ST. PETERSBURG, Fla. - Scientists say toxic red tide is back in the waters off the Florida southwest coast after fading away earlier this year following a 15-month bloom.

Biologists at the Florida Fish and Wildlife Research Institute said Friday that samples taken from the waters off the shore of Collier County found high concentrations of the toxic algae where they also received reports of dead fish and cases of respiratory irritation.

Red tide is a natural occurrence that happens due to the presence of nutrients in salt water and an organism called a dinoflagellate. The 15-month bloom caused respiratory irritation in people and killed sea turtles, manatees, dolphins and fish.

Scientists also observed low concentrations of the red tide algae in Lee County, according to the institute’s red tide status report.