50 Fun Facts About Freshwater Fish

Which famous thinker, as a young student, dissected hundreds of eels in a failed hunt for their male sex organs?

Sigmund Freud conceded failure in his first major research paper and turned to other subjects. The organs were not seen until some 20 years later, in an eel caught off Sicily.

Wels, the giant fish introduced to France's River Tarn, have been filmed lunging onto the bank to snatch which birds?

Pigeons bathing at the water's edge were caught in 28% of the lunges that Toulouse researchers filmed. The trick has never been reported in the native range of the wels, so it seems to be a new habit.

In the American South, 'noodling' means catching which fish with bare hands by reaching into its underwater hole?

Catfish guarding eggs in a hole will latch onto a hand pushed inside, and the noodler hauls the fish out by it. The sport is now illegal in some states where it was once a tradition.

In much of Central Europe, which fish is the traditional Christmas Eve dish, often kept in the bathtub for days first?

Carp is kept in the bathtub to flush out its muddy taste, then usually breaded and fried. Czechs and Slovaks slip one of its scales under a plate or into a wallet as a symbol of wealth.

Which sharp-toothed South American river fish has a plant-eating relative, the pacu, with squarer, straighter teeth?

Piranha have razor-sharp teeth set in a pronounced underbite, while the pacu's are built for a diet of plants. The pacu is also the bigger fish by far, growing to more than a metre long.

Which pet fish, often dumped in ponds after outgrowing its tank, is an invasive pest in North America and Australia?

Goldfish released into the wild typically reach 12 to 22 cm, far bigger than most bowls allow. One caught in an English pond in 2010 measured 41 cm and weighed 2.3 kg.

Mexico's famous cave fish, descended from ordinary river fish, has lost which body parts over many generations?

Its eyes have gone entirely, leaving empty sockets covered with skin. The fish steers instead with extra-sensitive organs along its body that pick up slight changes in water pressure.

Salmon returning from the ocean are thought to find the very stream where they hatched mainly by using which sense?

Smell leads salmon to the entrance of their home river and on to the gravel where they hatched, because each stream has its own scent. Out at sea they are thought to steer by magnetism instead.

When two kissing gouramis press their lips together in an aquarium, what are the fish thought to be doing?

Fighting is the usual reading: the 'kiss' is treated as ritualized aggression, a way for rivals to challenge each other. The same toothed lips are used to rasp algae off stones.

A 300 kg specimen of which fish, caught in the Mekong in 2022, is the largest strictly freshwater fish ever documented?

Stingray is the answer: the giant freshwater stingray pulled from the river measured 3.98 metres long and 2.2 metres across. The species is endangered, with numbers down by up to half in Thailand and Cambodia.

Largemouth and smallmouth bass, the black basses prized by North American anglers, belong to which fish family?

Sunfish is the family, the same one as the little bluegill that largemouths eat. The largemouth is the biggest of the black basses, with a top recorded length of 75 cm.

The Jack Dempsey, an aquarium cichlid known for its aggression, is named after a 1920s champion in which sport?

Boxing gave the fish its name: its aggressive nature and strong facial features were likened to the heavyweight champion. Both parents turn fiercely protective once there are eggs to defend.

Which widely farmed fish, raised in ponds since Ancient Egypt, is sometimes sold as 'St Peter's fish'?

Tilapia live in the Sea of Galilee, where the Gospel of Matthew has the apostle Peter catch a fish carrying a coin. The gospel never says what kind of fish it was.

Which African lake holds more fish species than any other lake on Earth, including at least 700 kinds of cichlid?

Malawi cichlids can have tiny ranges, with some restricted to stretches of rocky coastline only a few hundred metres long. The IUCN lists 117 of the lake's cichlids as threatened.

Which huge predator, released into Lake Victoria in the 1950s, is blamed for wiping out hundreds of native fish species?

Nile perch can reach nearly 2 metres and 200 kg, and several hundred native species went extinct or close to it after its release. It is still traded under the misleading name Victoria perch.

Which jawless parasite spread through the Great Lakes, probably via shipping canals, and attacked the native lake trout?

Sea lamprey clamp onto other fish with a round, toothed sucker, and each can kill about 40 pounds of fish in a feeding period of 12 to 18 months. The Erie and Welland canals are the likely way in.

