2013 Predicted Sockeye Returns

Can anyone explain why ODFW would make the following 2013 prediction considering so many fish came through last year, and recent years?.  Just seems a bit low considering the strong upward trend since 2007.:

Sockeye
2012: 515,700 counted at Bonneville
2013: 180,500 predicted at Bonneville

Biologists make predictions based on more than just previous counts.  There are a lot of other factors that play into the equation.

-Allen

http://nwsportsmanmag.com/2012/12/12/managers-unveil-more-columbia-salmon-forecasts-sharp-sockeye-dropoff/

After some searching...The above link explains it a bit...