained more giant bony fish species than the sea. Any suggestions why?  

lication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.

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