called a "jubilee" happens!
This only regularly happens in Mobile Bay, Alabama and Tokyo Bay in Japan. When the conditions are just right, and the tide comes in, it will push dense oxygen deprived water to the surface of the brackish bay. This makes all the bottom-dwelling marine life flock to the surface. Because of the lack of oxygen, they are very slow and can be scooped up with little effort. No need to fear, animals rarely die during this phenomenon (unless they're dinner that night)!