All those birds falling out of the sky like that, plus a whole lot of fish just washing up on the shore like that.
I'd guess solar flare. If they were baked out of the sky, and the water was flash boiled in a certain spot, it would explain a lot. Although I'm not sure if a solar flare is capable of doing that to water. The fall from the sky would explain the blunt force trauma, though.
I do remember watching mythbusters, and fish, or at least their gills are highly fragile. If the water had a flash freeze or a quick heating, it could rupture the blood vessels in the gills and they would die from lack of oxygen. If it were a disease, it would be more than just one species of fish that was affected.