There are a number of things that make the Chum more valuable than other species of salmon. With all salmon, the majority of spawning adults come back to their natal streams and die shortly after spawning. The timing of the Chum run occurs later in the year and extends into December, which is a critical time for eagles (and other wildlife like bears) to fatten up to make it through the winter.
Chum have a high fat content which is excellent for bulking up fast. Another characteristic that make Chum relatively more valuable is where they spawn. Chum have a strong preference for medium to large, shallow, and low gradient streams and rivers. This is significant because when the spawned out salmon die, a much higher percentage of the morts (dead salmon) get hung up on shore, or on islands in the river (compared to being washed out to sea in high energy rivers).
Eagles are primarily scavengers, so with very little energy expenditure (compared to active hunting) they can feast on the recently dead (or dying) salmon.
As with any event, it is likely a combination of factors that have contributed to the state of the eagles, of which one additional factor may be the cooler weather we are experiencing lately translating to the eagles energy reserves being used up to keep warm.
The good news is that the 2010 Chum run was the same brood year ocean conditions that affected the sockeye run collapse in 2009, and given the high returns of sockeye in 2010 (an indicator of good ocean conditions) we should expect a much larger Chum run in 2011.
The other good news is that the Herring spawn is not far away (which may not be soon enough for some eagles), and Herring numbers have rebounded significantly since the early 1990’s.