Run Size Forecast for Yakima River Adult Spring Chinook, 2019

Bill Bosch
Announcement

In 2018 the forecast was for a return of 6,340 adult (age-4 and age-5) spring Chinook to the mouth of the Yakima River. The actual return in 2018 was estimated to be 3,160 adult spring Chinook (50% of forecast).  However, the 2018 return was impeded by a thermal barrier in the lower Yakima River.  Hot spring and summer air temperatures combined with reduced summer and fall flows increased water temperatures. Maximum daily water temperatures near Prosser (rkm 76 from the mouth of the Yakima R.) reached 70o F on June 3 and exceeded 70o F every day from June 14 to Sept. 11, 2018 with 27 summer days having maximums above 80o F (source  U.S. BOR hydromet database). Since 1997, preseason forecasts have generally been based on examination of brood cohort relationships from 1982 to present projecting age-4 returns using the return survival of age-3 fish in a given brood, and similarly projecting age-5 returns from age-4 returns.  In many recent years, high age-3 jack return abundance has not always been a reliable predictor of age-4 return abundance.  Yakima River biologists have been collaborating with other scientists from the region to see if parameters such as NOAA ocean ecosystem indicators2, jack size or other variables can help explain and correct for ocean conditions or other factors that might affect the reliability of jacks as a predictor.

To produce the 2010-2018 Yakima River forecasts, I evaluated output from at least eight different methods all of which used jack abundance in some combination with traditional cohort relationships, NOAA ocean ecosystem indicators, or jack size.  These methods produced preliminary forecasts for 2019 age-4 and age-5 adult returns ranging from 2,300 to 10,900 spring Chinook.  Given the relatively poor reliability of jack-based predictor methods for high jack count years, I have also been evaluating a method which uses Prosser smolt estimates, juvenile survival estimates for natural- and hatchery-origin fish from Roza to McNary Dams, and NOAA ocean ecosystem indicator data as input variables to forecast combined age-4 and age-5 adult returns.  These data are only available dating back to the 1997 brood year (1999 migration year), but were used to produce Yakima River forecasts for 2011-2014 and resulted in relatively good forecasts (a 23% under-forecast in 2011 and over-forecasts within 9% of actual returns for 2012- 14).  This method produced a forecast of 5,850 adult spring Chinook for 2019.  Continued analysis of data such as Chandler flow and entrainment relationships and freshwater (smolt) survival indices has resulted in substantial year to year variation in the data used to populate this model. Given the wide variability in forecasts from the various methods for 2019, the low jack return in 2018, and the fact that data from NOAA’s data set as well as the 2018 actual return indicate recent years’ environmental conditions have not been favorable for Chinook productivity3, I chose to use a model with a forecast at the lower end of the range. The model uses jack counts, juvenile freshwater survival indices, and NOAA ocean ecosystem indicators as inputs and produces a 2019 forecast of 2,970 adult spring Chinook.  The 2019 forecast is 1,250 wild/natural and 1,720 hatchery-origin adult (age-4 and age-5) spring Chinook returns to the Yakima Basin (Table 1).  Hatchery-origin returns are from the Cle Elum Supplementation and Research Facility (CESRF) in the Upper Yakima River which has been in operation since 1997.

Table 1.  Forecasted return of adult Spring Chinook to the Yakima River mouth in 2019 by age and stock.

Stock

Age-4

Age-5

Total Adults

Upper Yakima Natural

620

20

640

Upper Yakima CESRF

1,720

0

1,720

Naches/American Wild

360

250

610

Total Run

2,700

270

2,970

Total Wild/Natural

980

270

1,250

Total CESRF

1,720

0

1,720

The total 2019 forecasted return of 2,970 adult spring Chinook is 36% of the recent 10-year (2009-2018) average adult return of 8,180 spring Chinook.

The full report can be downloaded at the link below.