Migrating Fish: Anadromous or Catadromous?

According to Merriam-Webster's online edition, the word "anadromous" means "ascending rivers from the sea for breeding."  This word is almost always used in conjunction with fish, as they are the main type of animal that do this.  There are a number of fish that are defined as anadromous, but of particular import for us are several types of salmon, including the Coho, steelhead, Chinook, and sockeye salmons.

You don't have to be in a school (of fish) to hypothesize that there might be a group of fish that behave in an opposite manner from the anadromous fish.  "I'll take the bait," you're saying.  "What's the opposite of an anadromous fish?"  Let's break the word apart.  First, we can break off the root "ana," a Greek root which means "up."  For you chemistry buffs out there, you might have felt an electric current running up your spine, as you've probably thought of the word "anion," a term used to describe an ion that contains more electrons than it does protons, giving the atom an overall negative charge.  If you follow the flow of my logic*, you might be thinking of the opposite of an anion: a "cation," or an ion that contains fewer electrons than it does protons, giving the atom an overall positive charge.  So judging from this chemistry example, you can either conclude that, A) The opposite of anadromous is likely something along the lines of "catadromous," or, B) I give really long and tangential comparisons that are neither helpful or correct.  Fortunately for you guys, option A is the correct one.  The opposite of an anadromous fish is, indeed, a catadromous fish.  To learn more about the life cycle of a catadromous fish, let's travel to the eastern coast of North America, and meet Anguilla rostrata, the North American eel.  (And yes, eels are a type of fish.)

To learn more about the life cycle of the North American eel, I consulted the website of the Penobscot River Restoration Trust.  According to the Trust, mature Anguilla rostrata leave their brackish or freshwater homes and migrate to the Sargasso Sea in the fall.  The Sargasso Sea is not actually a distinct sea, but instead a large region of the Atlantic Ocean where a holopelagic species of seaweed called Sargassum reproduces.  The term "holopelagic" simply refers to the fact that, unlike other seaweeds, Sargassum does not require the presence of the ocean floor to reproduce, and can instead reproduce while just floating around in the ocean, or in the pelagic zone.  In turn, the pelagic zone is simply defined as an area of a body of water that is neither close to shore nor close to the bottom.  I remember when my family and I went on the Tybee Island Ecology Tour with Dr. Joe Richardson several years ago in Georgia, Dr. Joe commented on the large amount of Sargassum that was on the beach that day.  I've recently talked with Dr. Joe about the Sargassum, as well as the North American eel, and this interview will be the subject of our next post.

As winter rolls around, the adult eel spawns....then dies.  Sad day for the grown up eels.  Their eggs, however, hatch after a few days, and the young develop into a larval stage (which are then called leptocephali) that simply drift around for a few months.  Their days of floating casually around the ocean end as they enter the Gulf Stream, and are carried north towards North America.  Once the larvae find themselves near the continental shelf, they transform into what are called "glass eels," miniature little eels that, as you could probably guess from the name, are transparent!

It's not over for these eels yet, though!  Next, the young reach the estuaries, transitional areas along the coast where the rivers meet the sea.  Once the summer rolls around, it is in the estuaries that the eels enter the next stage of their life.  In this phase, the young eels are called "elvers," a name for a juvenile fish that is specific to eels.  After making their way into their adult habitat, they finally develop into their adult morph, where some eels stay for 8-25 years before migrating back to spawn in the sea!  

So there's a perfect example of a fish with a catadromous lifestyle.  Or, rather, nearly perfect.  In more recent years, scientists have begun to discover that while some adult North American eels fit well under the catadromy heading, other members of the exact same species simply stay in the estuaries to mature, while still more travel back and forth between the estuaries and the fresh water habitats further upstream!  This has caused many scientists to reconsider the stance of a purely catadromous lifestyle for these eels, and revise their description to "facultative catadromy," which essentially means that these eels seem to be able to choose whether or not they want to commit to a fully catadromous lifestyle.  But what about an anadromous fish, like the Chinook and Coho salmon that we mentioned earlier?  Don't worry, I'm not going to flounder: I know that if you're still reading this, then you're probably hooked, and want to hear more.

Speaking of hooked: my fishing contact Larry Quilling with a spring Chinook salmon in the Trask River in Oregon.  Check back soon to see an interview with Larry about his experiences fishing salmon!  Photo Credit: Larry Quilling

When it comes to the lives of several of the species of Pacific salmon, the saying "When the going gets tough, the tough get going" really applies quite nicely.  One of the keystone species in the American northwest, the annual "salmon run" is incredibly important to the functioning of the ecosystem.  During the salmon run, millions upon millions of these fish travel upstream to their breeding grounds.  Some travel up to 900 miles, as is the case for many Chinook and steelhead salmon who make their home in Idaho, as seen below.  The Chinook females also build an enormous nest, called a redd, that can be one to four feet deep and around six feet in diameter!  

Here we have another one of my fishing contacts, Wallace Westfeldt, with a steelhead in Idaho.  Wallace was also kind enough to help me out a lot, and we will also be seeing an interview post with him soon!  Photo Credit: Wallace Westfeldt

Another fascinating thing to note is the incredible transformation of these salmon, especially the males, as they travel upstream.  My personal favorite is the transformation of the sockeye salmon, whose changes can be seen in the picture below.  

