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Could Fish Kill In Biscayne Bay Be A Sign Of Worsening Pollution?

Sunrise showing a fish kill in Biscayne Bay
Courtesy of Kathryn Mikesell
A fish kill spread across northern Biscayne Bay near Morningside on Monday.

When the Dolphins and Rainbows swim club headed out Monday morning for their regular swim in Biscayne Bay, they encountered a grisly sight: hundreds of dead fish in the Tuttle Basin.

“All of a sudden, we realized that we were literally surrounded by dead fish,” said Kathryn Mikesell, who’s been swimming with the group for five years and has lived in Morningside for two decades. “When we were getting back out, the number of fish was just increasing. The number was increasing by the minute.”

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Mikesell took pictures, tried to contact the U.S. Environmental Protection Agency and reported what she saw to Miami Waterkeeper, which had a crew nearby doing regular water monitoring for its swim advisories.

The group sent water samples to the Florida Department of Environmental Protection, which opened an investigation.

The kill was also documented by Miami-Dade environmental staff doing routine water monitoring near the Julia Tuttle Causeway. An inspector sent to the Venetian Causeway confirmed the kill stretched between the two causeways.

By Tuesday, pictures of the dead fish started turning up on social media as more and more residents spotted the fish.

Pollution in the bay has come under growing scrutiny as conditions worsen. Historically, the long shallow bay was low in nutrients and filled with clear water, which allowed seagrass to flourish. But increasing stormwater flushed down canals, leaky septic tanks and fertilizer have begun to permanently alter the bay. Those conditions are likely to worsen as climate change drives up sea level and worsens flooding.

Three years ago, despite having survived years of dredging and heavy boat traffic, more than 75 percent of the seagrass in the Tuttle Basin died.

And last year, the National Oceanic and Atmospheric Administration issued a dire report based on 20 years of data. The report warned the urban bay, one of the only places in the planet inhabited by all seven species of seagrass famed for flats fishing, was undergoing a regime shift as chlorophyll and algae begin to dominate.

Marine biologist and Miami Waterkeeper executive director Rachel Silverstein said Monday's kill was likely due to chronic pollution.

“We've had lots of warnings from scientists that Biscayne Bay is at a tipping point for nutrient pollution and that we're going from having our beautiful clear blue water to having green water that smells bad, that can kill fish and is devoid of seagrass in life,” she said.

Nutrients, like phosphorus from fertilizer in stormwater and nitrogen from leaky septic tanks, can fire up algae blooms which cloud water and prevent seagrass from growing. Warmer water also contains less oxygen. Overnight, conditions can worsen when algae gulp up what little oxygen remains, leaving fish to essentially suffocate.

It’s not uncommon to see fish kills in canals and areas with poor water circulation during the steamy summer, Silverstein said. But the bay is unusual.

“To see this in the open bay like this and over a widespread area with this number of fish is extremely concerning,” she said. “This is really the tipping point that we have been worried about. And this is what scientists and advocates like Miami Waterkeeper have been saying could happen to Biscayne Bay if we don't get this pollution under control.”

In an email, DERM spokeswoman Tere Florin said the staff measured water temperatures near 90 degrees along with extremely low oxygen levels at about 3 feet deep.

“While we cannot be absolutely certain at this time, it appears that very high temperatures and very low dissolved oxygen levels in the shallow waters where the dead fish were observed by DERM staff earlier are likely contributing to or driving the situation,” she wrote.

The county sent water samples and dead fish to state regulators looking into the spill.

In her twenty years living next to Biscayne Bay, Mikesell said she’s seen the level of pollution steadily rise.

“We pull out a bag of trash consistently every day we swim,” she said.

After a recent heavy rain, she said, they collected 20 bags. Up to now, she considered swimming a factor in her good health. That's been especially true during the COVID-19 pandemic, as she's had more time to swim.

"I'm literally flushed every day with bay water," she said.

Now she's rethinking that routine.

"But maybe I've just built up a serious immune system thanks to the bay water,” she said. “I will not be swimming again until I don't have to walk through a sea of dead fish to start my swim.”