Which Asian predator, now invasive in several US states, can breathe air and survive out of water for several days?

Snakehead gulp air using a special organ above the gills, and the young may even wriggle short distances overland. In its native Asia it is a valued food fish, which is how it came to be shipped abroad.

Many cichlids of Africa's great lakes protect their eggs and newly hatched young by carrying them where?

In their mouths, usually the mother's, the brood is held safe for long periods. In many species the male fertilises the eggs only after the female has already scooped them up.

What name is given to the chain of low pools that lets migrating salmon and trout get past a dam?

Fish ladder steps are low weirs with a resting pool between each one, so fish can climb in stages. Rough versions made from bundles of branches were in use in 17th-century France.

Which Robert Redford film follows two Montana brothers who learn fly fishing for trout from their minister father?

A River Runs Through It stars Brad Pitt and Craig Sheffer as the Maclean brothers and is based on Norman Maclean's novella about his own family. It won the Oscar for Best Cinematography.

Which long-lived, scaleless fish is armoured instead with five rows of bony plates called scutes?

Sturgeon fossils go back to the Late Cretaceous, when dinosaurs were still around. The adults have no teeth, so a feeding fish extends its lips and vacuums up soft food whole.

Which livebearing aquarium fish, named for a man who sent specimens from Trinidad, is also called the millionfish?

Guppy honours Robert John Lechmere Guppy, who sent the fish to London's Natural History Museum. It now lives on every continent except Antarctica, mostly after being released to eat mosquito larvae.

Which tiny South American aquarium fish has an iridescent blue stripe that fades to grey or black as it rests at night?

Neon tetra colours come from guanine crystals in the skin that respond to the light. Most of those sold in the United States are farm-raised in Southeast Asia, far from their home streams.

Water constantly diffuses into the bodies of freshwater fish, which shed the excess by producing plenty of what?

Dilute urine carries the surplus water out, while the gills actively pull salt back in. A sea fish has the opposite problem: it tends to lose water and gain salt.

Bettas and gouramis can gulp air at the surface thanks to which folded breathing structure above the gills?

Labyrinth organ tissue takes oxygen straight from the air. That lets a betta thrive in rice paddies, ditches and large puddles, where the water holds too little oxygen for most other fish.

The floating nest that a male betta builds at the surface to hold his eggs is made of what?

Bubbles blown with an oral secretion make up the raft, which the male then guards. Most fish that build these nests live in warm, standing water that is short of oxygen.

Despite its name, South America's electric eel is not a true eel but belongs to which order of fish?

Knifefish are South American fish that sense electricity, and the electric eel is the order's heavyweight, with shocks of up to 860 volts. It also has to come up regularly to breathe air.

Which Central American lake has sharks that were once thought to be a species of their own, trapped in fresh water?

Nicaragua's lake sharks are ordinary bull sharks that travel the rapids of the San Juan River, almost like salmon. Tagged ones have reached the open ocean in as little as seven days.

Which composer's 'Trout' Quintet takes its nickname from a movement based on his own earlier song about the fish?

Franz Schubert wrote the quintet at 22 and built its fourth movement as variations on his song Die Forelle. It swaps the usual second violin for a double bass, and was published only after his death.

George Perry's 1932 world-record largemouth bass was tied in 2009 by a fish from Lake Biwa in which country?

Japan's fish, landed by Manabu Kurita, was actually an ounce heavier than Perry's. The record is shared because a new one has to beat the old by at least two ounces.

Which fish, Minnesota's state fish, has giant statues in three towns that each claim to be its world capital?

Walleye see well in dim, murky water thanks to a reflective layer called the tapetum lucidum, which also gives them a pearly stare. That is why anglers so often go after them at night.

Which elusive apex predator of northern lakes, Wisconsin's state fish, is known as the 'fish of 10,000 casts'?

Muskellunge are hard to hook mainly because so few live in any one lake, not because they are crafty. They eat almost anything, including other muskies and the occasional duck or muskrat.

Which long, sharp-snouted ambush predator of northern lakes shares its English name with a medieval pole weapon?

Pike probably owes its name to a long, pointed jaw that recalls the tip of the weapon. Anglers have plenty of other names for it, among them jackfish, gator, slough shark and hammer handle.

Steelhead, which spend two or three years in the ocean before returning to spawn, are a form of which trout?

