21 migratory fish facts that'll make you say, "I never knew that!"

Rainbow smelt (Osmerus mordax) © NOAA Fisheries

Saturday, April 21 is World Fish Migration Day - a day to create awareness about the importance of open rivers and migratory fish. To celebrate World Fish Migration Day we're sharing a collection of migratory fish fact that'll make you say, "I never knew that!" From river herring and Atlantic salmon to rainbow smelt, even the most die-hard fish fans will learn something new.

1. Sea-run fish are also referred to as diadromous, meaning that they spend part of their life in freshwater and another part in saltwater. Diadromous fish are either:
   • Anadromous, spending most of their adult life at sea, but returning to freshwater to spawn, or
   • Catadromous, spending most of their adult life in freshwater, but returning to the sea to spawn
2. River herring are actually two different species of fish: alewives (Alosa pseudoharengus) and blueback herring (Alosa aestivalis).
3. To scale through rapids, river herring don't jump over them like salmon. They swim very fast and in short bursts.
4. Alewives prefer to lay their eggs at night in slow-moving water while blueback herring prefer to spawn over rocks during the day and in fast-moving water.
5. During the 1800s people ate most of the harvested alewives because they preserved well in salt or when smoked. As refrigeration became mainstream in the 20th century, the demand for alewives for human consumption decreased and was replaced by other fish species.
6. Atlantic sturgeon (Acipenser oxyrinchus) can grow to approximately 14 feet long, weigh up to 800 pounds, and live to about 60.
7. Atlantic sturgeon are cartilaginous, meaning their skeleton is mostly cartilage, not bone.
8. Atlantic sturgeon mouths are protrusible. They can be thrust out because their jaws are not really connected by any skeletal structure to their skulls. This allows them to essentially vacuum up prey items located on or in the bottom sediment. Sturgeons also have super muscular stomachs that are strong enough to crush and break up food for digestion. Handy, since they don't have any teeth!
9. Unlike Pacific salmon that spawn once and die, Atlantic salmon (Salmo salar) are iteroparous, meaning they don't die after spawning and can therefore spawn more than once.
10. Depending on the size of the female, Atlantic salmon produce about 2,500 to 7,000 eggs. That's about 600-800 eggs per pound of body weight! Why does it depend on the female's size? Typically, larger females produce more eggs.
11. In Norse mythology Loki is a trickster god who tricked Hodr into killing the much-adored Baldur. To escape the wrath of the other Norse gods, Loki transformed into a salmon. Thor foiled Loki's escape by catching him near his tail. Thor's grip was so strong it created the salmon's caudal peduncle, the narrow part of the fish's body where the tail fin attaches to the body.
12. Archaeological evidence seems to confirm what mythology touted -- that salmon have been venerated for tens of thousands of years. The French Abri du Poisson, or "Fish Rock Shelter", is adorned with a famous prehistoric bas-relief carving of an Atlantic salmon some 25,000 years old. It's one of the oldest known representations of a fish, the only known sculpture of a fish from the Paleolithic era, and the shelter is a UNESCO World Heritage Site.
13. Atlantic salmon in a various languages:
• Mi'kmaq (Micmac): Plamu
• English: Atlantic salmon
• Norwegian: Laks, Lax
• Swedish: Lax
• Finnish: Lohi
• Dutch: Zalm
• Danish: Atlantisk laks, Laks, Skaellaks
• Icelandic: Laxa, Lax
• Greenlandic: Kapisillit
• Gaelic/Irish: braddan and bradan
• Ancient Celtic: Iach
• French: saumon atlantique
• German: Echter lachs, Lachs, Las, Salm
• Spanish: Salmón, Salmón del Atlántico
• Portuguese: Salmao, Salmao do atlântico
• Greek: Salomós
• Turkish: Alabalik
• Polish: Losos, Losos szlachetny atlantycki
• Czech: Losos atlantsky
• Russian: Losos
14. American eels (Anguilla rostrata) are catadromous. Their eggs hatch in the Sargasso Sea and the Gulf Stream delivers them to river systems along East Coast of North America. This journey can take up to 12 months. They will grow and mature in rivers until they journey back to the Sargasso Sea to spawn. For some this journey can be close to 3000 miles! During their journey they don't eat and after they spawn, they die.
15. American eels not only absorb oxygen through their gills, but also through their skin! This help them travel over land, particularly in wet grass or mud.
16. During their riverine "yellow phase," American eels are nocturnal, swimming and feeding at night. They prey on a variety of things like insects, fish, fish eggs, crabs, worms, clams, and frogs. They'll even eat dead animals. Eels are strong and can move forward and backward quickly and easily. This helps them pull, twist and spin to tear apart large prey.
17. American eels -- like all eels -- have a leptocephalus larval stage where their bodies are long, very thin, and nearly see-through. Some scientists think that leptocephalus larvae mimic gelatinous zooplankton like jellyfish, ctenophores, siphonophores, and salps to escape predation. Why? Many gelatinous zooplankton species have stinging cells for defense and/or little food value. Leptocephalus larvae curl in response to life-threatening situations so they look like noxious gelatinous zooplankton. Mimicry like this could provide leptocephalus larvae a leg up on survival -- a cool ability when more than 99% of marine fish die during their early life stages!
18. Early records indicate that striped bass (Morone saxatilis) were once so plentiful that settlers used them to fertilize crops. But in 1649, the practice was banned by the General Court of the Massachusetts Bay Colony.
19. The striped bass is Maryland's state fish.
20. Rainbow smelt (Osmerus mordax) are anadromous and inhabit inshore waters along the North American Atlantic coast. In winter these waters are frequently ice-covered with temperatures as low as 28 degreesF. Rainbow smelt don't freeze because they have antifreeze proteins and glycerol -- a kind of alcohol -- in their blood, liver, muscle, and other tissues that prevents freezing.
21. Rainbow smelt are said to smell like freshly cut cucumber.

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