Red tide is concentrating on Florida's gulf coast. See the hot spots.

Dead fish are washing up on the southwest Florida coast thanks to a toxic algae known as red tide that can pose a risk to humans.

The algae, which is known formally as the single-cell Karenia brevis, has concentrated near Tampa and neighboring communities.

Scientists have found the algae at rates ranging from 10,000 cells per liter to more than 1 million cells per liter – levels that result in fish kills and breathing difficulties in exposed humans, according to the Florida Fish and Wildlife Conservation Commission.

The FWC said Friday that red tide was detected at concentrations greater than 100,000 cells per liter in samples from the following counties:

  • Pinellas

  • Manatee

The agency said red tide becomes harmful to people at 10,000 cells per liter. Red tide was also detected at background to low concentrations in the following Florida counties.

  • Sarasota County

  • Lee County

  • Charlotte County

  • Collier County

  • Bay County

  • Gulf County

  • Pasco County

  • Palm Beach County

Red tides produce a toxin called brevetoxin that can make humans ill if they breathe the toxin in through sea spray or get wet with contaminated water.

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The illness can cause a range of symptoms, according to the Centers for Disease Control and Prevention, including:

  • Coughing and sneezing

  • Shortness of breath

  • Eye, skin, and throat irritation

  • Asthma attacks

The FWC it had received multiple reports of dead fish in communities throughout Southwest Florida.

One community, Indian Rocks Beach, in Pinellas County, decided to cancel a beach festival slated for next month amid red tide concerns.

Red tides are a naturally occurring phenomenon that have been observed in the Gulf of Mexico since the 1800s. Nascent studies have connected nutrient-laden runoff from farms and developments to increased levels of red tide along the coast. They begin to form on the coast beginning in the fall, and typically clear up by spring.

Here’s where you can find red tide in Florida.

Florida red tide map

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This article originally appeared on USA TODAY NETWORK: Florida red tide map: Is there red tide right now? Check levels here.