Fish killed by red tide. Dead fish killed as a result of a bloom of Karenia brevis. K breves is a microscopic single-celled marine dinoflagellate that 'blooms' frequently along the Florida and Mexican coasts. These blooms look large like red algal blooms, hence the name red tide. It naturally produces a mix of potent neurotoxins (brevetoxins) that can kill marine animals such as fish and birds.