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Anadromous Fish: Definition, Species & Migration

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Many fish spend their lives in one place, but others prefer to see a bit more of the world. Anadromous fish are of the latter, and in this lesson you'll learn about the incredible journey they undertake in their lives.

What is Anadromous?

Imagine if I told you when you were a child that you had to move from your hometown to somewhere really far away, stay there until you were an adult, and then come all the way back to where you were born in order to have your own kids? Oh, and you had to walk both there and back. You'd probably tell me I was crazy. And then you'd go on living your life however and wherever you chose.

But if you are an anadromous fish, this is exactly what you have to do! These are fish that are born in freshwater, migrate out to the ocean to mature into adults, and then swim back to freshwater to spawn (reproduce). Crazy, but true! Anadromous fish are part of a broader classification of fish called diadromous. This includes not only anadromous fish but also their counterparts, catadromous fish like many eels that do the reverse - mature in freshwater and spawn in marine environments.

These names sound a bit weird but if we break them down we can see how they relate to the types of fish they describe. Anadromous can be broken down into 'ana' which means 'up' and 'dromous' which refers to 'running'. Up, like how they swim upstream from the sea to spawn. Catadromous is the opposite: 'down running'. And the overall term for both fish diadromous comes from 'dia' for 'across' since they run across or between two different types of waters.

A Long, Hard Journey

The journey of an anadromous fish is a long and arduous one. Unlike you and me, they can't just hop on a boat to head out to the ocean. Some fish will travel for days, months, or even years to reach their new destination. These distances may be very short but often they are very long, sometimes thousands of miles!

For anadromous fish, the time spent in freshwater before heading out to sea varies. Overall, this migration is an exhausting process that takes a lot of energy. For some fish, they can make this journey several times throughout their lives, while others, like salmon, die from exhaustion upon their first return.

Salmon that have returned to freshwater to spawn. The fish will die afterward from exhaustion.
sockeye salmon spawn

Why Migrate?

You may be wondering why in the world fish participate in this crazy activity. Well, the truth is we don't fully understand (have you ever asked a fish a question and gotten a meaningful response?). But there must be benefits of this migration that far outweigh the work it requires, and scientists have some theories.

For example, in more temperate latitudes (which is where we find many anadromous fish species) the ocean waters are more productive than freshwater, in other words they provide better feeding grounds during the growth stage of life.

The reverse is true for waters at tropical latitudes, which is where we find many catadromous species. There's also the thought that freshwater environments likely provide better protection for eggs and juveniles than ocean waters, which is why these fish choose to come back to freshwater to have their babies.

Species of Anadromous Fishes

There's a variety of anadromous fish species that live all over the world.

  • In New Zealand we find the common smelt, in the Black Sea we find the Pontic shad, the Arctic lamprey in Russia, Alaska, and northwest Canada, the Tasmanian whitebait in Tasmania, the three-spined stickleback in British Columbia, and so on and so forth.
  • On the east coast of the U.S. examples include Atlantic salmon, Atlantic sturgeon, striped bass, alewife, blueback herring.
  • Anadromous species on the west coast of the U.S. include pink, white and green sturgeon, steelhead trout, and the many salmon species such as coho, Chinook, and sockeye.

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