Rainbow trout that stay in streams average 2.5 kg at most, while the sea-going steelhead can reach 9 kg. The species has been introduced for food or sport in at least 45 countries.

Kokanee, which never go to sea and live out their whole lives in fresh water, are a form of which Pacific salmon?

Sockeye returning from the ocean spawn beside kokanee in some regions without the two interbreeding, a sign that they may be splitting into separate species. Kokanee also go by little redfish and silver trout.

Which giant fish of the southern United States has bone-hard scales that Native Americans once used as arrowheads?

Alligator gar scales have an enamel-like coating and are nearly impenetrable, so they also served as breastplates. The fish is named for its broad snout and long, sharp teeth.

The Chinese paddlefish, one of the largest freshwater fish until it was declared extinct in 2022, lived in which river?

Yangtze dam-building sealed its fate: the Gezhouba Dam, built in 1981, cut the fish off from its upriver spawning grounds. The last one seen alive was in 2003.

Which Asian aquarium fish, prized as a symbol of good luck, is called the dragonfish for its large metallic scales?

Arowana are endangered in the wild, and international trade in them has been under the strictest controls since 1975. Fish from registered breeders can still be exported, but in the United States it takes a permit just to keep one.

What are the whisker-like feelers, packed with taste buds, that many fish use to find food in murky water called?

Barbels put taste buds on the outside of the fish, raised on tiny ridges of skin. A bullhead can have 25 of them in a single square millimetre.

Which word describes water, like that of an estuary, that is saltier than fresh water but less salty than the sea?

Brackish water forms where seawater and fresh water mix, and it also turns up in ancient underground aquifers. The word covers a whole range of salt levels, not one fixed value.

Which English writer celebrated the art and spirit of river fishing in The Compleat Angler, first published in 1653?

Izaak Walton kept adding to the book for a quarter of a century. It came out just after the English Civil War and offered scenes of harmony, far from the political turbulence of his day.

Which cold-loving fish, which spawns under the ice in midwinter, is the only freshwater member of the cod order?

Burbot need frigid water to breed, spawning between December and March at just 1 to 5 °C. Plans to reintroduce the fish to British waters, where it may have died out, were being studied in 2020.

What do biologists call the gravel nest that a female salmon digs in a stream bed with sweeps of her tail?

Redd digging works by suction: the tail creates a low-pressure zone that lifts the gravel so the current can carry it off. The hatchlings then hide among the stones until their yolk is used up.

Which small, fleshy fin behind a trout's dorsal fin is often clipped off to mark hatchery-raised fish?

Adipose fins are still something of a mystery, but they are not useless. Trout that lose theirs beat their tails 8% faster, and the fin may help detect touch, sound and changes in pressure.

Which tiny fish reached the US Supreme Court in 1978 after its endangered status held up Tennessee's Tellico Dam?

Snail darter protections were upheld by the court, so Congress passed a law exempting the dam from the Endangered Species Act. The fish had first been recorded only in 1973.

The Devils Hole pupfish, a critically endangered fish, lives only in one water-filled cavern in which US state?

Nevada's Devils Hole gives the fish one shallow rock shelf to feed and spawn on. When nearby farms pumped water in the 1960s and 1970s, the level dropped and less and less of that shelf stayed submerged.

The golomyanka, a translucent, scaleless fish whose body is more than a third oil, lives only in which lake?

Baikal's golomyanka lays no eggs: the females give birth to swarms of 2,000 to 3,000 larvae. The fish is also the main food of the lake's seals.

The bitterling, a small European fish once used in human pregnancy tests, lays its eggs inside which living animals?

Mussels draw in the male's sperm with the water they breathe, so the eggs are fertilised inside the shell. The young stay protected there for about a month before swimming out.

GloFish, fluorescent pets first engineered to detect water pollution, were originally made from which striped fish?

Zebrafish embryos were given a jellyfish gene for green fluorescent protein by a team in Singapore. The glowing result became one of the first genetically modified animals sold to the public.

In a cycled aquarium, bacteria turn toxic ammonia from fish waste into nitrite and then into which safer compound?

Nitrate is far less toxic and can be soaked up by aquarium plants. Fishkeepers buy test kits to track all three compounds, since the ammonia they start from is dangerous in quantity.