How salmon return to their home waters after years in the Pacific Ocean is a fantastic two-part story involving chemical imprinting and geomagnetic imprinting.
Part I: Chemical Imprinting
Pacific salmon species have evolved an incredible adaptation for locating the home or natal waters they were once reared in as juveniles. Many years later as adults, they are able to find these home waters again to spawn.
It is in these home waters that the adults are intent on spawning at the end of their lives. They remember where they were raised by imprinting the smell of the fresh waters in which they lived during their early developmental stages.
Salmon actually have specialized neuro-receptor cells in the tissues associated with the paired nostril area in the upper head/snout area of the fish. These cells play a role in recording or becoming sensitive to the juvenile exposure to their environmental waters.
Water that makes its way from the Pipers Creek Watershed (Broadview, Blue Ridge, and Greenwood neighborhoods) to Puget Sound first accumulates and flows into the tributaries of Pipers Creek. The water carries with it the very specific chemicals and minerals that are dissolved into it during its route from the upper watershed to its lowest elevation in Puget Sound.
One of these tributaries is Venema Creek where CWCAP maintains a unique imprinting system to assist in re-establishing a spawning population of salmon.
This project began in 1979 when Carkeek Watershed Community Action Project (CWCAP) was formed. 501(c)(3) Non-profit status was established in 1984.
Over the decades, programs of education, demonstration, and outreach have grown along with CWCAP’s work in park development, forest and habitat restoration, and supplemental salmon stocking programs.
Each Spring from January through May, CWCAP keeps about 100,000 Chum salmon fry in four different batches long enough for these fish to imprint the mineral/chemical signature or smell of the waters that flow through the Pipers Creek Watershed.
After 3 weeks-to-2 months of imprinting, these fish have the best chance of returning in 3 to 5 years, to these safe home waters, to spawn and pass on their survival instincts to the next generation of salmon.
Part II: Geomagnetic Imprinting
As impressive as chemical environmental imprinting is, it is only effective once salmon have swam within 30 to 100 miles (depending on research results) of their home water systems. Getting close enough to pick up the chemical/mineral signature of a salmon’s home waters requires a sensitive preference for cold water currents and an imprinting of the pattern of geomagnetic strength and alignment that they have recorded during their migration from their smolting waters outbound to their adult feeding grounds.
Earth has a predictable, consistent geomagnetic field that weakens from the poles toward the equator. Salmon are sensitive to the gradient (change in magnitude) of the geomagnetic field, including the direction of alignment, and can record this pattern information in specialized cell structures.
After years of feeding as adults while following prey food based on cold water currents, the instinctual drive to spawn overwhelms the drive to feed. Prevailing water currents may provide a relative route back in the general direction of their home waters, but a more reliable geomagnetic route was recorded earlier in their lives as they were migrating outbound to their feeding grounds.
Matching the geomagnetic pattern of their outbound route is thought to give salmon accurate directions as they return and are finally able to pick up the chemical smell of their home water systems.
Why Chum salmon and not Coho or Chinook salmon?
Chum salmon (Oncorhynchus keta) is the only salmon species that CWCAP releases into Venema Creek, the main tributary of Pipers Creek.
Chum, along with Pink salmon (O. gorbuscha), are uniquely prepared to search for the estuary (the tidal mouth of a freshwater river or creek where the saltwater tide mixes with the stream) within days of either emerging from their gravelly nest as free-swimming fry or being released from imprinting facilities as fry.
Given the short time that Chum salmon are in their home waters as juveniles, their survival is not dependent on conditions that prevail during dry summer months when shallow creek waters cannot provide habitat and food sources. Chum salmon fry in the Pipers Creek system pass from the mouth of Pipers Creek into the eel grass areas off of Carkeek Beach long before their home waters become inhospitable.
Other salmon species such as Coho (O. kisutch) and Chinook salmon (O. tshawytscha) require up-to 6 months to a year or more as freshwater juveniles before entering the estuary. Therefore these species require suitable creeks, rivers and lakes that provide long-term freshwater habit and food sources during their early life stages. Summer waters in the Piper Creek Watershed are not hospitable to juvenile salmon species that are attempting to grow rapidly in preparation for migration to the Pacific Ocean.
Each Spring, 4 batches of Chum salmon are delivered to the Imprint Pond at Carkeek Park to be imprinted by CWCAP Salmon Imprint Stewards. Imprint Stewards care for, feed, and oversee the Imprinting System from January to May.
The Suquamish Tribe’s Grover’s Creek Salmon Hatchery provides about 100,000 Chum salmon fry and eggs to CWCAP. They also provide 5-6,000 salmon eggs to about two dozen local public, private, and home school groups in the Salmon in the Schools Program — all to be released in Venema Creek.
Imprinting will give these fish the best chance at returning to their home waters in the Pipers and Venema Creeks in 3 to 5 years to spawn.
Below is a summary of key imprinting results for 2018.
Imprinting Season: 126 days, January 7 to May 12, 2018
Total deliveries: 104,115 Chum salmon eggs and fry
Total releases: 98,501 imprinted Chum salmon fry
1st Batch Delivery – 30,000 eggs (fertilized Chum salmon eggs)
Delivery: January 7, 2018
Last release: April 2, 2018
Imprinted/Days until last fish exited Self-release tank: 86 days
Release number: 27,000 fish
2nd Batch Delivery – 35,000 fry (from hatchery trays, not yet fed by mouth)
Delivery: January 16, 2018 (feeding began on January 18 after fully buttoned up)
Release: March 2, 2018
Fed/Imprinted: 46 days
Release number: 34,318 fish
3rd Batch Delivery – 35,000 fry (from hatchery trays, not yet fed by mouth)
Delivery: March 7, 2018 (feeding began on March 10 after fully buttoned up)
Release: May 12, 2018
Fed/Imprinted: 67 days
4th Batch Deliveries – 4,115 fry (raised from eggs for 3 months by 23 schools in SIS Program)
Deliveries: March 26 through April 13, 2018 (weekdays)
Release: May 12, 2018
Fed/Imprinted: 30-48 days, depending on date of school delivery
3rd and 4th batches were combined as school fish were delivered between Mar 26 and Apr 13
Release: May 12, 2018 (final release of combined 3rd and 4th batches)
Release number: 37,183 fish (3rd and 4th batches combined)