‘Perfect storm’ of weather events led to fish kill in Siesta Key lagoon

The ghastly sight of thousands of mullet, bluegill, snook, bream, and pinfish belly up and ensnared in the mangroves of the brackish lagoon on the southern tip of Siesta Key was jarring to residents but not all that uncommon in Florida, air and water quality experts said.

It’s induced by changes in the water.

From Nov. 15 to Nov. 22, the Florida Fish and Wildlife Conservation Commission received 17 reports of fish kills from Heron Lagoon residents.

The Herald-Tribune took a water sample to experts at Mote Marine Laboratory, who said they did not find the red tide organism, Karenia brevis, or poor water salinity under a microscope. The FWC also took samples and confirmed those results.

The state wildlife agency, citing the absence of harmful algae, referred callers to the “Sarasota County Air and Water Pollution Department,” which also received multiple emails and calls and sent staff to the landlocked water body to investigate the situation. They noticed discoloration of water in the lagoon and fish gasping for air at the surface.

County sewer staff checked utilities in the area Nov. 15 and found the system operating as designed and no reports of sewer spills or illicit activities in the area.

A dissolved oxygen test was conducted Nov. 21, and the level measured in the lagoon was 1.36 milligrams/liter, and dead fish were observed. Dissolved oxygen levels below 5.0 mg per liter can stress aquatic life. Levels under 1 to 2 mg per liter for an extended period of time may result in fish kills, county officials said.

“It is more than likely that the lagoon had an algae bloom in recent weeks,” said county spokeswoman Brianne Grant. “The recent cold snap last week and several overcast days may have contributed to the algae dying off rapidly. When algae dies off, it sinks to the bottom of the water column and begins to decay. The process by which