Evan, Tweets,

There are many species of fish in Green Lake. Some native, some non-native.
In years past there have been similar events as described by Evan with different fish congregating by the thousands in the shallow areas around the base of cattails or bulrushes, usually around this time of year. Sometimes these events went on for a week or two. These events always attracted the attention from Double-crested Cormorants as well as Common Mergansers, with the occasional Hooded Merganser thrown in.
Most of the events I witnessed were on the northwestern side of the lake, near Aurora Avenue where the 'Red-winged Blackbird cattails' are, but not all (see below).
Initially I thought these congregations were mass spawning events, but some years they included carp (including the occasional bright orange goldfish), very uniform in size; about bite-size for a cormorant. Since then I have seen the late-spring and summer spawning activity of the carp among the water lilies, and I don't think that the mass congregations in the shallows are related to spawning (there is no jockeying for position around spawning females by males to fertilize newly released eggs). I have come to the conclusion that these fish were driven into the shallows by fish eating birds, like cormorants and/or mergansers and are basically disoriented and trapped.
In support of this theory: one Saturday morning in early April several years ago a group of us birdwatchers witnessed a flock of Common Mergansers (about 50 strong) in pursuit of fish in the bay near the Aqua Theater (SW corner of the park). The mergansers looked more like dolphins or penguins as they pursued the school of fish at high speed, porpoising to breathe. We were standing on the dock, which has water beneath it, and we could hear fish splashing beneath us the whole length of the dock. As long as people were standing on the dock the mergansers kept their distance, but when we retreated the birds went in for 'the kill.'

Martin Muller, Seattle
martinmuller at msn.com<mailto:martinmuller at msn.com>