Although some fish swim up the ladder on their own, others travel in schools or "flushes."
That's what I saw yesterday: Midway through my window, about 30 herring blasted up the ladder, passing the counting spot in 3-4 seconds. (That's why I have to say "about 30;" counting that many moving, weaving fish as they pass one small counting spot isn't easy.)
Here's why the fish travel in groups; large numbers of fish-eating birds gather at the base of the spillway, where the herring have to funnel into the ladder:
I've highlighted just the cormorants here:
It was interesting to see that the cormorants in the water were facing downstream, which meant the birds were having to paddle backwards, against the current, to maintain their positions. But that also meant that they could see the herring as they approached.
The cormorants weren't alone: Overhead, about 20 gulls were wheeling just over the spillway, too, looking for fish or scraps left by the cormorants.
It's quite a gauntlet the herring have to pass.
But pass they did, and I was glad to finally see my first flush.