The dead float back toward them.

Sockeye salmon photo by Phillip Colla. www.oceanlight.com

Sockeye salmon photo by Phillip Colla. http://www.oceanlight.com

Kim Addonizio is probably my favorite contemporary poet.  She can infuse such sadness, such wistful longing into her works, with a talent for such human description that is never too overwhelmingly emotional.  In her poem “Salmon,” she describes the experience of seeing adult salmon at the end of their long journey, laying eggs and then dying:

In this shallow creek
they flop back and forth and writhe forward as the dead
float back toward them.  Oh, I know
 
what I should say: fierce burning in the body
as her eggs burst free, milky cloud
of sperm as he quickens them.  I should stand
 
on the bridge with my camera,
frame the white froth of rapids where one
arcs up for an instant in its final grace.”  
 

The life of a salmon is unique among fish.  All Pacific salmon species are anadromous, which means that they are born in freshwater and spend their adult lives in saltwater, returning to breed in natal rivers.  After a 2-3 month incubation period, eggs in freshwater hatch into what are called alevin.  The alevin are still connected to the yolk sac, and continue to feed on it for the first few months while the mouth and digestive system develop.  See below for a great image of a freshly hatched alevin, complete with vascularized yolk sac:

411px-Salmonlarvakils

Once the alevin consumes the yolk sac, it is called a fry.  Fry emerge from the gravel nest in the stream bed in order to feed (some species take off for the ocean at this point; others remain in freshwater for a longer period of time).  The salmon fry will begin to develop vertical stripes, at which point it is called a parr.  The vertical stripes are thought to act as camouflage, allowing the parr to blend in among the rocks and stream  vegetation.  Again, depending on species, the salmon parr stay in freshwater from six months to three years, feeding and growing larger in preparation for the journey out to sea.  When it is time, the parr lose their markings and turn a silvery color, at which point they are called smolts.  Smolts begin to school in large groups and gradually allow their bodies to become accustomed to salt water, often spending time in brackish water before finally achieving the ocean.  After a few years, the smolts have become adult salmon, and live entirely in a saltwater environment for 1-5 years.

One thing that I find incredibly interesting about the salmon is its anadromous lifestyle; the ability to move from freshwater to saltwater and then back without major trauma.  From a biology standpoint, this ability is simply amazing.  Saltwater contains about 1000 times more ions than freshwater (salt, sodium chloride, is broken down into its most stable form in water: sodium ions and chloride ions).  This is a problem for an animal living in either environment.  The internal environment of a fish, for instance, is optimized to be at an intermediate salinity.  The fish must have what is called an osmoregulatory system in order to maintain the balance of water and ion in its aquatic environment.

So, in freshwater, the fish is in an environment where its body is ‘saltier’ (contains more ions) than its surroundings.  To deal with it, its kidneys work to dump water; in other words, it urinates frequently.  At the same time, the fish takes in ions (there are a few, even in freshwater!) through its gills.

In saltwater, on the other hand, it is the opposite.  The surrounding water is much saltier (more ions!) than its body, and has the effect of being dehydrating to the fish.  To deal with this, a fish drinks a lot of water, but does not urinate.  Salt is stripped away from the water it drinks and is secreted by the gills.

Possessing the capacity to osmoregulate in either environment is a physiological miracle in itself.  Being anadromous means that a salmon can actually switch from one mode to another and back.  The first transition, from fresh to saltwater, is arguably the most incredible since during this period the smolts actually restructure their entire physiology prior to changing environments.

The return journey, back to freshwater, is no less amazing than the journey out.  Salmon return to the same rivers they hatched in to spawn.  A salmon is considered mature when it begins to change to a deeper color.  Male salmon prepare for combat by developing other striking features: a distinct hump, canine teeth, and a kype, or pronounced curvature of the jaws.  These mature salmon spend anywhere from a day to a week at the mouth of the river  and then begin to travel upriver, using the same route taken as a smolt.  This homing ability is thought to be facilitated in large part by the salmon’s sense of smell; they can actually smell where they were born and navigate towards it.

Salmon can travel hundreds of miles upstream to reach a spawning ground.  Very few survive the journey, and those that do manage to lay or fertilize eggs are beset by accelerated aging.  Their bodies begin to rapidly deteriorate right after spawning as a result of the release of massive amounts of corticosteroid hormones.  It is at this point that Addonizio observes the salmon, having fought hundreds of miles upstream to lay eggs and pass on their genes to the next generation, their bodies wasted and spent, rotting in the sunshine.  She refuses to shy away from this aspect of their lives, writing,

“I have to study the small holes
gouged into their skin, their useless gills, 
their gowns of black flies.  I can’t
 
make them sing.  I want to,
but all they do is open
their mouths a little wider
 
so the water pours in
until I feel like I’m drowning.  
On the bridge the tour bus waits
 
and someone waves, and calls down
it’s time, and the current keeps lifting
dirt from the bottom to cover the eggs.”
 
 

REFERENCES:

“About Pacific Salmon.” Pacific Salmon Commission.  Link.

“Salmon,” by Kim Addonizio.  Read it here.

“Salmon Biology”.  2010.  Salmon Fishing Now.  Link.

“Salmon”.  Wikipedia.  Link.

THANKS:

Jon Velotta was instrumental in writing an understandable explanation of osmoregulation.  Please go check out his research on osmoregulation in the alewife (a river herring).  His webpage is here.

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