Why are dead fish in the water? What’s in your throat? How red tide hits Florida beaches

A red tide has persisted in Southwest Florida and Tampa Bay area waters since October 2022.

If you’re wondering why there are dead fish in the water or irritation in your throat when you visit the beach, look no further than Karenia brevis, the microscopic alga responsible for red tide. It produces toxins that can cause trouble for marine life and people.

The 2022-23 red tide has so far been mild compared to extremely potent bouts that have hit Florida’s Gulf coast over the past decade, including severe blooms between 2017-19 and in 2021.

But even at lower levels, red tide can cause a nuisance. When will it go away?

Here’s what science tells us about how red tides start, how long they last and what causes them to end.

What is red tide?

Toxic blooms: Red tides are a naturally occurring phenomena in the Gulf of Mexico.

“Red tides, also called harmful algal blooms (HABs), occur when microscopic algae multiply to higher-than-normal concentrations, often discoloring the water,” the Florida Fish and Wildlife Conservation Commission (FWC)’s Fish and Wildlife Research Institute says.

Anecdotal records of red tide date back to the writings of Spanish explorers in the 16th century, and it was first officially documented in Florida in 1844.

Other parts of the world have red tides caused by different algae species.

The trouble with K. brevis is the powerful neurotoxins that it produces. They can kill fish, seabirds and other marine life, including large mammals like manatees and dolphins, at high concentrations. The toxins are a respiratory irritant for people and can pose a danger to those with pre-existing conditions like asthma.

Seafood sickness: Red tide can also poison shellfish and cause people who eat it to get sick. Recent research points to further complications for human health from breathing the algae.

Growing evidence suggests that human pollution can make red tides more intense and long-lasting.

Why is there red tide in winter?

Arriving by winds and currents: There is a bloom of red tide in the Gulf of Mexico almost every year, according to Cynthia Heil, director of Mote Marine Laboratory’s Red Tide Institute in Sarasota.

The blooms begin far offshore before they are transported to the coast by winds and currents. Many factors influence the duration and strength of a red tide bloom, including water temperature, salinity, nutrient availability and the position of the Loop Current in the Gulf of Mexico.

The blooms typically start in the late summer or fall, and it’s not unusual for them to last into the winter or early spring.

How long does red tide last?

Time span: Southwest Florida’s red tides can last anywhere from a few days to weeks to months, depending on environmental conditions.

The FWC’s Fish and Wildlife Research Institute says most blooms last three to five months. Heil with Mote Marine Laboratory said it’s not uncommon for the blooms to endure up to seven months.

Much like the weather, scientists can predict red tide’s movement in the short term with some accuracy.

Longer-term predictions are tough due to the many factors at play.

Strength: If K. brevis becomes concentrated enough to start killing fish, it usually leads to a longer and stronger red tide event, says Robert Weisberg, an emeritus oceanography professor who helped build the University of South Florida’s red tide tracking program.

The decaying marine life provides the algae with a built-in nutrient supply and allows the bloom to thrive, as long as other factors permit.

What makes a red tide end?

Ocean pattern: Scientists know how red tides start and approximately how long they last. What causes them to end is less certain.

A recent study led by the University of South Florida’s Ocean Circulation Lab director Yonggang Liu found evidence that a particular ocean pattern may play a key role.

The study found that a “persistent upwelling circulation” — that is when deeper ocean water is continuously brought to the surface by forces including wind and currents — can quickly end a bloom by flushing out the waters where red tide is concentrated. Surface currents carry away the red tide algae that remain at the surface, and the bloom gradually dissipates.

But there is a caveat. If there is an offshore source of K. brevis cells, the upwelling current can bring them to shore and make red tide worse, the study says.

Should I avoid the beach during red tide?

Risks: Just because there is red tide in the Gulf doesn’t mean that you have to stay away from the beach.

Red tides are typically patchy and affect different beaches at different times. Beach conditions can be poor in one location and pristine just a few miles away. Conditions can improve or worsen as winds and currents shift.

Advice: Health officials advise visitors to be conscious of where red tide is present and avoid swimming near dead fish. Informational signs are posted at public beaches during red tide events with safety tips.

People with respiratory conditions should be especially careful to avoid impacted areas.

How can I find red tide conditions near me?

Resources: Several online resources make it easy to check if your favorite beach is being impacted by red tide.

NOAA’s Red Tide Respiratory Forecast is updated several times a day with the level of red tide risk at specific beaches along the Gulf Coast, from Pinellas County through Collier County. Visit habforecast.gcoos.org.

Mote Marine Laboratory’s Beach Conditions Report is updated frequently by lifeguards and includes reports of red tide conditions, including respiratory irritation and dead fish, as well as other hazards on local beaches. Visit visitbeaches.org.

The Florida Fish and Wildlife Conservation Commission issues weekly red tide reports with conditions by county and maintains a map of red tide sampling around the state. Visit myfwc.com/research/redtide/statewide.

University of South Florida’s Red Tide Prediction and Tracking center provides short-term forecasts of red tide’s movement around Tampa Bay and the west coast of Florida. Visit ocgweb.marine.usf.edu/hab_tracking.