When they’re chased! When not swimming!
The Pacific Salmon spawn is highly variable, depending upon individual historic timing for a stream, the species and the geographic location. Still not much help!
In general terms the earliest salmon runs are in the north and as the season progresses the runs are maturing and spawning farther south. So the earliest runs occur in NW Alaska — some of the fish coming into the rivers as early as April and May, immediately after the rivers thaw. Then the peak spawn happens during the summer months and into the early fall. However, by late fall in the north you are starting to get early freeze-ups, both preventing the salmon accessing the shallows and, more importantly, preventing the hatching fingerlings finding any insects upon which to feed.
So once the freeze ups start putting the dead and dying carcasses out of reach under the ice, the eagles are generally forced south. This means that in the years of early and strong early freeze ups we get thousands more eagles forced early to our southern waters that are still open. In most years the rivers around southern BC, and particularly our Harrison River complex, the heavy die-off is not available until late October or early November. But nature is full of variation. The northern freeze up can be severe and yet the Chilkat River, at least the 4 miles bordering the “Council Grounds”, stays open most years due to a very unusual and local phenomena. During the ice age 10,000 or more years ago, the glaciers carved out a 1000 meter deep trench 20 miles from the river mouth. This then filled in with large boulders causing a large sink that slowly circulates the warmer waters from down deep keeping the surface waters open all winter or at least until the new year.
This phenomena has encouraged runs of salmon to come out of the ocean until late into December. These late runs make fresh fish and spawned out carcasses available from July through January — the great bonanza that attracts 4000 or more northern eagles each winter. This phenomena remains an interesting conflict for nesting eagles that have to cope with all these intruders through their nesting season and an opportunity for immature eagles to feed freely throughout the winter at this site.
In most areas the sockeye come in early but different genetic populations of sockeye can be spread out over 3 months in the same river — and this applies to all species, some of which don’t start the upstream run until months after the earlier species. The result of the genetic variation, within the species and between the species, can yield dead carcasses to eagles for 6 or more months on some rivers.
To the south the focus is a little earlier. In Goldstream River, a little stream just NW out of Victoria, BC on Vancouver Island, there are chum, coho and springs starting in early October but peaking in December. The famous Chehalis — Harrison River complex, where we support the annual Fraser Valley Bald Eagle Festival, has peak runs of all 6 salmon species starting as early as October and is largely eaten out by February.
So the real answer to the question is; The salmon can be in the rivers from Alaska to northern California from May through February — which is most of the year. And for the other months some salmon accumulate off the river mouths or as fry and yearlings (some species stay in fresh water a year) are available in the adjacent waters. The west coast eagles, like the native populations, are the products of the salmon.