I can’t really describe what it is like to witness this. It doesn’t happen very often. It’s based entirely on when water levels in the marshes are being changed by opening a series of trunks, water gates, across multiple marshlands. In general a series of events triggers this.
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When the water spills through the open gates fish are forced through also, into the low water side. It becomes an open dining feast for every big Alligator in the area. They congregate by the flooding stream and just eat fish, lots of fish.
This photograph was taken by standing on top of the water gate, looking down with a wide angle lens. Most likely about 24 mm.