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Forecasting Oregon Coho Marine Survival

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April 2005 archive

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Marine survival rates for Oregon coho salmon are influenced by several sequential environmental processes that affect coastal ocean food webs:

  1. Winter climate prior to smolt migration from rivers to the ocean;
  2. Date of occurrence of the “spring transition”, the period when alongshore winds (off the Pacific Northwest (PNW) coast) shift from being mostly northward to mostly southward;
  3. Total coastal upwelling during the spring;
  4. Ocean conditions during the maturing cohos’ only winter at sea.

By monitoring conditions during each cohort’s lifecycle, we can provide an experimental forecast of Oregon coho marine survival rates.

Recent and Forecasted Conditions

Environmental Index Observed/
Forecasted
Conditions		 Relative Condition Implication for coho marine survival
Oregon Coastal Sea Surface Temperature, January-February-March 2004 (before ocean entry) 10.5° C A little above average

Slightly
UNFAVORABLE
Spring Transition Date, 2004 Day 110 About 20 days later than average UNFAVORABLE
Neah Bay Coastal Sea Level (proxy for upwelling and alongshore transport), April-May-June 2004 -75.02 mm About average NEUTRAL
Forecasted Oregon Coastal Sea Surface Temperature, January-February-March 2005(after ocean entry) 10.74° C Above average UNFAVORABLE

Current Forecast

Issue date: April 25, 2005 (update of February 2005 forecast)

FORECAST RETURN RATE FOR ADULTS RETURNING FALL 2005 (mean rate): 0.5% (+/- 0.5%)

Observed Jan-March 2005 Sea Surface Temperature
Forecasted Return Rate for March 2005
10.74°C
0.5 +/- 0.5% (0 to 1%)

This winter's ocean temperatures (t1, after ocean entry) ended up being very warm despite cool temperatures in January (March 2005 temperature was 11.35,
among the warmest on record), and this late winter warming is also unfavorable for over-winter survival for maturing coho (and smolt survival this spring).

Next Forecast Update:
The early long-lead forecast for 2006 adult returns will be available in September 2005 (requires a forecast for January-February-March 2006 Oregon coastal sea surface temperatures).

Past Forecast Performance

Coho Returns (Year)
2002
2003
2004
2005
2006
Forecast
4-8%
5-7%
0-2%
0-1%
  
Observed
2.6%
3.9%
2.5%
   

See Figure 1 for the model’s performance at hindcasting for the period 1969-1998.

click image to enlarge

model’s performance at hindcasting for the period 1969-1998

Figure 1

Forecast Methodology

To better understand and predict Oregon coho marine survival, we developed a conceptual model (Figure 2) of key environmental processes that influence coastal ocean food webs and ultimately marine survival rates for Oregon coho salmon. The key processes are sequential:

  1. winter climate prior to smolt migration from rivers to the ocean;
  2. date of occurrence of the spring transition, when alongshore winds (off the PNW coast) shift from being mostly northward to mostly southward;
  3. total coastal upwelling during the spring;
  4. ocean conditions during the maturing cohos’ only winter at sea.

We then parameterized a general additive model (GAM) with Oregon Production Index (OPI) coho smolt-to-adult survival estimates from 1970-2001 and the environmental processes listed above. For the model training period (smolt year data from 1969-2000), the GAM explained 75% of the variance in observed OPI smolt-to-adult survival rates.

click image to enlarge

conceptual model of key environmental processes that influence coastal ocean food webs

Figure 2

Selected References

Logerwell, E. A., N. J. Mantua, P. Lawson, R. C. Francis, and V. Agostini. 2003. Tracking environmental processes in the coastal zone for understanding and predicting Oregon coho (Oncorhynchus kisutch) marine survival. Fisheries Oceanography 12(3):1-15.