Sunday, November 13, 2016

Chinook Salmon in the Stanislaus River

Over the last several weeks, an incredible phenomenon has been taking place in California's Great Central Valley: the fall salmon run.  Chinook salmon (Oncorhynchus tshawytscha) have been traveling many miles from the Pacific Ocean upstream to their spawning grounds in freshwater rivers, like the Stanislaus.  This weekend, the salmon were celebrated at the annual Knight's Ferry Salmon Festival.
Female Chinook Salmon
Something about watching these large fish, this magnificent keystone species, swim slowly up-river to suitable gravel spawning grounds is captivating, mesmerizing even, as you realize you are witnessing a timeless dance in the circle of life, an event that has taken place in our local rivers for thousands of years.  And these days, many of us have very little idea of the fascinating and intricate circle of events playing out in our nearby streams.  It almost seems like a miracle that in this age of development and habitat alteration, an ancient species like the salmon, so dependent on clear, cold, unobstructed rivers, is able to persist.
Male Chinook Salmon
Chinook salmon are anadromous, meaning they spend the early part of their life in freshwater before migrating to the sea and undergoing physiological changes to be able to live in salt water.  These fish spend between two to five years of their lives at sea before returning to the same freshwater river of their birth in order to spawn and then die.  It is a fascinating life cycle that continues to intrigue citizens and scientists alike.  Adult chinook salmon reach three or more feet in length, weighing upwards of 100 pounds (an average is probably closer to 40 or 50 pounds).
Adult male Chinook Salmon, about one meter in length.
At 2 to 4 years of age, adult salmon leave their marine home and find their way through a maze of rivers to the very stream where they were born, a life strategy known as anadromy.  Females search for a suitable nest site, dependent upon the size and composition of the streambed gravel, as well as water depth and velocity.  Females use their fins and bodies to create a shallow depression in the gravel, a nest known as a redd.  A female salmon may deposit thousands of eggs in several individual "nesting pockets" within her redd.  The male salmon then releases sperm over the eggs, in a process known as external fertilization. 
After mating, the male and female will work together to territorially guard their redd for from several days to a month before dying.  Migrating many miles upriver to spawning grounds, breeding and guarding the redd requires a massive amount of energy and the salmons' last bit of strength; the salmon will die before their eggs hatch, their bodies returning rich nutrients to the ecosystem.  The act of spawning once in a lifetime and then dying is known as semelparity.  Historically, bears and eagles lined the riverbanks of the Great Central Valley to feast on spawning salmon in the fall.
Salmon carcass
Salmon eggs are preyed upon by other species of fish, raccoons and even ducks.  Eggs are susceptible to pollution, as well as suffocation from silt accumulation due to stream bank erosion.  Between 3 and 5 months from the time of deposition, salmon eggs will begin to hatch.  The exact time of hatching is dependent on water temperature, though the salmon lifecycle is timed to ensure that their hatching corresponds with the abundant productivity of spring, when there will be plenty of food available for the young fry.  Juvenile salmon eat insects, crustaceans and other invertebrates, and in turn are preyed upon by fish-eating birds, such as herons and egrets, as well as mammals like the river otter.
Stages of Chinook Salmon egg development
Salmon eggs thrive in cold water; the upper water temperature threshold is around 50-55 degrees Fahrenheit.  It is critical that our relatively shallow valley streams remain cold enough for salmon, even as November daytime temperatures creep toward 80 degrees (as the temperature has the past few days).

After spending several months to two years in freshwater, chinook salmon make their way to the sea where they will spend the next few years of life as marine adults, preying largely on other fish, as well as squid and shrimp.  While at sea, salmon are a favorite food item for seals and sea lions, orcas, sharks, and, of course, humans (which is okay as long as harvesting is done sustainably!)  After a few years at sea, they begin their long and final journey upriver to return to their birthplace.  And the cycle continues.
Look closely: Chinook salmon in the Stanislaus River (A red male can be seen in the lower left of the photo)
Like much of our wildlife and many of our wild places, both salmon and the habitat they depend on is under threat.  Most of California's waterways have been altered, and since the life cycle of salmon  is so intrinsically linked to many miles of pristine aquatic habitat, these changes have been detrimental to salmon populations.  The construction of dams, water storage, withdrawal and diversion for agriculture and urban use, as well as for flood control and hydroelectric power, has done severe damage to habitat that has been historically available to salmon for millennia.
The Stanislaus River, looking west
The NOAA Fisheries website states,
"Land use activities associated with logging, road construction, urban development, mining, agriculture, and recreation have significantly altered fish habitat quantity and quality. Associated impacts of these activities include: alteration of streambanks and channel morphology; alteration of ambient stream water temperatures; degradation of water quality; reduction in available food supply; elimination of spawning and rearing habitat; fragmentation of available habitats; elimination of downstream recruitment of spawning gravels and large woody debris; removal of riparian vegetation resulting in increased stream bank erosion; and increased sedimentation input into spawning and rearing areas resulting in the loss of channel complexity, pool habitat, suitable gravel substrate, and large woody debris. Studies indicate that in most western states, about 80 to 90 percent of the historic riparian habitat has been eliminated." 

In recent years, efforts have been made to bring back the salmon and restore habitat in order to ensuring their conservation and success as a species.  Efforts include captive rearing in hatcheries and habitat protection and rehabilitation.  Sections of key habitat have been protected in order to guard against habitat fragmentation.  Dams that obstruct salmon migration have been modified or in some cases removed.  Degraded habitat has been restored and water quality and flow improved.  In many cases, efforts have been made to curb streamside erosion and restore streambed gravel deposits suitable for nesting and critical for the reproductive success of the salmon.
The Stanislaus River, looking east
It was encouraging to see curious and concerned citizens gathered at the river during the festival to learn about our native Chinook Salmon.  And it was inspiring to see close to record numbers of salmon in the shallow waters of the Stanislaus River, performing the age-old dance: migrate, select a nest site, spawn, vigilantly guard the nest with the last of their strength, then die, their bodies returning to the same river gravels from which they came. 

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