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Thousands of fish turn up dead in Biscayne Bay. FWC will test water for algae blooms.

For the past five years, Kathryn Mikesell has taken a daily morning swim with friends in Biscayne Bay just in front of Morningside Park. On Monday, she quickly got out of the water when she noticed hundreds of dead fish floating nearby.

“I had never seen anything like that in this area before,” Mikesell said. “We were swimming in the middle of hundreds and hundreds of dead fish.”

Over the weekend, she said she and her swimming group collected 20 bags of garbage from the Bay after the recent rains. She suspects that decaying trash, septic tank and stormwater runoff mixed with the heat created “the perfect storm” for the fish kill, which included puffer fish, eels and some larger fish. She may be right.

Morningside residents were concerned after observing thousands of dead fish in Biscayne Bay on Monday.
Morningside residents were concerned after observing thousands of dead fish in Biscayne Bay on Monday.

Thousands of dead fish have been spotted floating in different locations in Biscayne Bay since Monday as water temperatures reached about 90 degrees and dissolved oxygen dropped to very low levels. They’ve been documented in photos from Miami Waterkeeper and concerned residents.

A Miami Waterkeeper sampling team first observed the fish kill near Morningside Park earlier Monday, alerting environmental authorities. By Tuesday, dead fish had also been reported south of the Venetian Causeway and in the Julia Tuttle Causeway area, according to Miami Dade County’s Department of Environmental and Resources Management (DERM).

“What we think happened is low oxygen levels in the water, which is very hot right now,” said Miami Waterkeeper executive director Rachel Silverstein. “And there are probably other factors reducing the oxygen that’s available for fish to use, like runoff after the recent rains, nutrients from fertilizers, septic effluent and other things like pet waste going into the Bay.”

Fish kills are not uncommon in the summer, when warmer water and higher salinity levels lead to a drop in oxygen dissolved in the Bay, especially in shallow areas. If algae blooms occur, there’s even more competition for oxygen: during the day, the algae produce oxygen through photosynthesis; but at night, when that process stops, algae and other organisms like sargassum and bacteria continue to use up oxygen.

Residents believe increased pollution may have contributed to the fish kill that was first observed on Monday.
Residents believe increased pollution may have contributed to the fish kill that was first observed on Monday.

It’s unclear if algae blooms are contributing to the fish kill, but Waterkeeper is sending water samples to the Fish and Wildlife Conservation Commission for analysis. DERM is also investigating the event, as staff were out doing its monthly sampling when inspectors observed the fish kill, according to spokeswoman Tere Florin. They also collected specimens to send to FWC.

She said DERM documented water depth of about 3 feet and temperature of nearly 90 degrees in the western Julia Tuttle basin on Monday. The dissolved oxygen levels were “extremely low” in the shallow water column, she said, adding staff didn’t note any turbidity, smells or discoloration or other indications of a possible cause of the fish kill.

“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,” Florin said in an emailed statement.

Staff is conducting a monthly surface water monitoring program today and will try to understand what might have caused the kill and send water samples to FWC to test for harmful algal bloom.

According to FWC’s Fish Kill Hotline, three reports of a fish kill in Biscayne Bay were filed yesterday.