[env-trinity] CBB: Wild Chinook Study: Smolt Size When Leaving Freshwater A Determining Factor For Return Age

[Sari Sommarstrom]

THE COLUMBIA BASIN BULLETIN: Weekly Fish and Wildlife News

www.cbbulletin.com      December 4, 2015           Issue No. 773

 

* 10-Year John Day Wild Chinook Study: Smolt Size When Leaving Freshwater A
Determining Factor For Return Age

 

The age at which a salmon returns to its native stream to spawn is
determined before the smolt-sized fish leaves freshwater, according to a
study of John Day River wild chinook salmon.

 

The huge study that tagged over 24,000 wild chinook salmon over a ten-year
period, concluded that smolts that are smaller yet healthy when they leave
their native stream more than likely will return as four and five year old
adults to spawn.

 

Those are the adults that are most fecund, which in turn boosts demographic
productivity, according to the report. Conversely, larger smolts in
freshwater tend to return as three-year-old adults, commonly known as jack
salmon.

 

"Our study adds to growing evidence indicating that the age at which each
salmon or steelhead will eventually mature (provided that they survive
migration and rearing) is established before leaving freshwater," said Ian
Tattam, supervisory fish and wildlife biologist, Northeast-Central Oregon
Research and Monitoring, with the Oregon Department of Fish & Wildlife.
"Fisheries managers therefore have an opportunity to influence adult age via
actions which change freshwater conditions."  

 

Furthermore, the study concluded that John Day River chinook salmon smolts
from more abundant cohorts had a shorter average fork length than those from
less abundant cohorts.  

 

"Allowing high spawner escapements, which can produce abundant smolt
cohorts, will maintain older-aged chinook salmon in the John Day River,"
Tattam said, and that very likely would apply to populations of chinook
salmon in other rivers.  "We hypothesize that maintaining high spawner
density will contribute to older aged chinook salmon in other populations as
well."

 

High spawner density results in a smaller smolt. Larger smolt sizes,
according to the study, result in adults maturing and returning at younger
ages.

 

"Because age-3 Chinook Salmon are predominantly male (Scheuerell 2005),
maturation at age 3 versus ages 4 or 5 has implications for population
demographics," the study says. Age 4 adults (2 years in freshwater, two
years in saltwater) dominated the return of chinook salmon on the John Day
River in most years.

 

Researchers also found that earlier, as well as larger, migrating smolts
tend to return as younger adults, and that ocean conditions did not
influence the age at which an adult returns. "Individual age-at-maturity
trajectories appear to be established strictly prior to ocean entry," the
report says.

 

"Length and Condition of Wild Chinook Salmon Smolts Influence Age at
Maturity" was published online Oct. 22, 2015, in Transactions of the
American Fisheries Society
(http://www.tandfonline.com/doi/abs/10.1080/00028487.2015.1082503?journalCod
e=utaf20#.VmBsFfmrTIU) 

 

Tattam's co-authors are Jim Ruzycki, Mid-Columbia program manager,
Northeast-Central Oregon Research and Monitoring, ODFW; Josh McCormick,
fisheries biometrician, ODFW; and 

Rich Carmichael (retired), program director, Northeast-Central Oregon
Research and Monitoring, ODFW.

 

"Larger smolts had an increased probability of returning at age 3 (commonly
referred to as a 'jack') instead of age 4 or age 5," Tattam said.  "Ocean
conditions were an important factor in determining the number of returning
salmon, but ocean conditions did not influence the age at which surviving
salmon returned to spawn."

 

"Freshwater rearing conditions (e.g., population density of juvenile chinook
salmon) experienced by juvenile salmon prior to emigrating to the ocean did
influence smolt size and, therefore, the age of chinook salmon which
survived to return," Tattam concluded.

 

The John Day River in Oregon is one of the few tributaries of the Columbia
River (it enters the Columbia at river kilometer 351 or river mile 218) that
is undammed. Hatchery chinook salmon have never been introduced to the
stream and stray hatchery fish amount to less than 2 percent of spawning
salmon. Some 2,000 to 7,000 wild chinook return to the river each year.

 

The study looked at three populations of wild chinook salmon in the John Day
River system: North Fork, Middle Fork and the upper Main Fork. Researchers
counted redds on the spawning grounds and later seined further downriver to
capture and PIT-tag a sampling of migrating smolts. Overall, they tagged
24,240 wild chinook salmon smolts.

 

Tattam said it is possible that the results of the study could be applicable
to other wild chinook salmon populations in other rivers of the interior
Columbia River basin which have a predominantly yearling age at
smoltification. But for steelhead there is evidence smolt size and age at
maturation are inversely related.  

 

"Steelhead in the interior Columbia River basin (with the exception of a few
Idaho populations) have a narrower maturation spectrum (i.e., they mature at
two ocean ages, instead of three like chinook salmon), which may reduce the
ultimate importance of smolt size on population demographics," Tattam said.

 

Results in the study also have implications about how salmon populations
survive in years when spawner numbers are down.

 

"Our results lead to a hypothesis that during periods of low spawner
abundance, John Day River Chinook Salmon shift towards younger adults
(decreased cohort mean age). We theorize that this shift in age structure
maximizes the odds of adult return by reducing the number of years in the
ocean, thereby allowing a quicker population-scale response to low
freshwater abundance," the report says.

 

"Long-term monitoring studies such as this, while expensive and challenging
to conduct year-after-year, are vital to the constant evolution of salmonid
management," Tattam concluded.  "This study captured and tagged more than
24,000 wild juvenile salmon across 10 years; during peaks and troughs in
both population abundance and ocean productivity.  Only with such a broad
data set were we able to discern the influence of smolt size, cohort
abundance, and ocean productivity on age at maturation.  A study of only a
few years in duration would not have encompassed a broad spectrum of
conditions, nor allowed us to provide useful guidance for management of John
Day River chinook salmon."

 

The study was funded by the Bonneville Power Administration and the Oregon
Watershed Enhancement Board.

 

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