Earlier this summer, reports came flooding in of dead fish floating on the surface of Biscayne Bay and washing up on its shores. Concerned citizens reported these sightings to their local government and environmental organizations, who confirmed their fears: what they were seeing was the beginning of Miami’s fourth major fish kill.
What is a fish kill?
A fish kill is defined as a localized die-off of several species of marine organisms, and the number of mortalities can vary greatly. The fish kill reported this June resulted in the death of around 2,000 fish, a small event compared to the August 2020 fish kill that saw over 27,000 dead fish wash up on the shores of Biscayne Bay.
What causes these mass mortalities?
Fish kills are typically attributed to natural causes, but pollution and climate change induced by humans can make these events more frequent and deadly.
The June 2024 fish kill in Biscayne Bay has been reported as the result of several days of intense rainfall that flooded South Florida homes and highways, creating chaos in low-lying areas like Miami Beach and Fort Lauderdale. Local 10 News reported low salinity levels in the bay following these rainstorms, indicating a strong freshwater influx into this brackish environment. Floodwater is notoriously highly contaminated with pollutants such as litter, oil and gas, sewage, and pet waste, and the introduction of these pollutants to Biscayne Bay creates less than favorable water quality.
Caption: Dead fish at Morningside Park, Miami in June 2024 (Photographed by Sandy Moise)
A study published in the Journal of Aquatic Plant Management found that fish kills in Florida were largely due to low dissolved oxygen levels in nutrient-rich water, with 64% of studied fish kills attributed to low dissolved oxygen levels (Hoyer et al., 2009). Dissolved oxygen levels may drop after an algal bloom, in which excess nutrients cause algae to grow rapidly, creating a sludge-like film on the water’s surface. The density of this algae can prevent sunlight from reaching photosynthetic organisms on the seafloor and creates anoxic conditions as it decays. A study published in the journal Harmful Algae found that one fish kill in a river in Florida occurred after an algal bloom; decomposition was attributed to hemolytic bacteria introduced by the decomposition, while dissolved oxygen levels remained normal (Landsberg et al., 2020).
According to Miami Waterkeeper’s report, the devastating fish kill in August 2020, which resulted in the deaths of marine creatures from 56 species, has been linked to low dissolved oxygen levels, likely due to nutrient pollution.
Caption: Harmful algal bloom (Photographed by Jennifer L Graham, Ph.D.)
How can we prevent human-induced fish kills?
Identified as a cause of low dissolved oxygen levels, nutrient pollution is often the result of human influence. Sewage leaks, due to broken pipes and decaying septic tanks, may cause strong outflows of raw sewage directly into the ocean or slow leaks into our groundwater and waterways that flow into the bay. Excess nitrogen and phosphorus can also enter the ocean due to fertilizer runoff, a phenomenon that pushed Miami-Dade County to pass the Florida-friendly fertilizer use on urban landscapes ordinance in 2021 that bans fertilizer usage from May 15 - October 31. Aligning with the rainy season in Miami, fertilizer does not typically have time to saturate soil before a rainstorm during this period. Homeowners can also prevent nutrient runoff by using nitrogen-only and slow-release fertilizers and not fertilizing within 20 feet of a storm drain or body of water.
Green Thumb Strategies has a solution for nutrient-rich waters: Phytoflora Floating Wetlands. These installations consist of plants growing on buoyant platforms with their roots dangling in a polluted body of water. These plants perform phytoremediation, a process by which their roots filter the water, removing excess nutrients through absorption. Phytoflora is unique from other floating wetlands in that they grow commercially valuable plants that can be harvested and sold, with the proceeds supporting the installation of more wetlands.
To report a fish kill, please contact the Division of Environmental Resource Management (DERM) and Miami Waterkeeper.
References:
Aguirre, L., & Brenman, A. P. (2024, June 19). Another fish kill hits Biscayne Bay after record flooding: “we’re devastated.” WPLG.
Fertilizer Awareness Toolkit - Miami Waterkeeper. Miami Waterkeeper. (n.d.).
Cleaning Water and Creating Opportunity with Floating Wetlands. Phytoflora by Green Thumb Strategies. Green Thumb Strategies. (n.d.).
Hoyer, M., Watson, D., Willis, D., & Canfield, D. (2009). Fish Kills in Florida's Canals, Creeks/Rivers, and Ponds/Lakes. Journal of Aquatic Plant Management, 47, 8-11.
Landsberg, J. H., Hendrickson, J., Tabuchi, M., Kiryu, Y., Williams, B. J., & Tomlinson, M. C. (2020). A large-scale sustained fish kill in the St. Johns River, Florida: A complex consequence of cyanobacteria blooms.” Harmful Algae, 92, Article 101771.
Morejon, S., Silverstein, R., & Kelly, E. (2022, May 24). Fish Kill in Biscayne Bay: A report and a Plan.” Miami Waterkeeper.
Image sources:
Graham, J. L. (n.d.). Harmful Algal Blooms. U.S. Geological Survey.”