With their fisshin accomplished, most salmon species die once they reach their spawning areas, just like the North American eel.  However, unlike the Pacific salmon, some individual Salmo salar, or Atlantic salmon, live to tell their gilling tale to future generations of salmon.  Not all Atlantic salmon die from their ordeal, as the bodies of these fish don't deteriorate post-spawning.  For the Pacific salmon, their method of reproduction is sometimes referred to as "semelparity," a situation in which the organism is physiologically incapable of spawning more than once prior to kicking the chum bucket.  The opposite of semelparity is iteroparity, where the animal can reproduce multiple times prior to death.  Almost all extant vertebrates (and, indeed, a large portion of extant organisms) are iteroparous, including us humans.  Notable examples of this semelparous lifestyle include some types of spiders, the genus of marsupial mice Antechinus, some types of bamboo, the aptly named century plant (or agave), and of course, several types of salmon.  

So in the course of this blog post, I've thrown a whole lot of new words at you.  Let's stop to reconsider them real fast, in the order that they were introduced in the post.

• Anadromous: Refers to a type of animal that goes from the sea into rivers to breed.  Many types of salmon fit this ticket.
• Catadromous: This term refers to a type of animal (typically fish) that goes from rivers into the sea to breed.  An example would be the North American eel (Anguilla rostrata).
• Holopelagic: An organism that remains in a pelagic area for its entire life.  An example would be the Sargassum seaweed.
• Pelagic: An area of a body of water that is neither close to shore nor close to the bottom.
• Leptocephali: A name given to the larval stage of eels.
• Glass Eel: The next stage in the life cycle of an eel, following leptocephali, but preceding the elver stage.  
• Estuary: The transition zone between the ocean and a river.  The portion of a river that is saltier than the rest of the river, but not quite salty enough to be the ocean, and is influenced by the tides.  Essentially, the last portion of a river prior to its arrival in the ocean.  
• Elver: A name referring to an eel in its postlarval stage following the leptocephali and glass eel stages.  
• Facultative Catadromy: A term used to refer to an animal that can choose (facultative) whether or not they want to commit to a fully catadromous lifestyle.  A good example is the North American eel (Anguilla rostrata).  
• Keystone Species: A species of animal that is integral for the functioning of a healthy ecosystem. 
• Redd: A nest made by a fish, such as that of the female Chinook salmon pictured above.
• Semelparity: A term which refers to organisms that can only reproduce a single time prior to their death.  Examples include some spiders, several types of Australian marsupials and salmon.
• Iteroparity: A term which refers to organisms that can reproduce many times prior to their death.  Examples include humans, pigs, and the Atlantic salmon.

That's a lot of new vocab for just one post!  While I'm not sure if we will be returning to the life cycles of eels again in the future, make sure you hold on to the terms "anadromous" and "catadromous" (as that was the whole meaning of this post), as well as "keystone species."  We will hopefully be taking a broad look at several keystone species sometime in the near future!  And make sure to check back in during the next several days so you can hear from Larry Quilling, Wallace Westfeldt, and Dr. Joe Richardson!

*I'm sorry, I am really proud of that pun.  Not ONLY does it work in the context of fish in streams, but it also works in the context of electric current and chemistry.  Dang I'm good.

References:

Braithwaite, Richard W., and Anthony K. Lee. "A Mammalian Example of Semelparity." The American Naturalist. no. 1 (1979): 151. http://www.jstor.org/discover/10.2307/2459950?uid=3739568&uid=2&uid=4&uid=3739256&sid=21103001478577 (accessed November 26, 2013).
California Department of Fish and Wildlife, "What is an anadromous fish?." Accessed November 21, 2013. http://www.dfg.ca.gov/fish/resources/AnadromousDef.asp.

Idaho Fish and Game, "http://fishandgame.idaho.gov/public/fish/?getPage=316." Last modified 0004 19, 2012. Accessed November 22, 2013. http://fishandgame.idaho.gov/public/fish/?getPage=316.

Keefer, M. L. , C. A. Peery, and C. C. Caudill. "Long distance downstream movements by homing adult chinook salmon." Journal of Fish Biology. . doi:10.1111/j.1095-8649.2006.00958.x (accessed November 22, 2013).

Merriam-Webster, "Anadromous Definition." Accessed November 21, 2013. http://www.merriam-webster.com/dictionary/anadromous.

NOAA, "Sargasso Sea." Accessed November 22, 2013. http://oceanservice.noaa.gov/facts/sargassosea.html.

Penobscot River Restoration Trust, "Catadromous Fish." Accessed November 22, 2013. http://www.penobscotriver.org/content/4028/catadromous-fish.

Richard, B. (2004). The encyclopedia of north american animals. New York: Barnes & Noble Books.

U.S. Fish and Wildlife Service Newsroom, "The American Eel." Accessed November 22, 2013. http://www.fws.gov/northeast/newsroom/facts.html.

Young, Truman P. The Nature Education Knowledge Project, "Semelparity and Iteroparity." Accessed November 22, 2013. http://www.nature.com/scitable/knowledge/library/semelparity-and-iteroparity-13260334.

"2010 Late Run Sockeye Salmon Preliminary Escapement Estimates." Salmon Society. . http://www.salmonsociety.com/DFO salmon count.pdf (accessed November 22, 2013).